Harper's Biochemistry Chapter 33 - Metabolism of Purine & Pyrimidine Nucleotides PDF

C H A P T E R Metabolism o Purine & Pyrimidine Nucleotides Victor W. Rodwell, PhD 33 O B J E C TI V E S Compare and contrast the roles o dietary nucleic ac...

C H A P T E R Metabolism o Purine & Pyrimidine Nucleotides Victor W. Rodwell, PhD 33 O B J E C TI V E S Compare and contrast the roles o dietary nucleic acids and o de novo biosynthesis in the production o purines and pyrimidines destined or After studying this chapter, polynucleotide biosynthesis. you should be able to: Explain why antiolate drugs and analogs o the amino acid glutamine inhibit purine biosynthesis. Outline the sequence o reactions that convert inosine monophosphate (IMP), rst to AMP and GMP, and subsequently to their corresponding nucleoside triphosphates. Describe the ormation rom ribonucleotides o deoxyribonucleotides (dNTPs). Indicate the regulatory role o phosphoribosyl pyrophosphate (PRPP) in hepatic purine biosynthesis and the specic reaction o hepatic purine biosynthesis that is eedback inhibited by AMP and GMP. State the relevance o coordinated control o purine and pyrimidine nucleotide biosynthesis. Identiy the reactions discussed that are inhibited by anticancer drugs. Write the structure o the end product o purine catabolism. Comment on its solubility and indicate its role in gout, Lesch-Nyhan syndrome, and von Gierke disease. Identiy reactions whose impairment leads to modied pathologic signs and symptoms. Indicate why there are ew clinically signicant disorders o pyrimidine catabolism. BIOMEDICAL IMPORTANCE that involve abnormalities in purine metabolism include gout, Lesch-Nyhan syndrome, adenosine deaminase deiciency, and Despite a diet that may be rich in nucleoproteins, dietary purines purine nucleoside phosphorylase deiciency. Diseases o pyrim- and pyrimidines are not incorporated directly into tissue nucleic idine biosynthesis are rarer, but include orotic acidurias. Unlike acids. Humans synthesize the nucleic acids and their derivatives the low solubility o uric acid ormed by catabolism o purines, ATP, NAD+, coenzyme A, etc, rom amphibolic intermediates. the end products o pyrimidine catabolism (carbon diox- However, injected purine or pyrimidine analogs, including ide, ammonia, β-alanine, and γ-aminoisobutyrate) are highly potential anticancer drugs, may nevertheless be incorporated water soluble. One genetic disorder o pyrimidine catabolism, into DNA. The biosyntheses o purine and pyrimidine ribo- β-hydroxybutyric aciduria, is due to total or partial deiciency nucleotide triphosphates (NTPs) and dNTPs are precisely regu- o the enzyme dihydropyrimidine dehydrogenase. This disorder lated events. Coordinated eedback mechanisms ensure their o pyrimidine catabolism, also known as combined uraciluria- production in appropriate quantities and at times that match thyminuria, is also a disorder o β-amino acid biosynthesis, varying physiologic demand (eg, cell division). Human diseases since the ormation o β-alanine and o β-aminoisobutyrate is 337 338 SECTION VII Structure, Function, & Replication o Inormational Macromolecules impaired. A nongenetic orm can be triggered by administra- Avian tissues also served as a source o cloned genes that tion o 5-luorouracil to patients with low levels o dihydropy- encode enzymes o purine biosynthesis and the regulatory pro- rimidine dehydrogenase. teins that control the rate o purine biosynthesis. The three processes that contribute to purine nucleotide biosynthesis are, in order o decreasing importance: PURINES & PYRIMIDINES ARE 1. Synthesis rom amphibolic intermediates (synthesis de DIETARILY NONESSENTIAL novo) Normal human tissues can synthesize purines and pyrimi- 2. Phosphoribosylation o purines dines rom amphibolic intermediates in quantities and at times 3. Phosphorylation o purine nucleosides appropriate to meet variable physiologic demand. Ingested nucleic acids and nucleotides thereore are dietarily non- essential. Following their degradation in the intestinal tract, INOSINE MONOPHOSPHATE the resulting mononucleotides may be absorbed or converted to purine and pyrimidine bases. The purine bases are then (IMP) IS SYNTHESIZED FROM oxidized to uric acid, which may be absorbed and excreted in AMPHIBOLIC INTERMEDIATES the urine. While little or no dietary purine or pyrimidine is The initial reaction o purine biosynthesis, transer o incorporated into tissue nucleic acids, injected compounds are two phosphoryl groups rom ATP to carbon 1 o ribose incorporated. The incorporation o injected [3H] thymidine 5-phosphate orming phosphoribosyl pyrophosphate into newly synthesized DNA thus can be used to measure the (PRPP), is catalyzed by PRPP synthetase, EC 2.7.6.1. The end rate o DNA synthesis. product o the ten subsequent enzyme-catalyzed reactions is IMP (Figure 33–2). Following synthesis o IMP, separate branches lead to BIOSYNTHESIS OF PURINE AMP and GMP (Figure 33–3). Subsequent phosphoryl trans- NUCLEOTIDES er rom ATP converts AMP and GMP to ADP and GDP, respectively. Conversion o GDP to GTP involves a second With the exception o parasitic protozoa, all orms o lie syn- thesize purine and pyrimidine nucleotides. Synthesis rom phosphoryl transer rom ATP, whereas conversion o ADP to amphibolic intermediates proceeds at controlled rates appro- ATP is achieved primarily by oxidative phosphorylation (see priate or all cellular unctions. To achieve homeostasis, intra- Chapter 13). cellular mechanisms sense and regulate the pool sizes o NTPs, which rise during growth or tissue regeneration when cells are Multifunctional Catalysts rapidly dividing. Purine and pyrimidine nucleotides are synthesized in Participate in Purine Nucleotide vivo at rates consistent with physiologic need. Early investiga- Biosynthesis tions o nucleotide biosynthesis irst employed birds, and later In prokaryotes, each reaction o Figure 33–2 is catalyzed by Escherichia coli. Isotopic precursors o uric acid ed to pigeons a dierent polypeptide. By contrast, the enzymes o eukary- established the source o each atom o a purine (Figure 33–1) otes are polypeptides that possess multiple catalytic activities and initiated study o the intermediates o purine biosynthesis. whose adjacent catalytic sites acilitate channeling o interme- diates between sites. Three distinct multiunctional enzymes catalyze reactions ➂, ➃, and ➅; reactions ➆ and ➇; and reac- Respiratory CO 2 tions ➉ and 11 o Figure 33–2. Glycine Aspartate C 6 N Antifolate Drugs & Glutamine N1 C 7 5 8 C Analogs Block Purine Nucleotide C 2 3 4 C 9 N N 5,N10 -Methenyl- tetrahydrofolate Biosynthesis N10 -Formyl- N tetrahydro- H The carbons added in reactions ➃ and ➉ o Figure 33–2 are folate contributed by derivatives o tetrahydroolate. Purine dei- ciency states, while rare in humans, generally relect a dei- ciency o olic acid. Compounds that inhibit ormation o Amide nitrogen of glutamine tetrahydroolates and thereore block purine synthesis have FIGURE 33–1 Sources of the nitrogen and carbon atoms been used in cancer chemotherapy. Inhibitory compounds of the purine ring. Atoms 4, 5, and 7 (blue highlight) derive rom and the reactions they inhibit include azaserine (reaction ➄, glycine. Figure 33–2), diazanorleucine (reaction ➁, Figure 33–2),  FIGURE 33–2 Purine biosynthesis from ribose 5-phosphate and ATP. See the text or explanations. ( P , PO32– or PO2–.) 339 340 SECTION VII Structure, Function, & Replication o Inormational Macromolecules H – H – – – OOC C C COO OOC C C COO H2 H2 H H O + GDP NH – – NH 2 NH 3 OOC C C COO N GTP +Pi N N HN 12 N 13 N + GTP, Mg 2 N N N N Adenylosuccinase N N Adenylosuccinate R-5- P synthetase R-5- P R-5- P Inosine monophosphate Adenylosuccinate Adenosine monophosphate (IMP) (AMPS) (AMP) + NAD H 2O 14 IMP Dehydrogenase NADH +H+ O Glutamine Glutamate O N N HN 15 HN ATP O N N H2N N N H Transamidinase R-5- P R-5- P Xanthosine monophosphate Guanosine monophosphate (XMP) (GMP) FIGURE 33–3 Conversion of IMP to AMP and GMP. 6-mercaptopurine (reactions 13 and 14 , Figure 33–3), and Liver, the major site o purine nucleotide biosynthesis, mycophenolic acid (reaction 14 , Figure 33–3). provides purines and purine nucleosides or salvage and or utilization by tissues incapable o their biosynthesis. Human brain tissue has a low level o PRPP glutamyl amidotranser- “SALVAGE REACTIONS” CONVERT ase, EC 2.4.2.14 (reaction ➁, Figure 33–2) and hence depends PURINES & THEIR NUCLEOSIDES in part on exogenous purines. Erythrocytes and polymorpho- nuclear leukocytes cannot synthesize 5-phosphoribosylamine TO MONONUCLEOTIDES (structure III, Figure 33–2), and thereore also utilize exog- Conversion o purines, their ribonucleosides, and their deoxyri- enous purines to orm nucleotides. bonucleosides to mononucleotides involves “salvage reactions” that require ar less energy than de novo synthesis. The more important mechanism involves phosphoribosylation by PRPP HEPATIC PURINE BIOSYNTHESIS (structure II, Figure 33–2) o a ree purine (Pu) to orm a IS STRINGENTLY REGULATED purine 5′-mononucleotide (Pu-RP): AMP & GMP Feedback Regulate PRPP Pu + PR–PP → Pu–RP + PPi Glutamyl Amidotransferase Phosphoryl transer rom PRPP catalyzed by adenosine- Biosynthesis o IMP is energetically expensive. In addition to and hypoxanthine-phosphoribosyl transerases (EC 2.4.2.7 & ATP, glycine, glutamine, aspartate, and reduced tetrahydroolate EC 2.4.2.8, respectively), converts adenine, hypoxanthine, and derivatives all are consumed. Thus, it is o survival advantage to guanine to their mononucleotides (Figure 33–4). closely regulate purine biosynthesis in response to varying physi- A second salvage mechanism involves phosphoryl transer ologic need. The overall determinant o the rate o de novo purine rom ATP to a purine ribonucleoside (Pu-R): nucleotide biosynthesis is the concentration o PRPP. This, in Pu-R + ATP → PuR-P + ADP turn, depends on the rate o PRPP synthesis, utilization, degra- dation, and regulation. The rate o PRPP synthesis depends on Phosphorylation o the purine nucleotides, catalyzed by the availability o ribose 5-phosphate and on the activity o PRPP adenosine kinase (EC 2.7.1.20), converts adenosine and synthetase, EC 2.7.6.1 (reaction ➁ Figure 33–5), an enzyme deoxyadenosine to AMP and dAMP. Similarly, deoxycyti- whose activity is eedback inhibited by AMP, ADP, GMP, and dine kinase (EC 2.7.1.24) phosphorylates deoxycytidine and GDP. Elevated levels o these nucleoside phosphates thus signal a 2′-deoxyguanosine, orming dCMP and dGMP, respectively. physiologically appropriate overall decrease in their biosynthesis.  CHAPTER 33 Metabolism o Purine & Pyrimidine Nucleotides 341 NH 2 NH 2 Ribose 5-phosphate + ATP PRPP PP i N N N N 1 N N N N H P O H2C PRPP Adenine O Adenine 2 phosphoribosyl – transferase H H H H 5-Phosphoribosylamine OH OH AMP O O PRPP PP i N N HN HN – – N N N N H Hypoxanthine P O H2C O H H IMP Hypoxanthine-guanine H H phosphoribosyltransferase OH OH IMP O O AMP GMP N N HN HN H2N N N H2N N N H Guanine ADP GDP PRPP PP i P O H2C O ATP GTP H H H H FIGURE 33–5 Control of the rate of de novo purine nucleo- OH OH tide biosynthesis. Reactions ➀ and ➁ are catalyzed by PRPP synthe- GMP tase and by PRPP glutamyl amidotranserase, respectively. Solid lines represent chemical low. Broken red lines represent eedback inhibi- FIGURE 33–4 Phosphoribosylation of adenine, hypoxan- tion by intermediates o the pathway. thine, and guanine to form AMP, IMP, and GMP, respectively. IMP AMP & GMP Feedback Regulate Their Formation From IMP – + – In addition to regulation at the level o PRPP biosynthesis, additional mechanisms that regulate conversion o IMP to ATP and GTP are summarized in Figure 33–6. AMP eed- AMPS XMP back inhibits adenylosuccinate synthetase, EC 6.3.4.4 (reac- + tion 12 , Figure 33–3), and GMP inhibits IMP dehydrogenase, EC 1.1.1.205 (reaction 14 , Figure 33–3). Furthermore, conver- AMP GMP sion o IMP to adenylosuccinate en route to AMP (reaction 12 , Figure 33–3) requires GTP, and conversion o xanthinylate ADP GDP (XMP) to GMP requires ATP. This cross-regulation between the pathways o IMP metabolism thus serves to balance the biosynthesis o purine nucleoside triphosphates by decreas- ATP GTP ing the synthesis o one purine nucleotide when there is a deiciency o the other nucleotide. AMP and GMP also FIGURE 33–6 Regulation of the conversion of IMP to ade- inhibit hypoxanthine-guanine phosphoribosyltranserase, nosine nucleotides and guanosine nucleotides. Solid lines repre- sent chemical low. Broken green lines represent positive eedback which converts hypoxanthine and guanine to IMP and GMP loops , and broken red lines represent negative eedback loops . (Figure 33–4), while GMP also eedback inhibits PRPP glu- (AMPS, adenylosuccinate; XMP, xanthosine monophosphate; their tamyl amidotranserase (reaction ➁, Figure 33–2). structures are given in Figure 33–3.) 342 SECTION VII Structure, Function, & Replication o Inormational Macromolecules Purine nucleotides PRPP ATP + Ribose 5-phosphate + – Aspartate FIGURE 33–7 Reduction of ribonucleoside diphosphates to 2′-deoxyribonucleoside diphosphates. ATP + CO2 + Glutamine CAP – CAA REDUCTION OF RIBONUCLEOSIDE – – DIPHOSPHATES FORMS DHOA DEOXYRIBONUCLEOSIDE PRPP OA DIPHOSPHATES OMP Reduction o the 2′-hydroxyl o purine and pyrimidine ribo- UMP nucleotides, catalyzed by the complex that includes ribonu- UDP cleotide reductase, EC 1.17.4.1 (Figure 33–7), provides the CTP UTP deoxyribonucleoside diphosphates (dNDPs) needed or both the synthesis and repair o DNA (see Chapter 35). The enzyme complex is unctional only when cells are actively synthesizing UDP DNA. Reduction requires reduced thioredoxin, thioredoxin TDP reductase (EC 1.8.1.9), and NADPH. The immediate reduc- dUDP TMP tant, reduced thioredoxin, is produced by NADPH-dependent reduction o oxidized thioredoxin (Figure 33–7). The reduc- FIGURE 33–8 Regulatory aspects of the biosynthesis of tion o ribonucleoside diphosphates (NDPs) to dNDPs is purine and pyrimidine ribonucleotides and reduction to their subject to complex regulatory controls that achieve balanced respective 2′-deoxyribonucleotides. The broken green line repre- production o dNTPs or synthesis o DNA (Figure 33–8). sents a positive eedback loop. Broken red lines represent negative eedback loops. Abbreviations are provided or the intermediates in the biosynthesis o pyrimidine nucleotides whose structures are given in Figure 33–9. (CAA, carbamoyl aspartate; DHOA, dihydrooro- BIOSYNTHESIS OF PYRIMIDINE tate; OA, orotic acid; OMP, orotidine monophosphate; PRPP phospho- NUCLEOTIDES ribosyl pyrophosphate.) Figure 33–9 illustrates the intermediates and enzymes o pyrimidine nucleotide biosynthesis. The catalyst or the initial reaction is cytosolic carbamoyl phosphate synthetase II (EC polypeptide named or the the irst letters o its enzyme 6.3.5.5), a dierent enzyme rom the mitochondrial carbamoyl activities, catalyzes the irst three reactions o Figure 33–9. A phosphate synthetase I o urea synthesis (see Figure 28–16). second biunctional enzyme catalyzes reactions ➄ and ➅ o Compartmentation thus provides an independent pool o car- Figure 33–9. The close proximity o multiple active sites on a bamoyl phosphate or each process. Unlike in purine biosyn- multiunctional polypeptide acilitates eicient channeling o thesis where PRPP serves as a scaold or assembly o the purine the intermediates o pyrimidine biosynthesis. ring (Figure 33–2), PRPP participates in pyrimidine biosynthesis only subsequent to assembly o the pyrimidine ring. As or the biosynthesis o pyrimidines, purine nucleoside biosynthesis is THE DEOXYRIBONUCLEOSIDES energetically costly. OF URACIL & CYTOSINE Multifunctional Proteins Catalyze ARE SALVAGED Adenine, guanine, and hypoxanthine released during the the Early Reactions of Pyrimidine turnover o nucleic acids, notably messenger RNAs, are recon- Biosynthesis verted to nucleoside triphosphates via so-called salvage path- Five o the irst six enzyme activities o pyrimidine biosyn- ways. While mammalian cells reutilize ew free pyrimidines, thesis reside on multifunctional polypeptides. CAD, a single “salvage reactions” convert the pyrimidine ribonucleosides  CHAPTER 33 Metabolism o Purine & Pyrimidine Nucleotides 343 CO 2 + Glutamine + ATP Carbamoyl phosphate 1 synthetase II O Aspartate O O –O 4 transcar- –O Dihydro- +H N C bamoylase C orotase C 3 3 5 CH 2 4 CH 2 + H2 N 3 5 CH 2 HN 2 C 6 H O 1 C 2 C1 2 CH 6 3 C CH O P + H3 N COO – O N – O N COO – COO H Pi H H2O Carbamoyl Aspartic Carbamoyl Dihydroorotic phosphate acid aspartic acid acid (DHOA) (CAP) NAD + (CAA) Dihydroorotate dehydrogenase 4 NADH + H + O O O CO2 PP i PRPP 4 6 5 HN 3 5 HN HN 2 1 6 O N Orotidylic acid O N COO – Orotate O N COO – decarboxylase phosphoribosyl- H R-5- P R-5- P transferase Orotic acid UMP OMP (OA) ATP 7 ADP NADPH + H+ NADP+ 10 UDP dUDP (deoxyuridine diphosphate) Ribonucleotide H2O ATP reductase 8 11 ADP Pi UTP dUMP ATP P N 5,N10 -Methylene H4 folate Glutamine Glutamin ne Thymidylate 12 CTP synthase synthetase 9 H2 folate ADP P Glutamate mate +Pi NH 2 O CH 3 N HN O N O N R-5- P - P - P dR-5- P CTP TMP FIGURE 33–9 The biosynthetic pathway for pyrimidine nucleotides. uridine and cytidine and the pyrimidine deoxyribonucleosides Methotrexate Blocks Reduction of thymidine and deoxycytidine to their respective nucleotides. Dihydrofolate Guanine + PRPP → GMP + PPi The reaction catalyzed by thymidylate synthase, EC 2.1.1.45 Hypoxanthine + PRPP → IMP + PPi (reaction 12 , Figure 33–9) is the only reaction o pyrimidine Phosphoryltranserases (kinases) catalyze transer o the nucleotide biosynthesis that requires a tetrahydroolate deriv- γ-phosphoryl group o ATP to the diphosphates o the dNDPs ative. During this reaction, the methylene group o N5,N10- 2′-deoxycytidine, 2′-deoxyguanosine, and 2′-deoxyadenosine, methylene-tetrahydroolate is reduced to the methyl group converting them to the corresponding nucleoside triphosphates. that is transerred to the 5-position o the pyrimidine ring, as well as dihydroolate, the oxidized orm o tetrahydroolate. NDP + ATP → NTP + ADP For urther pyrimidine synthesis to occur, dihydroolate must be dNDP + ATP → dNTP + ADP reduced back to tetrahydroolate. This reduction, catalyzed by 344 SECTION VII Structure, Function, & Replication o Inormational Macromolecules dihydroolate reductase (EC 1.5.1.3), is inhibited by methotrexate. Dividing cells, which must generate TMP and dihydroolate, are especially sensitive to inhibitors o dihydroolate reductase such as the anticancer drug methotrexate. Certain Pyrimidine Analogs Are Substrates for Enzymes of Pyrimidine Nucleotide Biosynthesis Allopurinol and the anticancer drug 5-fluorouracil (see Figure 32–13) are alternate substrates or orotate phospho- ribosyltranserase, EC 2.4.2.10 (reaction ➄, Figure 33–9). Both drugs are phosphoribosylated, and allopurinol is con- FIGURE 33–10 Regulation of the conversion of purine verted to a nucleotide in which the ribosyl phosphate is and pyrimidine NDPs to NTPs. Solid lines represent chemical low. attached to N1 o the pyrimidine ring. Broken lines indicate targets o positive or negative  eedback inhibition. REGULATION OF PYRIMIDINE higher primates, uricase (EC 1.7.3.3) converts uric acid to NUCLEOTIDE BIOSYNTHESIS the water-soluble product allantoin. However, since humans lack uricase, the end product o purine catabolism in humans Gene Expression & Enzyme Activity is uric acid. Both Are Regulated CAD represents the primary ocus or regulation o pyrimi- dine biosynthesis. Expression o the CAD gene is regulated at DISORDERS OF PURINE the level both o transcription and o translation. At the level METABOLISM o enzyme activity, the carbamoyl phosphate synthetase II Various genetic deects in PRPP synthetase (reaction ➀, (CPS) activity o CAD is activated by PRPP and is eedback Figure 33–2) present clinically as gout. Each deect—or inhibited by UTP. The eect o UTP is, however, abolished by example, an elevated Vmax, increased ainity or ribose phosphorylation o serine 1406 o CAD. 5-phosphate, or resistance to eedback inhibition—results in overproduction and overexcretion o purine catabo- lites. When serum urate levels exceed the solubility limit, Purine & Pyrimidine Nucleotide sodium urate crystalizes in sot tissues and joints where it Biosynthesis Are Coordinately causes an inlammatory reaction, gouty arthritis. However, Regulated most cases o gout relect abnormalities in renal handling Purine and pyrimidine biosynthesis parallel one another o uric acid. quantitatively, that is, mole or mole, suggesting coordinated While purine deiciency states are rare in human subjects, control o their biosynthesis. Several sites o cross-regulation there are numerous genetic disorders o purine catabolism. characterize the pathways that lead to the biosynthesis o Hyperuricemias may be dierentiated based on whether purine and pyrimidine nucleotides. PRPP synthetase (reaction patients excrete normal or excessive quantities o total urates. Some hyperuricemias relect speciic enzyme deects. Others are ①, Figure 33–2), which orms a precursor essential or both processes, is eedback inhibited by both purine and pyrimi- secondary to diseases such as cancer or psoriasis that enhance dine nucleotides, as is the conversion o both pyrimidine and tissue turnover. purine nucleotides NDPs to NTPs (Figure 33–10). Lesch-Nyhan Syndrome The Lesch-Nyhan syndrome, an overproduction hyperurice- HUMANS CATABOLIZE PURINES mia characterized by requent episodes o uric acid lithiasis and a bizarre syndrome o sel-mutilation, relects a deect TO URIC ACID in hypoxanthine-guanine phosphoribosyl transferase, an Humans convert adenosine and guanosine to uric acid enzyme o purine salvage (Figure 33–4). The accompanying (Figure 33–11). Adenosine is irst converted to inosine by rise in intracellular PRPP results in purine overproduction. adenosine deaminase, EC 3.5.4.4. In mammals other than Mutations that decrease or abolish hypoxanthine-guanine  CHAPTER 33 Metabolism o Purine & Pyrimidine Nucleotides 345 NH2 phosphoribosyltranserase activity include deletions, rame- N shit mutations, base substitutions, and aberrant mRNA N splicing. N N HO H 2C von Gierke Disease O Purine overproduction and hyperuricemia in von Gierke H H H H disease (glucose-6-phosphatase deficiency) occurs second- OH OH ary to enhanced generation o the PRPP precursor ribose Adenosine 5-phosphate. An associated lactic acidosis elevates the renal H2O threshold or urate, elevating total body urates. Adenosine deaminase NH + 4 Hypouricemia O O Hypouricemia and increased excretion o hypoxanthine and N N xanthine are associated with a deiciency in xanthine oxidase, HN HN EC 1.17.3.2 (Figure 33–11) due to a genetic deect or to severe H2N N N liver damage. Patients with a severe enzyme deiciency may N N HO H 2C HO H 2C exhibit xanthinuria and xanthine lithiasis. O O H H H H Adenosine Deaminase & Purine H H H H OH OH Nucleoside Phosphorylase OH OH Inosine Guanosine Deficiency Pi Purine nucleoside Pi Adenosine deaminase deficiency (Figure 33–11) is asso- phosphorylase ciated with an immunodeiciency disease in which both Ribose 1-phosphate thymus-derived lymphocytes (T cells) and bone marrow– derived lymphocytes (B cells) are sparse and dysunctional. O O Patients suer rom severe immunodeiciency. In the absence N N HN HN o enzyme replacement or bone marrow transplantation, NH H 2N NH inants oten succumb to atal inections. Deective activity N N Hypoxanthine Guanine o purine nucleoside phosphorylase (EC 2.4.2.1) is associ- ated with a severe deiciency o T cells, but apparently nor- H2O + O2 mal B-cell unction. Immune dysunctions appear to result rom accumulation o dGTP and dATP, which inhibit ribo- O HN3 H 2O 2 nucleotide reductase and thereby deplete cells o DNA pre- N cursors. Table 33–1 summarizes known disorders o purine HN metabolism. O NH NH Xanthine H2O + O2 PYRIMIDINE CATABOLITES ARE Xanthine oxidase WATER SOLUBLE H2O2 Unlike the low solubility products o purine catabolism, catabo- O lism o the pyrimidines orms highly water-soluble products— HN CO2, NH3, β-alanine, and β-aminoisobutyrate (Figure 33–12). HN O Excretion o β-aminoisobutyrate increases in leukemia and O NH NH severe x-ray radiation exposure due to increased destruction o Uric acid DNA. However, many persons o Chinese or Japanese ancestry routinely excrete β-aminoisobutyrate. FIGURE 33–11 Formation of uric acid from purine nucleo- Disorders o β-alanine and β-aminoisobutryrate metabo- sides by way of the purine bases hypoxanthine, xanthine, and guanine. Purine deoxyribonucleosides are degraded by the same lism arise rom deects in enzymes o pyrimidine catabolism. catabolic pathway and enzymes, all o which exist in the mucosa o These include β-hydroxybutyric aciduria, a disorder due the mammalian gastrointestinal tract. to total or partial deiciency o the enzyme dihydropyrimi- dine dehydrogenase, EC 1.3.1.2 (Figure 33–12). The genetic disease relects an absence o the enzyme. A disorder o 346 SECTION VII Structure, Function, & Replication o Inormational Macromolecules TABLE 33–1 Metabolic Disorders of Purine & Pyrimidine Matabolism Enzyme Catalog Figure and Defective Enzyme Number OMIM Reference Major Signs and Symptoms Reaction Purine Metabolism Hypoxanthine-guanine 2.4.2.8 308000 Lesch-Nyhan syndrome. 33-4 ➁ phosphoribosyl transerase Uricemia, sel-mutilation PRPP synthase 2.7.6.1 311860 Gout; gouty arthritis 33-2 ➀ Adenosine deaminase 3.5.4.6 102700 Severely compromised immune 33-1 ➀ system Purine nucleoside 2.4.2.1 164050 Autoimmune disorders; benign 33-11 ➁ phosphorylase and opportunistic inections Pyrimidine Metabolism Dihydropyrimidine 1.3.1.2 274270 Can develop toxicity to 33-12 ➁ dehydrogenase 5-fuorouracil, also a substrate or this dehydrogenase Orotate phosphoribosyl 2.4.2.10 and 258900 Orotic acid aciduria type 1; 33-9 ➄ and ➅ transerase and orotidylic acid 4.1.1.23 megaloblastic anemia decarboxylase Orotidylic acid decarboxylase 4.1.1.23 258920 Orotic acid aciduria type 2 33-9 ➅ pyrimidine catabolism, known also as combined uraciluria- mitochondria to utilize carbamoyl phosphate, which then thyminuria, is also a disorder o β-amino acid metabolism, becomes available or cytosolic overproduction o orotic acid. since the formation o β-alanine and o β-aminoisobutyrate is Type I orotic aciduria relects a deiciency o both orotate impaired. When caused by a genetic error, there are serious phosphoribosyltranserase (EC 2.1.3.3) and orotidylate decar- neurologic complications. A nongenetic orm is triggered by boxylase, EC 4.1.1.23 (reactions ➄ and ➅, Figure 33–9). The the administration o the anticancer drug 5-luorouracil (see rarer Type II orotic aciduria is due to a deiciency only o Figure 32–13) to patients with low levels o dihydropyrimidine orotidylate decarboxylase (reaction ➅, Figure 33–9). dehydrogenase. Deficiency of a Urea Cycle Enzyme Pseudouridine Is Excreted Unchanged Results in Excretion of Pyrimidine No human enzyme catalyzes hydrolysis or phosphorolysis o the pseudouridine (ψ) derived rom the degradation o RNA Precursors molecules. This unusual nucleotide thereore is excreted Increased excretion o orotic acid, uracil, and uridine accom- unchanged in the urine o normal subjects. Pseudouridine was panies a deiciency in liver mitochondrial ornithine transcar- indeed irst isolated rom human urine (Figure 33–13). bamoylase (see reaction ➁, Figure 28–16). Excess carbamoyl phosphate exits to the cytosol, where it stimulates pyrimidine nucleotide biosynthesis. The resulting mild orotic aciduria is OVERPRODUCTION OF increased by high-nitrogen oods. PYRIMIDINE CATABOLITES Since the end products o pyrimidine catabolism are highly Drugs May Precipitate Orotic Aciduria water soluble, pyrimidine overproduction results in ew clini- Allopurinol (see Figure 32–13), an alternative substrate or cal signs or symptoms. Table 33–1 lists exceptions. In hyper- orotate phosphoribosyltranserase (reaction ➄, Figure 33–9), uricemia associated with severe overproduction o PRPP, there competes with orotic acid. The resulting nucleotide product also is overproduction o pyrimidine nucleotides and increased inhibits orotidylate decarboxylase (reaction ➅, Figure 33–9), excretion o β-alanine. Since N5,N10-methylene-tetrahydroo- resulting in orotic aciduria and orotidinuria. 6-Azauridine, ol- late is required or thymidylate synthesis, disorders o olate lowing conversion to 6-azauridylate, also competitively inhibits and vitamin B12 metabolism result in deiciencies o TMP. orotidylate decarboxylase (reaction ➅, Figure 33–9), enhancing excretion o orotic acid and orotidine. Four genes that encode Orotic Aciduria urate transporters have been identiied. Two o the encoded The orotic aciduria that accompanies the Reye syndrome proteins are localized to the apical membrane o proximal probably is a consequence o the inability o severely damaged tubular cells.  CHAPTER 33 Metabolism o Purine & Pyrimidine Nucleotides 347 O HN NH HO O O OH OH FIGURE 33–13 Pseudouridine, in which ribose is linked to C5 of uridine. IMP is a precursor both o AMP and GMP. Glutamine provides the 2-amino group o GMP, and aspartate the 6-amino group o AMP. Phosphoryl transer rom ATP converts AMP and GMP to ADP and GDP. A second phosphoryl transer rom ATP orms GTP, but ADP is converted to ATP primarily by oxidative phosphorylation. Hepatic purine nucleotide biosynthesis is stringently regulated by the pool size o PRPP and by eedback inhibition o PRPP glutamyl amidotranserase by AMP and GMP. Coordinated regulation o purine and pyrimidine nucleotide biosynthesis ensures their presence in proportions appropriate or nucleic acid biosynthesis and other metabolic needs. Humans catabolize purines to uric acid (pKa 5.8), present as the relatively insoluble acid at acidic pH or as its more soluble sodium urate salt at a pH near neutrality. Urate crystals are diagnostic o gout. Other disorders o purine catabolism include Lesch-Nyhan syndrome, von Gierke disease, and hypouricemias. Since pyrimidine catabolites are water soluble, their overproduction does not result in clinical abnormalities. Excretion o pyrimidine precursors can, however, result rom a defciency o ornithine transcarbamoylase because excess carbamoyl phosphate is available or pyrimidine biosynthesis. REFERENCES Brassier A, Ottolenghi C, Boutron A, et al: Dihydrolipoamide dehydrogenase defciency: a still overlooked cause o recurrent acute liver ailure and Reye-like syndrome. Mol Genet Metab FIGURE 33–12 Catabolism of pyrimidines. Hepatic 2013;109:28. β-ureidopropionase catalyzes the ormation o both β-alanine and Fu R, Jinnah HA: Genotype-phenotype correlations in Lesch-Nyhan β-aminoisobutryrate rom their pyrimidine precursors. disease: moving beyond the gene. J Biol Chem 2012;287:2997. Fu W, Li Q, Yao J, et al: Protein expression o urate transporters in renal tissue o patients with uric acid nephrolithiasis. Cell Biochem Biophys 2014;70:449. SUMMARY Moyer RA, John DS: Acute gout precipitated by total parenteral nutrition. J Rheumatol 2003;30:849. Degradation o ingested nucleic acids yields ree purines Uehara I, Kimura T, Tanigaki S, et al: Paracellular route is the major and pyrimidines. Purines and pyrimidines are ormed rom urate transport pathway across the blood-placental barrier. amphibolic intermediates and thus are dietarily nonessential. Physiol Rep 2014;20:2. Several reactions o IMP biosynthesis require olate derivatives Wu VC, Huang JW, Hsueh PR, et al: Renal hypouricemia is an and glutamine. Consequently, antiolate drugs and glutamine ominous sign in patients with severe acute respiratory syndrome. analogs inhibit purine biosynthesis. Am J Kidney Dis 2005;45:88.