Modern Nutrition in Health & Disease PDF

A. M A J O R D I E TA R Y C O N S T I T U E N T S Proteins and Amino Acids1 1 DWIGHT E. MAT T H E W S ce...

A. M A J O R D I E TA R Y C O N S T I T U E N T S Proteins and Amino Acids1 1 DWIGHT E. MAT T H E W S centers to facilitate the biochemical reactions of life that AMINO ACIDS.................................. 4 either would run out of control or not run at all without Basic Definitions............................. 4 them. Life could not have begun without these enzymes, Amino Acid Pools and Distribution............... 7 thousands of different types of which are found in the Amino Acid Transport......................... 8 body. Proteins are prepared and secreted to act as cell–cell PATHWAYS OF AMINO ACID SYNTHESIS signals in the form of hormones and cytokines. Plasma AND DEGRADATION............................. 9 proteins produced and secreted by the liver stabilize the Amino Acid Degradation Pathways.............. 9 blood by forming a solution of the appropriate viscosity Synthesis of Dispensable Amino Acids........... 12 and osmolarity. These secreted proteins also transport a Incorporation of Amino Acids variety of compounds through the blood. into Other Compounds....................... 13 The largest source of protein in higher animals resides TURNOVER OF PROTEINS IN THE BODY.............. 14 in muscle. Through complex interactions, entire sheets METHODS OF MEASURING PROTEIN TURNOVER of proteins slide back and forth to form the basis of AND AMINO ACID KINETICS...................... 15 muscle contraction and all aspects of our mobility. Muscle Nitrogen Balance........................... 15 contraction provides for pumping oxygen and nutrients Arteriovenous Differences to throughout the body, for inhalation and exhalation of our Define Organ Balances....................... 17 lungs, and for movement. Many of the underlying causes Tracer Methods Defining Amino Acid Kinetics..... 17 of noninfectious diseases are the result of derangements CONTRIBUTION OF SPECIFIC ORGANS TO in the sequence of proteins. The incredible advances PROTEIN METABOLISM.......................... 23 in molecular biology provided tremendous information Whole Body Metabolism of Protein............. 23 about DNA and RNA and introduced the field of genom- Role of Skeletal Muscle in Whole Body ics. This research is not driven to understand DNA itself, Amino Acid Metabolism...................... 25 but rather to understand the purpose and function of Metabolic Adaptation to Fasting and Starvation... 25 the proteins that are translated from the genetic code. The Fed State.............................. 26 The emerging field of proteomics studies the expression, PROTEIN AND AMINO ACID REQUIREMENTS......... 26 modification, and regulation of proteins. Protein Requirements........................ 27 Three major classes of substrates are used for energy: Amino Acid Requirements.................... 29 carbohydrates, fat, and protein. Protein differs from the Assessment of Protein Quality................. 31 other two primary sources of dietary energy by inclusion Protein and Amino Acid Needs in Disease........ 32 of nitrogen (N). Protein on average is 16% by weight N. The component amino acids of proteins contain one N in Proteins are associated with all forms of life, and much the form of an amino group and additional N, depend- of the effort to determine how life began has centered ing on the amino acid. When amino acids are oxidized to on how proteins were first produced. Amino acids joined carbon dioxide (CO2) and water to produce energy, N is together in long strings by peptide bonds form proteins also produced as a waste product that must be eliminated that twist and fold in three-dimensional space and produce via incorporation into urea. Conversely, N must be avail- able when the body synthesizes amino acids de novo. The 1 Abbreviations: ATP, adenosine triphosphate; AV; arteriovenous; BCAA, synthetic routes of other N-containing compounds in the branched-chain amino acid; CO2, carbon dioxide; CoA, coenzyme A; body (e.g., nucleic acids for DNA and RNA synthesis) DAAO, direct amino acid oxidation; EAR, estimated average require- obtain their N during synthesis from donation of N from ment; FAO/WHO/UNU, Food and Agriculture Organization/World amino acids. Therefore, when we think of amino acid Health Organization/United Nations University; IAAO, indicator amino acid oxidation; IDAA, indispensable amino acid; KIC, -ketoisocaproate; metabolism in the body, we really mean N metabolism. N, nitrogen; NH3, ammonia; PER, protein efficiency ratio; RDA, recom- Protein and amino acids are also important to the mended dietary allowance; TCA, tricarboxylic acid; TML, trimethyllysine. energy metabolism of the body. As Cahill pointed out (1), 3 MNHD11e_CH001.indd 3 09/24/12 5:00 PM  4 PART I SPE CI F I C DI E T ARY COM P O N E N T S TABLE 1.1 BODY COMPOSITION OF A NORMAL MAN (b) uncharged but polar, (c) acidic (negatively charged), IN TERMS OF ENERGY COMPONENTS and (d) basic (positively charged) groups. Within any class are considerable differences in shape and physical prop- MASS ENERGY AVAILABILITYa erties. Thus, amino acids are often grouped into other COMPONENT (kg) (kcal) (d) functional subgroups. For example, amino acids with an Body water and 49.0 0 0 aromatic group—phenylalanine, tyrosine, tryptophan, and minerals Protein 6.0 24,000 13.0 histidine—are often associated together, although tyrosine Glycogen 0.2 800 0.4 is clearly polar and histidine is also basic. Other common Fat 15.0 140,000 78.0 groupings are the aliphatic or neutral amino acids (glycine, Total 70.0 164,800 91.4 alanine, isoleucine, leucine, valine, serine, threonine, and a Availability is the duration for which the energy supply would last proline). Proline is different in that its functional group based on 1800 kcal/day resting energy consumption. is also attached to the amino group, thus forming a five- Data from Cahill GF. Starvation in man. N Engl J Med 1970;282:668–75, membered ring. Because of the ring, proline is actually with permission. an imino acid, not an amino acid. Serine and threonine contain hydroxyl groups. There is also another impor- protein is the second largest store of energy in the body tant subgroup: the branched-chain amino acids (BCAAs: after adipose tissue fat stores (Table 1.1). Carbohydrate isoleucine, leucine, and valine), which share common is stored as glycogen, and although it is important for enzymes for the first two steps of their degradation. The short-term energy needs, it is of very limited capacity for acidic amino acids, aspartic acid and glutamic acid, are providing for energy needs beyond a few hours. Amino often referred to as their ionized, salt forms: aspartate acids from protein are converted to glucose by the process and glutamate. These amino acids become asparagine and called gluconeogenesis to provide a continuing supply of glutamine when an amino group is added in the form of an glucose after the glycogen is consumed during fasting. amide group to their carboxyl tails. Conversely, however, protein stores must be conserved The sulfur-containing amino acids are methionine and for the numerous critical roles in which protein func- cysteine. Cysteine is often found in the body as an amino tions in the body. Loss of more than approximately 30% acid dimer called cystine in which the thiol groups (the of body protein results in reductions in muscle strength two sulfur atoms) are connected to form a disulfide bond. for breathing, immune function, organ function and, ulti- Particular attention should be paid when reading the lit- mately, in death. Hence, the body must adapt to fasting erature to note the distinction between the names cysteine by conserving protein, as is seen by a dramatic decrease and cystine, because the former is a single amino acid, in N excretion within the first week of onset of starvation. and the latter is a dimer with different properties. Other Body protein is made up of 20 different amino acids, amino acids that contain sulfur, such as homocysteine, are each with different metabolic fates in the body, with not incorporated into protein. diverse activities in different metabolic pathways in differ- All amino acids exist as charged particles in solution: in ent organs, and with varying compositions in different pro- water, the carboxyl group rapidly loses a hydrogen to form teins. When amino acids are liberated after absorption of a carboxyl anion (negatively charged), whereas the amino dietary protein, the body makes a complex series of deci- group gains a hydrogen to become positively charged. sions concerning the fate of those amino acids: to oxidize The amino acids therefore become “bipolar” (often called them for energy, to incorporate them into proteins in the a zwitterion) in solution, but without a net charge (the body, or to use them in the formation of a number of other positive and negatively charges cancel). The attached N-containing compounds. The purpose of this chapter is functional group may distort that balance, however. The to elucidate the complex pathways and roles amino acids acidic amino acids lose the hydrogen on the second car- play in the body, with a focus on nutrition. boxyl group and become negatively charged in solution. In contrast, the basic group amino acids in part accept a AMINO ACIDS hydrogen on the second N and form a molecule with a net positive charge. Although the other amino acids do Basic Definitions not specifically accept or donate additional hydrogens in The amino acids that we are familiar with and all those neutral solution, their functional groups do influence the incorporated into mammalian protein are “”-amino relative polarity and acid–base nature of the bipolar por- acids. By definition, they have a carboxyl-carbon group tion of the amino acids and give each amino acid different and an amino N group attached to a central -carbon properties in solution. (Fig. 1.1). Amino acids differ in structure by the sub- The functional groups of amino acids also vary by size. stitution of one of the two hydrogens on the -carbon The molecular weights of the amino acids are shown in with another functional group. Amino acids can be Table 1.2. Amino acids range from the smallest, glycine, characterized by their functional groups, which are often to the large and bulky molecules (e.g., tryptophan). Most organized at neutral pH into the classes of (a) nonpolar, amino acids crystallize as uncharged molecules when they MNHD11e_CH001.indd 4 09/24/12 5:00 PM  CHAPTER 1 P R O T E I N S A N D A M INO AC IDS 5 O R CH C-OH Aromatic amino acids NH2 Phenylalanine CH2 Neutral amino acids Tyrosine HO CH2 Glycine H CH2 Alanine CH3 Tryptophan N CH3 H Valine CH3-CH Histidine H-N CH2 CH3 N Leucine CH3-CH-CH2 Basic amino acids CH3 Isoleucine CH3-CH2-CH Lysine H2N-CH2-CH2-CH2-CH2 Serine HO-CH2 Ornithinine H2N-CH2-CH2-CH2 OH NH Threonine Arginine CH3-CH H2N C-NH-CH2-CH2-CH2 Fig. 1.1. Structural formulas of the 21 common -amino acids. The Sulfur amino acids Acidic amino acids and amides -amino acids all have a carboxyl Cysteine HS-CH2 O group, an amino group, and a differ- Glutamic Acid entiating functional group attached HO C-CH2-CH2 to the -carbon. The generic struc- O Methionine CH3-S-CH2-CH2 Glutamine ture of amino acids is shown in the upper left corner with the differ- NH2 C-CH2-CH2 entiating functional group marked O by R. The functional group for each Cyclic amino acids Aspartic Acid amino acid is shown below. Amino HO C-CH2 acids have been grouped by func- O tional class. Proline is actually an Proline COOH Asparagine imino acid because of its cyclic N NH2 C-CH2 structure involving its nitrogen (N). H are purified and dried. The molecular weights shown center: the -carbon. The term “chiral” comes from Greek in Table 1.2 reflect their molecular weight as crystalline for hand in that these molecules have a left (“levo” or “L”) amino acids. The basic and acidic amino acids tend to form and right (“dextro” or “D”) handedness to them around the much more stable crystals as salts, however, rather than -carbon atom. Because of the tetrahedral structure of the as free amino acids. Glutamic acid can be obtained as the carbon bonds, there are two possible arrangements of a free amino acid with a molecular weight of 147 and as its carbon center with the same four different groups bonded sodium salt, monosodium glutamate, which has a crystal- to it that are not superimposable; the two configurations, line weight of 169. Lysine is typically found as a salt con- called stereoisomers, are mirror images of each other. The taining hydrogen chloride. Therefore, when amino acids body recognizes only the L-form of amino acids for most are represented by weight, it is important to know whether reactions in the body, although some enzymatic reactions the weight is based on the free amino acid or on its salt. will operate with a lower efficiency when given the D-form. Another important property of amino acids is optical Because we do encounter some D-form amino acids in activity. Except for glycine, which has its functional group the foods that we eat, the body has some mechanisms for as a single hydrogen, all amino acids have at least one chiral clearing these amino acids through renal filtration. MNHD11e_CH001.indd 5 09/24/12 5:00 PM  6 PART I SPE CI F I C DI E T ARY COM P O N E N T S TABLE 1.2 COMMON AMINO ACIDS IN THE BODY STANDARD ABBREVIATION 3-LETTER 1-LETTER MOLECULAR WEIGHTa Indispensable (Essential) Amino Acids Isoleucine Ile I 131.2 Leucine Leu L 131.2 Lysine Lys K 146.2 Methionine Met M 149.2 Phenylalanine Phe F 165.2 Threonine Thr T 119.1 Tryptophan Trp W 204.2 Valine Val V 117.2 Histidineb His H 155.2 Dispensable (Nonessential) Amino Acids Alanine Ala A 89.1 Arginine Arg R 174.2 Aspartic acid Asp D 133.2 Asparagine Asn N 132.2 Glutamic acid Glu E 147.2 Glutamine Gln Q 146.2 Glycine Gly G 75.1 Proline Pro P 115.1 Serine Ser S 105.1 Conditionally Dispensable Amino Acids Cysteine Cys C 121.2 Tyrosine Tyr Y 181.2 Some Special Amino Acids Citrulline 175.2 Homocysteine Hcy 135.2 Hydroxylysine Hyl 162.2 Hydroxyproline Hyp 131.2 3-Methylhistidine 169.2 Ornithine Orn 132.2 a Molecular weight (daltons) is rounded to the nearest tenth and represents the number of grams per mole of amino acid. Because glutamine is degraded to glutamate when proteins are hydrolyzed, the sum of the glutamine and glutamate together is often abbreviated Glx. The same is true also for the sum of asparagine and aspartate: Asx. The one-letter abbreviations are often used to indicate protein sequences. b The essentiality for histidine has been shown only for infants, but probably small amounts are needed for adults as well. To date, the indispensability of histidine has not been documented in healthy adults (6). Any number of molecules could be designed that com- these amino acids are essential or indispensable to the plete the basic definition of an amino acid: a molecule diet. The classification of amino acids as nondispensable/ with a central carbon to which an amino group, a carboxyl dispensable or essential/nonessential for humans is shown group, and a functional group are attached. A relatively in Table 1.2. Both the standard three-letter abbreviation limited variety appears in nature, however, and only 20 and the one-letter abbreviation used in representing are incorporated directly into mammalian protein. Amino amino acid sequences in proteins are also presented in acids are selected for protein synthesis when coupled Table 1.2 for each amino acid. Some dispensable amino to transfer RNA (tRNA). To synthesize protein, strands acids may become conditionally indispensable under con- of DNA are transcribed into messenger RNA (mRNA). ditions when synthesis becomes limited or when adequate tRNA binds to mRNA in 3-base groups. Different com- amounts of precursors are unavailable to meet the needs binations of 3 consecutive RNA molecules in the mRNA of the body (2–4). The history and rationale of the classifi- code for different tRNA molecules. However, the 3-base cation of amino acids in Table 1.2 are discussed in greater combinations of mRNA are recognized by only 20 differ- detail later in this chapter. ent tRNA molecules, and 20 different amino acids are Besides the 20 amino acids that are recognized by tRNA incorporated into protein during protein synthesis. for incorporation into protein, other amino acids appear Of these 20 amino acids in proteins, some are synthe- commonly in the body. These amino acids have important sized de novo in the body either from other amino acids metabolic functions. Examples are ornithine and citrul- or from simpler precursors. These amino acids may be line, which are linked to arginine through the urea cycle. deleted from our diet without impairing health or blocking Other amino acids appear as modifications of amino acids growth. These amino acids are nonessential and dispens- after they have been incorporated into proteins. Examples able from the diet. No pathways exist for the synthesis of are hydroxyproline and hydroxylysine, which are produced several other amino acids in humans, however, and hence when proline and lysine residues in collagen protein are MNHD11e_CH001.indd 6 09/24/12 5:00 PM  CHAPTER 1 P R O T E I N S A N D A M INO AC IDS 7 hydroxylated, and 3-methylhistidine, which is produced by the composition of amino acids in hen egg protein, mam- posttranslational methylation of select histidine residues malian muscle and liver proteins (5), and human milk (6). of actin and myosin proteins. Because no tRNA exists to The data are expressed as moles of amino acid. The his- code for these amino acids, they cannot be reused when torical expression of amino acids is on a weight basis (e.g., a protein containing them is broken down (hydrolyzed) to grams of amino acid). Comparing amino acids by weight its individual amino acids. skews the comparison toward the heaviest amino acids and makes them appear more abundant than they are. For example, tryptophan (molecular weight 204) appears Amino Acid Pools and Distribution almost three times as abundant as glycine (molecular The distribution of amino acids is complex. Not only are weight 75) when quoted in terms of weight. different amino acids incorporated into a variety of differ- An even distribution of all 20 amino acids would be 5% ent proteins in many different organs in the body, but also per amino acid per protein, and the median amino acid amino acids are consumed in the diet from numerous pro- content centers around this value for the proteins shown tein sources. In addition, each amino acid is maintained in in Table 1.3. Tryptophan is the least common amino acid part as a free amino acid in solution in blood and inside in many proteins. Considering the effect of tryptophan’s cells. Overall, a wide range of concentrations is found large size on the conformation of proteins, it is not surpris- among amino acids across the various protein and free ing to find less tryptophan in protein. Other amino acids of pools that exist. We consume protein in food that is enzy- modest size and limited polarity, such as alanine, leucine, matically hydrolyzed in the alimentary tract, thus releasing serine, and valine, are relatively abundant in protein (8% to free individual amino acids that are then absorbed by the 10% per amino acid). Although the abundance of the indis- gut lumen and are transported into portal blood. Amino pensable amino acids (IDAAs) is similar across the protein acids then pass into the systemic circulation and are sources in Table 1.3, various vegetable proteins are defi- extracted by different tissues. Although the concentrations cient or low in some IDAAs. In the body, certain proteins of individual amino acids vary among different free pools are particularly rich in specific amino acids to produce such as plasma and intracellular muscle, the abundance of specific functions in the protein. For example, collagen is a individual amino acids is relatively constant in a variety of fibrous protein abundant in connective tissues in tendons, proteins throughout the body and nature. Table 1.3 shows bone, and muscle. Collagen fibrils are arranged in differ- ent ways, depending on the functional type of collagen. Glycine comprises approximately one third of collagen, and there is also considerable proline and hydroxyproline TABLE 1.3 AMINO ACID COMPOSITION OF SEVERAL (proline converted after it has been incorporated into DIFFERENT PROTEIN SOURCES collagen). The glycine and proline residues allow the col- COMPOSITION (mol/g PROTEIN) lagen protein chain to turn tightly and intertwine, and the MAMMALIAN hydroxyproline residues provide for hydrogen bond cross- AMINO ACID HEN EGG MUSCLE LIVER HUMAN MILK linking. Generally, the alterations in amino acid concentra- Alanine 810 730 750 426 Arginine 360 380 328 132 tions do not vary dramatically among proteins as they do Aspartate  530 600 600 679 in collagen, but such examples demonstrate the diversity Asparagine and functionality of the different amino acids in proteins. Cysteine 190 120 140 182 The abundance of amino acids varies among amino acids Glutamate  810 990 800 1,206 over a far wider range in the free pools of extracellular and Glutamine Glycine 450 670 610 306 intracellular compartments. Typical values of free amino Histidine 150 180 170 148 acid concentrations in plasma and in intracellular muscle Isoleucine 490 360 380 434 are shown in Table 1.4. The primary points of Table 1.4 are Leucine 650 610 690 770 as follows: (a) amino acid concentrations vary widely among Lysine 425 580 510 472 amino acids, and (b) free amino acids are generally concen- Methionine 200 170 170 107 Phenylalanine 340 270 310 242 trated inside cells. Although the correlation between plasma Proline 350 430 430 695 and muscle free intracellular amino acid levels is significant, Serine 770 480 510 476 the relationship is not linear (7). Concentrations of plasma Threonine 410 390 390 395 amino acids range from a low of approximately 20 M for Tryptophan 80 55 80 88 Tyrosine 220 170 200 259 aspartic acid and methionine to a high of approximately Valine 600 470 520 538 500 M for glutamine. The median level for plasma amino Data from Block RJ, Weiss KW. Amino Acid Handbook: Methods and acids is 100 M. No defined relationship exists between Results of Analysis. Springfield, IL: Charles C Thomas, 1956:343–4; the nature of amino acids (IDAAs versus dispensable amino and Food and Agriculture Organization/World Health Organization/ acids) and amino acid concentrations or type of amino acids United Nations University. Protein and Amino Acid Requirements in Human Nutrition. Geneva: World Health Organization, 2007:1–256, (e.g., plasma concentrations of the three BCAAs range from with permission. 50 to 250 M). One notable point is that the concentrations MNHD11e_CH001.indd 7 09/24/12 5:00 PM  8 PART I SPE CI F I C DI E T ARY COM P O N E N T S TABLE 1.4 TYPICAL CONCENTRATIONS OF FREE AMINO Table 1.4, it is clear that different transport mechanisms ACIDS IN THE BODY must exist for different amino acids to produce the range of concentration gradients observed. Many different CONCENTRATION (mM) GRADIENT transporters exist for different types and groups of amino INTRACELLULAR INTRACELLULAR/ acids (10–12). Amino acid transport is probably one of the AMINO ACID PLASMA MUSCLE PLASMA Aspartic acid D 0.02 more difficult areas of amino acid metabolism to quan- Phenylalanine I 0.05 0.07 1.4 tify and characterize. The affinities of the transporters Tyrosine CI 0.05 0.10 2.0 and their mechanisms of transport set the intracellular Methionine I 0.02 0.11 5.5 levels of the amino acids. Generally, the IDAAs have Isoleucine I 0.06 0.11 1.8 Leucine I 0.12 0.15 1.3 lower intracellular/extracellular gradients than do the Cysteine CI 0.11 0.18 1.6 dispensable amino acids (Table 1.4) and are transported Valine I 0.22 0.26 1.2 by different carriers. The amino acid transporters are Ornithine 0.06 0.30 5.0 membrane-bound proteins that recognize different amino Histidine I 0.08 0.37 4.6 Asparagine D 0.05 0.47 9.4 acid shapes and chemical properties (e.g., neutral, basic, Arginine D 0.08 0.51 6.4 or anionic). Transport occurs both into and out of cells. Proline D 0.17 0.83 4.9 Transport may be thought of as a process that sets the Serine D 0.12 0.98 8.2 intracellular/extracellular gradient, or the transporters Threonine I 0.15 1.03 6.9 Lysine I 0.18 1.15 6.4 may be thought of as processes that set the rates of amino Glycine D 0.21 1.33 6.3 acid influx into and efflux from cells, which then define Alanine D 0.33 2.34 7.1 the intracellular/extracellular gradients (10). Perhaps the Glutamic acid D 0.06 4.38 73.0 more dynamic concept of transport defining flows of Glutamine D 0.57 19.45 34.1 Taurinea 0.07 15.44 221.0 amino acids is more appropriate, but in real life the gradi- Cl, conditionally indispensable; D, dispensable; I, indispensable. ent (e.g., intracellular muscle amino acid levels) is measur- a able, not the rates. Taurine is not an amino acid itself, but is highly concentrated in free form in muscle. The transporters fall into two classes: sodium- Data from Bergström J, Fürst P, Norée LO et al. Intracellular free independent and sodium-dependent carriers. The sodium- amino acid concentration in human muscle tissue. J Appl Physiol dependent carriers cotransport a sodium atom into the 1974;36:693–7, with permission. cell along with the amino acid. The high extracellular/ intracellular sodium gradient (140 mEq outside and 10 mEq inside) facilitates the inward transport of amino acids by of the acidic amino acids, aspartate and glutamate, are very the sodium-dependent carriers. These transporters gener- low outside cells in plasma. In contrast, the concentration of ally produce larger gradients and accumulations of amino glutamate is among the highest inside cells, such as muscle acids inside cells than outside them. The sodium entering (Table 1.4). the cell may be transported out via the sodium–potassium Important to bear in mind are the differences in the pump that transports a potassium ion in for the removal of relative amounts of N contained in extracellular and a sodium ion. intracellular amino acid pools and in protein itself. A physi- Few of the transporter proteins have been identified; ologically normal person has approximately 55 mg of amino most information concerning transport has been accrued acid N/L outside cells in extracellular space and approxi- through kinetic studies of membranes using amino acids mately 800 mg of amino acid N/L inside cells; this means and competitive inhibitors or amino acid analogs to define that free amino acids are approximately 15-fold more abun- and characterize individual systems. Table 1.5 lists the dant inside cells than outside cells (7). The second point is different amino acid transporters characterized to date that the total pool of free amino acid N is small compared and the amino acids they transport. The neutral and bulky with protein-bound amino acids. Multiplying the free pools amino acids (the BCAAs, phenylalanine, methionine, by estimates of extracellular water (0.2 L/kg) and intra- and histidine) are transported by system L. System L is cellular water (0.4 L/kg) provides a measure of the total sodium independent and operates with a high rate of amount of N present in free amino acids: 0.33 g N/kg body exchange and produces small gradients. Other important weight. In contrast, body composition studies have shown transporters are systems ASC and A. These transporters that the N content of the body is 24 g N/kg body weight use the energy available from the sodium-ion gradient as (8, 9). Therefore, free amino acids make up approximately a driving force to maintain a steep gradient for the various 1% of the total amino N pool versus more than 99% of the amino acids transported (e.g., glycine, alanine, threonine, amino acids that reside in proteins. serine, and proline) (10, 11). The anionic transporter (XAG-) also produces a steep gradient for the dicarboxylic Amino Acid Transport amino acids, glutamate and aspartate. Other important The gradient of amino acids within and outside cells is carriers are system N and Nm for glutamine, asparagine, maintained by active transport. From a simple scan of and histidine. System y handles much of the transport MNHD11e_CH001.indd 8 09/24/12 5:00 PM  CHAPTER 1 P R O T E I N S A N D A M INO AC IDS 9 TABLE 1.5 AMINO ACID TRANSPORTERS SYSTEM AMINO ACID TRANSPORTED TISSUE LOCATION pH DEPENDENCE Sodium dependent A Most neutrals (Ala, Ser) Ubiquitous Yes ASC Most neutrals Ubiquitous No B Most neutrals Intestinal brush border Yes N Gln, Asn, His Hepatocytes Yes Nm Gln, Asn Muscle No Gly Gly, sarcosine Ubiquitous XAG- Glu, Asp Ubiquitous Sodium independent L Leu, Ile, Val, Met, Phe, Tyr, Trp, His Ubiquitous Yes T Trp, Phe, Tyr Red blood cells, hepatocytes No y Arg, Lys, Orn Ubiquitous No asc Ala, Ser, Cys, Thr Ubiquitous Yes Data from references 10 to 12, with permission. of the basic amino acids. Some overall generalizations can For mammals, urea production is a means of removal of be made in terms of the type of amino acid transported waste N from the oxidation of amino acids in the form of by a given carrier, but the system is not readily simplified a nontoxic, water-soluble compound. because individual carrier systems transport several dif- More detailed descriptions of the amino acid path- ferent amino acids, whereas individual amino acids are ways can be found in standard textbooks of biochemistry. often transported by several different carriers with differ- Keep in mind when consulting such texts that pathways ent efficiencies. Thus, amino acid gradients are formed for nonmammalian systems (e.g., Escherichia coli and and amino acids are transported into and out of cells via a yeast) are often presented, and these pathways often have complex system of overlapping carriers. little importance to human biochemistry. When consult- ing reference material, the reader needs to be aware of PATHWAYS OF AMINO ACID SYNTHESIS the system of life from which the metabolic pathways and AND DEGRADATION enzymes are being discussed. The discussion here is rel- evant to human biochemistry. Presented first is a discus- Several amino acids have their metabolic pathways linked sion of the routes of degradation of each amino acid when to the metabolism of other amino acids. These codepen- the pathway is directed toward oxidation of the amino acid dencies that link the pathways of amino acids become for energy. Next follows a discussion of pathways of amino important when nutrient intake is limited or when meta- acid synthesis, and finally the use of amino acids for other bolic requirements are increased. Two aspects of metabo- important compounds in the body is described. lism are reviewed here: (a) synthesis of amino acids and (b) amino acid degradation. Degradation serves two use- Amino Acid Degradation Pathways ful purposes: (a) production of energy from the oxidation of individual amino acids (艐4 kcal/g protein, almost the Complete amino acid degradation ends up with the pro- same energy production as for carbohydrate) and (b) con- duction of N, which is removed by incorporation into urea. version of amino acids into other products. The latter Carbon skeletons are eventually oxidized as CO2 via the is also related to amino acid synthesis: the degradation TCA cycle (also known as the Krebs cycle or the citric pathway of one amino acid may be the synthetic pathway acid cycle). The inputs to the cycle are acetyl-coenzyme A of another amino acid. The other important aspect of (CoA) and oxaloacetate forming citrate, which is degraded amino acid degradation is production of other nonamino to -ketoglutarate and then to oxaloacetate. The carbon acid, N-containing compounds in the body. The need for skeletons from amino acid may enter the Krebs cycle via synthesis of these compounds may also drain the pools of acetate as acetyl-CoA or via oxaloacetate/-ketoglutarate. their amino acid precursors and thus increase the need These latter two precursors are direct metabolites of the for these amino acids in the diet. When amino acids amino acids aspartate and glutamate. An alternative to are degraded for energy rather than converted to other the complete oxidation of the carbon skeletons to CO2 compounds, the ultimate products become CO2, water, is the use of these carbon skeletons for the formation of and urea. The CO2 and water are produced through fat and carbohydrate. Fat is formed from elongation of classical pathways of intermediary metabolism involving acetyl units, and so amino acids whose carbon skeletons the tricarboxylic acid (TCA) cycle. The urea is produced degrade to acetyl-CoA and ketones may alternatively be because other forms of waste N, such as ammonia (NH3), used for synthesis of fatty acids. Glucose is split in glycoly- are toxic if their levels rise in the blood and inside cells. sis to pyruvate, and pyruvate is the immediate product of MNHD11e_CH001.indd 9 09/24/12 5:00 PM  10 PART I SPE CI F I C DI E T ARY COM P O N E N T S alanine. Pyruvate may be converted back to glucose by elongation to oxaloacetate. Amino acids whose degradation Leucine Valine Isoleucine pathways go toward formation of pyruvate, oxaloacetate, or -ketoglutarate may be used for synthesis of glucose. Therefore, the degradation pathways of many amino acids can be partitioned into two groups with respect to the dis- posal of their carbon: amino acids whose carbon skeleton NH3 + GLUTAMATE ALANINE may be used for synthesis of glucose (gluconeogenic amino α-Ketoglutarate acids) or those whose carbon skeletons degrade for poten- tial use for fatty acid synthesis. The amino acids that degrade directly to the primary gluconeogenic and TCA cycle precursors, pyruvate, oxalo- acetate, and -ketoglutarate, do so by rapid and reversible GLUTAMINE ASPARTATE transamination reactions: L-glutamate  oxaloacetate ↔ -ketoglutarate  Fig. 1.2. Movement of amino-nitrogen (N) around glutamic acid. L-aspartate Glutamate undergoes reversible transamination with several amino acids. Nitrogen is also removed from glutamate by glutamate dehy- by the enzyme aspartate aminotransferase, which, of course, drogenase, thus producing an -ketoglutarate and an ammonia. also can be In contrast, the enzyme glutamine synthetase adds an ammonia to glutamate to produce glutamine. Glutamine is degraded back to glu- L-aspartate  -ketoglutarate ↔ oxaloacetate  tamate by liberation of the amide-N to release ammonia by a different L-glutamate enzymatic pathway (glutaminase). NH3, ammonia. and L-alanine  -ketoglutarate ↔ pyruvate  A similar process occurs for formation and degradation L-glutamate of asparagine from aspartate. In terms of N metabolism, by the enzyme alanine aminotransferase. What is quickly Figure 1.2 shows that the center of N flow in the body is apparent is that the amino-N of these three amino acids through glutamate. This role becomes even clearer when may be rapidly exchanged and each amino acid rapidly con- we look at how urea is synthesized in the liver. The inputs verted to and from a primary compound of gluconeogenesis into the urea cycle are a CO2, adenosine triphosphate and the TCA cycle. As described later, compartmentation (ATP), and NH3 to form carbamoyl phosphate, which among different organ pools is the only limiting factor for condenses with ornithine to form citrulline (Fig. 1.3). The complete and rapid exchange of the N of these amino acids. second N enters via aspartate to form argininosuccinate, The IDAAs leucine, isoleucine, and valine are grouped which is then cleaved into arginine and fumarate. The together under the heading of the BCAAs because the arginine is hydrolyzed by arginase to ornithine, thus lib- first two steps in their degradation pathway are common erating urea. The resulting ornithine can reenter the urea to all three amino acids: cycle. As mentioned briefly later, some amino acids may liberate NH3 directly (e.g., glutamine, asparagine, and Leucine ⎫ ⎧-ketoisocaproate glycine), but most transfer through glutamate first, which Isoleucine⎬  -ketoglutarate ↔ glutamate  ⎨-keto--methylvalerate Valine ⎭ ⎩-ketovalerate is then degraded to -ketoglutarate and NH3. The pool of aspartate is small in the body, and aspartate cannot be the The reversible transamination to keto acids is followed primary transporter of the second N into urea synthesis. by irreversible decarboxylation of the carboxyl group Rather, aspartate must act like arginine and ornithine as to liberate CO2. The BCAAs are the only IDAAs that a vehicle for the introduction of the second N. If so, the undergo transamination and therefore are unique among second N is delivered by transamination via glutamate, IDAAs. again placing glutamate at another integral point in the Together, the BCAAs, alanine, aspartate, and glutamate degradative disposal of amino acid N. make up the pool of amino-N that can move among amino An outline of the degradative pathways of the various acids via reversible transamination. As shown in Figure 1.2, amino acids is presented in Table 1.6. Rather than show glutamic acid is central to the transamination process. In individual reaction steps, the major pathways for degrada- addition, N can leave the transaminating pool by removal tion, including the primary end products, are presented. of the glutamate N via glutamate dehydrogenase, or it can The individual steps may be found in current textbooks enter by the reverse process. The amino acid glutamine is of biochemistry or in older reviews on the subject (13). intimately tied to glutamate as well: all glutamine is made Because of the importance of transamination, the major- from amidation of glutamate, and glutamine is degraded ity of the N from amino acid degradation appears via N by removal of the amide-N to form NH3 and glutamate. transfer to -ketoglutarate to form glutamate. In some MNHD11e_CH001.indd 10 09/24/12 5:00 PM  CHAPTER 1 P R O T E I N S A N D A M INO AC IDS 11 Amino Acid ATP Degradation CO2 NH3 Glutamate glutamate Carbamoyl dehydrogenase Phosphate Citrulline Aspartate α-Ketoglutarate Urea Argininosuccinate Ornithine Cycle Fig. 1.3. Urea cycle disposal of amino acid nitrogen (N). Urea synthesis Arginine Fumarate Glutamate incorporates one N from ammonia (NH3) and another from aspartate. Ornithine, citrulline, and arginine sit in the middle of the cycle. Glutamate is the primary source for the aspartate N; Urea glutamate is also an important source O of the ammonia into the cycle. ATP, adenosine triphosphate; CO2, carbon C dioxide; NH2, amine. H2N NH2 TABLE 1.6 PATHWAYS OF AMINO ACID DEGRADATION METABOLIC PATHWAY IMPORTANT ENZYMES NITROGEN END PRODUCTS CARBON END PRODUCTS Amino acids converted to other amino acids Asparagine Asparaginase Aspartate  NH3 Glutamine Glutaminase Glutamate  NH3 Arginine Arginase Ornithine  Urea Phenylalanine Phenylalanine hydroxylase Tyrosine Proline Glutamate Serine Serine hydroxymethyltransferase Glycine Cysteine Taurine Amino acids transaminating to form glutamate Alanine Glutamate Pyruvate Aspartate Glutamate Oxaloacetate Cysteine Glutamate Pyruvate  SO42 Isoleucine Glutamate Succinate Leucine Glutamate Ketones Ornithine Glutamate -Ketoglutarate Serine Glutamate 3-Phosphoglycerate Valine Glutamate Succinate Tyrosine Glutamate Ketone  fumarate Other pathways Glycine NH3 CO2 Histidine NH3 Urocanate Methionine NH3 Ketobutyrate Serine Serine dehydratase NH3 Pyruvate Threonine Serine dehydratase NH3 Ketobutyrate Tryptophan NH3 Kynurenine Lysine 2 Glutamates Ketones CO2, carbon dioxide; NH3, ammonia; SO42, sulfate. MNHD11e_CH001.indd 11 09/24/12 5:00 PM  12 PART I SPE CI F I C DI E T ARY COM P O N E N T S cases, the aminotransferase catalyzes the transamination contrast, the former group is rarely rate limited in synthe- reaction with glutamate bidirectionally, as indicated in sis because of the ample precursor availability of carbon Figure 1.2, and these enzymes are distributed in multiple skeletons from the TCA cycle and from the labile amino-N tissues. In other cases, the transamination reactions are pool of transaminating amino acids. liver specific, are compartmentalized, and act specifically The pathways of dispensable amino acid synthesis are to degrade N, not reversibly exchange it. For example, shown in Figure 1.4. As with amino acid degradation, when leucine labeled with the stable isotope tracer 15N glutamate is central to the synthesis of several amino acids was infused into dogs for 9 hours, considerable amounts by providing the N for synthesis. Glutamate, alanine, and of the 15N tracer were found in circulating glutamine  aspartate may share amino-N transaminating back and glutamate, alanine, the other two BCAAs, but not in tyro- forth among them (see Fig. 1.2). As Figure 1.4 is drawn, sine (14)—a finding indicating that the transamination of glutamate derives its N from NH3 with -ketoglutarate, tyrosine did not proceed backward. and that glutamate goes on to promote the synthesis of Another reason that the entries in Table 1.6 do not other amino acids. Kitagiri and Nakamura (17) argued show individual steps is that the specific pathways of the that we have little capacity to form glutamate from NH3 metabolism of all the amino acids are not clearly defined. and that the primary source of glutamate N comes from For example, two pathways for cysteine are shown. Both other amino acids via transamination. These amino acids are active, but how much cysteine is metabolized by ultimately result from dietary protein intake. Under cir- which pathway is not as clear. Methionine is metabolized cumstances of adequate dietary intake, the transaminating by conversion to homocysteine. The homocysteine is not amino acids shown in Figure 1.2 supply more than ade- directly converted to cysteine; rather, homocysteine con- quate amino-N to glutamate. The transaminating amino denses with a serine to form cystathionine, which is then acids act to provide a buffer pool of N that can absorb broken apart to liberate cysteine, NH3, and ketobutyrate. an increase in N from increased degradation or supply N The original methionine molecule appears as NH3 and when there is a drain. From this pool, glutamate provides ketobutyrate, however. The cysteine carbon skeleton material to maintain synthesis of ornithine and proline, of comes from the serine. So the entry in Table 1.6 shows which proline is particularly important in protein synthesis methionine degraded to NH3, yet this degradation path- of collagen and related proteins. way is the major synthesis pathway for cysteine. Because of the importance of the sulfur-containing amino acids, a more extensive discussion of the metabolic pathways of Pyruvate +Glu Alanine these amino acids may be found in a later chapter. Glycine is degraded by more than one possible path- +Glu +NH3 Oxaloacetate Aspartate Asparagine way, depending on the text used for reference. The pri- mary pathway, however, appears to be the glycine cleavage α-Ketoglutarate enzyme system that breaks glycine into CO2 and NH3 and +NH3 + transfers a methylene group to tetrahydrofolate (15). This GLUTAMATE Glutamine pathway has been shown to be the prominent pathway in NH3 rat liver and in other vertebrate species (16). Although this Glutamate Urea Cycle reaction degrades glycine, its importance is the production +Glu Ornithine Arginine semialdehyde of a methylene group that can be used in other metabolic reactions. Proline Synthesis of Dispensable Amino Acids Glucose +Glu or Serine Glycine The IDAAs are those amino acids that cannot be synthe- Glycerol or +Ala sized in sufficient amounts in the body and therefore must be in the diet in sufficient amounts to meet the body’s Methionine Homocysteine Cystathionine needs. Therefore, discussion of amino acid synthesis applies only to the dispensable amino acids. Dispensable Cysteine amino acid synthesis falls into two groups: (a) amino acids that are synthesized by transferring an N to a carbon skeleton precursor that has come from the TCA cycle or Phenylalanine Tyrosine from glycolysis of glucose and (b) amino acids that are syn- Fig. 1.4. Pathways of the synthesis of dispensable amino acids. thesized specifically from other amino acids. Because this Glutamate is produced from ammonia (NH3) and -ketoglutarate. latter group of amino acids depends on the availability of That glutamate becomes the nitrogen source added to carbon pre- cursors (pyruvate, oxaloacetate, glycolysis products of glucose, and other, specific amino acids, these amino acids are particu- glycerol) to form most of the other dispensable amino acids. Cysteine larly vulnerable to becoming indispensable if the dietary and tyrosine are different in that they require indispensable amino acid supply of a precursor amino acid becomes limiting. In input for their production. MNHD11e_CH001.indd 12 09/24/12 5:00 PM  CHAPTER 1 P R O T E I N S A N D A M INO AC IDS 13 Serine is produced from 3-phosphoglycerate that comes TABLE 1.7 IMPORTANT PRODUCTS SYNTHESIZED from glycolysis of glucose. Serine may then be used to FROM AMINO ACIDS produce glycine through a process that transfers a methy- lene group to tetrahydrofolate. This pathway is listed in AMINO ACID INCORPORATED INTO Arginine Creatine Table 1.6 as a degradative pathway for serine, but it is also Nitric oxide a source of glycine and one-carbon unit generation (15, Aspartate Purines and pyrimidines 16). Conversely, this pathway actively operates backward Cysteine Glutathione to form serine from glycine in humans. When [15N]glycine Taurine is given orally, the primary transfer of 15N is to serine (18). Glutamate Glutathione Neurotransmitters Therefore, significant reverse synthesis of serine from gly- Glutamine Purines and pyrimidines cine occurs. The other major place where 15N appears was Glycine Creatine in glutamate and glutamine, a finding indicating that the Glutathione NH3 released by glycine oxidation is immediately picked Porphyrins (hemoglobin and cytochromes) up and incorporated into glutamate and the transaminat- Purines Histidine Histamine ing N-pool via glutamate dehydrogenase. Lysine Carnitine All the amino acids shown in Figure 1.4 have active Methionine One-carbon methylation/transfer reactions routes of synthesis in the body (13), in contrast to the Creatine IDAAs for which no routes of synthesis exist in humans. Choline This statement should be a simple definition of “indis- Serine One-carbon methylation/transfer reactions Ethanolamine and choline pensable” versus “dispensable.” In nutrition, however, Tyrosine Catecholamines we define a dispensable amino acid as an amino acid that Thyroid hormone is dispensable from the diet (3). This definition is differ- Tryptophan Serotonin ent from defining the presence or absence of enzymatic Nicotinic acid pathways for an amino acid’s synthesis. For example, two of the dispensable amino acids depend on the degrada- tion of IDAAs for their production: cysteine and tyrosine. before fatty acids can be oxidized and is synthesized Although serine provides the carbon skeleton and amino from -N,N,N-trimethyllysine (TML) (26). TML synthe- group of cysteine, methionine provides the sulfur through sis occurs from posttranslational methylation of specific condensation of homocysteine and serine to form cysta- lysine residues in specific proteins. TML is liberated when thionine (19). From the foregoing discussion, neither the the proteins containing it are broken down (26). TML can carbon skeleton nor the amino group of serine is likely to also arise from hydrolysis of ingested meats. In contrast be in short supply, but provision of sulfur from methio- to 3-methylhistidine, TML can be found in proteins of nine may become limiting. Therefore, cysteine synthesis both muscle and other organs such as liver (27). In rat depends heavily on the availability of the IDAA methio- muscle, TML is approximately one eighth as abundant as nine. The same is also true for tyrosine. Tyrosine is pro- 3-methylhistidine. duced by the hydroxylation of phenylalanine, which is also Amino acids are the precursors for a variety of neu- the degradative pathway of phenylalanine. The availability rotransmitters that contain N. Glutamate may be an excep- of tyrosine strictly depends on the availability of phenyl- tion in that it serves both as a precursor for neurotransmitter alanine and the liver’s ability to perform the hydroxylation. production and is itself a primary neurotransmitter (28). Glutamate appears important in numerous neurodegen- Incorporation of Amino Acids into erative diseases from amyotrophic lateral sclerosis to Other Compounds Alzheimer disease. (29). Tyrosine is the precursor for catecholamine synthesis. Tryptophan is the precursor for Table 1.7 lists some of the compounds that amino acids serotonin synthesis. Various studies have reported the are converted directly into or are used as important parts importance of plasma concentrations of these and other of the synthesis of other compounds in the body. The list amino acids on the synthesis of their neurotransmitter is not inclusive, and it is meant to highlight important products. The most common putative relationship cited compounds in the body that depend on amino acids for is the administration of tryptophan, thus increasing brain their synthesis. Other important uses of amino acids are serotonin levels. for the synthesis of taurine (20, 21) that is the “amino acid–like” 2-aminoethanesulfonate, found in far higher Creatine and Creatinine concentrations inside skeletal muscle than any amino Most of the creatine in the body is found in muscle, where acid (7). Another important, sulfur-containing compound it exists primarily as creatine phosphate (30). When mus- is glutathione (22–24), a tripeptide composed of glycine, cular work is performed, creatine phosphate provides the cysteine, and glutamate. energy through hydrolysis of its “high-energy” phosphate Carnitine (25) is important in the transport of long- bond that forms creatine with transfer of the phosphate to chain fatty acids across the mitochondrial membrane create an ATP. The reaction is reversible and is mediated MNHD11e_CH001.indd 13 09/24/12 5:00 PM  14 PART I SPE CI F I C DI E T ARY COM P O N E N T S by the enzyme ATP-creatine transphosphorylase (also Creatinine is not retained by muscle, but it is released into known as creatine phosphokinase). body water, is then removed by the kidney from blood, The original pathway of creatine synthesis from amino and is excreted into urine (32). acid precursors was defined by Bloch and Schoenheimer The daily rate of creatinine formation is remarkably in an elegant series of experiments using 15N-labeled constant (艐1.7% of the total creatine pool per day) and compounds (31). Creatine is synthesized outside muscle depends on the mass of the creatine/creatine-phosphate in a two-step process (Fig. 1.5). The first step occurs in pool, which is proportional to muscle mass (33). Thus, daily the kidney and involves the transfer of guanidino group urinary output of creatinine has been used as a measure of of arginine onto the amino group of glycine to form orni- total muscle mass in the body. Urinary creatinine excretion thine and guanidinoacetate. Methylation of the guanidi- increases within a couple days after a creatine load has noacetate occurs in the liver via S-adenosylmethionine to been added to the diet, and several more days are required create creatine. Although glycine donates a N and carbon after removal of creatine from the diet before urinary cre- backbone to creatine, arginine must be available to pro- atinine excretion returns to baseline—a finding indicating vide the guanidino group, as well as methionine for dona- that creatine in the diet itself affects creatinine production tion of the methyl group. Creatine is then transferred to (34). Therefore, consumption of creatine and creatinine in muscle, where creatine is phosphorylated. When creatine meat-containing foods increases urinary creatinine mea- phosphate is hydrolyzed in muscle to form creatine, most surements. Although urinary creatinine measurements of the creatine is recycled back to the phosphate form. have been used primarily to estimate the adequacy of A nonenzymatic process forming creatinine continually 24-hour urine collections, with adequate control of food dehydrates some of the muscle creatine pool, however. composition and intake, creatinine excretion measure- ments are useful indices of body muscle mass (35, 36). Purine and Pyrimidine Biosynthesis KIDNEY The purines (adenine and guanine) and the pyrimidines Glycine Arginine (uracil, cytosine, and thymine) form the building blocks of DNA and RNA. Purines are heterocyclic double-ring Guanidinoacetic Acid Ornithine compounds that require incorporation of two glutamine NH molecules (donation of the amide-N), a glycine molecule, H2N C NH-CH2-COOH a methylene group from tetrahydrofolate, and the amino-N of aspartic acid for their synthesis as inosine monophos- LIVER phate. Adenine and guanine are formed from inosine monophosphate by the addition of another glutamine NH S-Adenosyl- amide-N or aspartate amino-N. H2N C NH-CH2-COOH methionine Pyrimidines are synthesized after an amide-N of glu- methylation tamine is condensed with a CO2 to form carbamoyl phos- NH CH3 S-Adenosyl- phate, which is further condensed with aspartic acid to H2N C N-CH2-COOH homocysteine make orotic acid, the pyrimidine’s heterocyclic 6-member Creatine ring. The enzyme that forms this carbamoyl phosphate is present in many tissues for pyrimidine synthesis, but it is not the enzyme found in the liver that makes urea MUSCLE (see Fig. 1.3). A block in the urea cycle causing a lack NH CH3 of adequate amounts of arginine to prime urea synthe- H2N C N-CH2-COOH sis cycle in the liver, however, will result in diversion of ATP unused carbamoyl phosphate to orotic acid and pyrimi- H3C N O dine synthesis (37). Uracil is synthesized from orotic acid, ADP NH BLOOD and cytosine is synthesized from uracil by adding an amide NH CH3 HN group of glutamine to uridine triphosphate to form cyti- HN C N-CH2-COOH dine triphosphate. Creatinine URINE PO3– Phosphocreatine TURNOVER OF PROTEINS IN THE BODY Fig. 1.5. Synthesis of creatine and creatinine. Creatine is synthesized As indicated earlier, proteins in the body are not static. in the liver from guanidinoacetic acid, and that is synthesized in the Just as every protein is synthesized, it is also degraded. kidney. Creatine taken up by muscle is primarily converted to phospho- The concept that proteins are continually made and creatine. Although there is some limited direct dehydration of creatine directly to creatinine, the majority of the creatinine comes from dehy- degraded in the body at different rates was first described dration of phosphocreatine. Creatinine is rapidly filtered by the kidney by Schoenheimer and Rittenberg, who first applied iso- into urine. ADP, adenosine diphosphate; ATP, adenosine triphosphate. topically labeled tracers of amino acids to the study of MNHD11e_CH001.indd 14 09/24/12 5:00 PM  CHAPTER 1 P R O T E I N S A N D A M INO AC IDS 15 amino acid metabolism and protein turnover in the 1930s. from a variety of proteins. Approximately one third of the We now know that the rate of turnover of proteins in the amino acids will appear from the large, but slowly turning body spans a broad range and that the rate of turnover of over, pool of muscle protein. In contrast, considerably individual proteins tends to follow their function in the more amino acids will appear and disappear from proteins body; that is, those proteins whose concentrations need in the visceral and internal organs. These proteins make to be regulated (e.g., enzymes) or that act as signals (e.g., up a much smaller proportion of the total mass of protein peptide hormones) have relatively high rates of synthesis