Nota Chemical Pathology I PDF

Nota Chemical Pathology I Topic 1: Laboratory Testing Group 1.​ Basic metabolic panel (BMP) -​ Kidneys -​ Acid/ Base balance -​ Electrolyte -​ Blood glucose level 2.​ Comprehensive Metabolic Panel (CMP) -​ Kidneys -​ Liv...

Nota Chemical Pathology I Topic 1: Laboratory Testing Group 1.​ Basic metabolic panel (BMP) -​ Kidneys -​ Acid/ Base balance -​ Electrolyte -​ Blood glucose level 2.​ Comprehensive Metabolic Panel (CMP) -​ Kidneys -​ Liver -​ Electrolyte -​ acid/ base balance -​ Blood glucose -​ Blood proteins 3.​ Liver Tests -​ Protein (total protein & albumin) -​ ALP (alkaline phosphatase) -​ ALT (alanine aminotransferase) @ SGPT -​ AST (aspartate aminotransferase) @ SGOT -​ Bilirubin (total bilirubin & Direct bilirubin) 4.​ Renal profile -​ Albumin -​ Urea/ BUN (blood urea nitrogen) -​ Electrolytes and anion gap ​ Sodium ​ Potassium ​ Chloride ​ Bicarbonate ​ Phosphorus ​ calcium 1  -​ Creatinine -​ Glucose -​ eGFR 5.​ Cardiac panel profile/ Coronary risk profile/ Lipid profile -​ Cholesterol -​ HDL-cholesterol -​ LDL-cholesterol -​ Triglycerides 6.​ Diabetes mellitus profile -​ Screening: Urinalysis -​ Confirmation: ​ Fasting blood glucose = >7.0 mmol/L ​ Random glucose = >11.0 mmol/L ​ OGTT/ MGTT = >11.0 mmol/L ​ Monitoring = HbA1c > 6.5% 7.​ Anemia profile -​ Total iron binding capacity -​ Iron -​ Vitamin B12 -​ Folate, RBC -​ Transferrin -​ Ferritin 8.​ Hypertension profile -​ Urinalysis, urine protein -​ Urinary albumin, BUN, creatinine. eGFR -​ Calcium -​ TSH (thyroid stimulating hormone) and T4 -​ Fasting glucose, A1c -​ Potassium -​ Basic metabolic panel (BMP) -​ Lipid profile 2  -​ Aldosterone and renin -​ Cortisol and dexamethasone test -​ Catecholamines and d metanephrines -​ Complete blood picture 9.​ Arthritis Profile -​ C-reactive protein (CRP) -​ Sedimentation rate -​ Uric Acid 10.​Bone/ joint profile -​ Calcium -​ Uric acid -​ RA Factor (rheumatoid arthritis) -​ Phosphate 11.​Pancreatic profile -​ Amylase -​ Insulin fasting -​ Lipase -​ C-peptide 12.​Thyroid profile -​ TSH Tests ​ T3 bound to protein ​ FT3 13.​Parathyroid profile -​ Vitamin D tests -​ Calcium -​ Phosphorus 3  14.​Drug toxicology profile -​ Therapeutic drug monitoring ​ Barbiturates ​ Phencyclidine (PCP) -​ Illegal drug ​ Amphetamines ​ Cocaine ​ Methamphetamines ​ Marijuana ​ Opiates Topic 2: Amino Acids and Proteins 1.​ Protein -​ Large molecules -​ Made up of chains of amino acids -​ Are found in every cell in the body -​ Are involved in most of the body’s functions and life processes -​ The sequence of amino acids is determined by DNA 2.​ Amino acid structure= 4 3.​ Describe the general structure of amino acids -​ Each amino acid has an amino group and a carboxyl group, joined by a single carbon atom -​ In addition, each amino acid has a characteristic ‘side chain’ often called the R-group -​ Each amino acid (except glycine) can occur in two isomeric forms, L and D, because of the possibility of forming two different enantiomers (mirror image) around the central carbon atom. -​ Only L-amino acids are found in proteins in all organism -​ Some D-amino acids are found in bacterial cell walls -​ ‘R’ side group can be any of 20 different chemicals 4.​ Properties of amino acids -​ Building blocks of proteins -​ Group of organic compounds -​ Containing 2 functional groups (zwitterion and amphoteric) 5.​ 2 types of amino acids -​ Non-essential -​ Essential 6.​ Non-essential amino acids -​ Considered non-essential because our body produce these -​ Liver produces nearly 80% of the amino acids needed for sustaining life 7.​ Essential amino acids -​ Body cannot produce these, so must be obtained from diet -​ 20% of amino acids are needed to build specific proteins 5 8.​ 20 types of amino acids Non polar, aliphatic R group Glycine, alanine, proline, valine, leucine, isoleucine, methionine Polar, uncharged R group Serine, threonine, cysteine, asparagine, glutamine Aromatic R group Phenylalanine, Tyrosine, tryptophan Negatively charged R group Aspartate, glutamate Positively charged R group Lysine, arginine, histidine 9.​ Protein bond = -​ Dipeptide (2 amino acid) -​ Tripeptide (3 amino acid) -​ Oligopeptide -​ Polypeptide 10.​Structure of protein -​ Primary = collagen -​ Secondary = albumin -​ Tertiary = Hb -​ Quaternary = Protein 11.​Specimen -​ Serum -​ Plasma -​ Urine -​ Cerebrospinal fluid 6 12.​Method -​ Kjeldahl -​ Biuret -​ Refractometry -​ Electrophoresis -​ Bromocresol green (BCG) -​ Immunoassay 13.​Functions of proteins -​ Contractile - Contractile proteins are essential for the movement of muscles and various cellular structures. They enable the contraction and relaxation of muscles, allowing for activities such as body movement, organ function, and cellular motility. -​ Hormones - Hormonal proteins, also known as signaling proteins, serve as chemical messengers that regulate and coordinate various physiological processes in the body. They help in communication between cells and tissues. -​ Structural - Structural proteins provide support and stability to cells and tissues. They contribute to the maintenance of the overall structure and shape of cells, tissues, and organs. -​ Enzymes - Enzymes are proteins that act as biological catalysts, facilitating and accelerating chemical reactions in living organisms. They play a crucial role in metabolism, allowing various biochemical processes to occur at a faster rate. -​ Protection - Some proteins are involved in the defense and protection of the body against pathogens, toxins, and other harmful agents. They contribute to the immune system's response and play a role in maintaining overall health. Ex = antibodies -​ Transport - Transport proteins facilitate the movement of substances, such as ions, molecules, and gasses, across cell membranes and throughout the body. They play a crucial role in the transportation of nutrients, oxygen, and waste products. 7 Topic 4: Non-protein nitrogenous compound 1.​ Functions of amino acids Amino acids Function alanine Use for energy, constituent of vitamin 65 and coenzyme A arginine 1.​ Enhancing immune function= -​ Retards growth of tumor and cancer -​ Healing and repair damage tissue 2.​ Aids in liver detoxification by neutralizing ammonia 3.​ Important for muscle metabolism asparagine Maintain balance in CNS Aspartic acid 1.​ Increase stamina 2.​ Help to protect liver - aid in removal excess ammonia Cysteine 1.​ Aid in production of collagen 2.​ Promotes proper elasticity and texture of the skin Glycine Retards muscle degeneration - supply additional creatine Histidine 1.​ Need for production of RBC and WBC 2.​ For growth and repair of tissues 2.​ Transamination -​ Transfer of amino acid group from an amino acid to a keto acid, with the formation of a new amino acid & new keto acid -​ Catalyzed by a group of enzymes called ‘transaminases (aminotransferases)’ 3.​ Relation between transamination and these organs -​ Kidney = In the kidney, transamination is involved in the regulation of acid-base balance and the excretion of nitrogenous waste products. The conversion of amino acids to keto acids through transamination allows for the removal of excess nitrogen in the form of urea, which is then excreted in urine. -​ Brain = Transamination in the brain is essential for the synthesis of neurotransmitters, which are chemical messengers that transmit signals between nerve cells. Amino acids, such as glutamate and aspartate, are involved in transamination processes to produce neurotransmitters like GABA (gamma-aminobutyric acid). 8  -​ Heart = In the heart, transamination is involved in energy metabolism. Amino acids can be used as substrates for energy production -​ Liver = The liver plays a central role in amino acid metabolism, and transamination is a key process in this organ. The liver is responsible for synthesizing, modifying, and catabolizing amino acids based on the body's needs. Transamination reactions contribute to both the synthesis of new amino acids and the conversion of amino acids into other metabolites. 4.​ Deamination -​ The removal of amino group from the amino acids as ammonia -​ Results in the releasing of ammonia for urea synthesis -​ The carbon skeleton of amino acids is converted to keto acids -​ May be either oxidative or non-oxidative 5.​ Defect in metabolism of amino acids can cause: -​ Phenylketonuria (PKU) -​ Tyrosinemia, alkaptonuria, and albinism -​ Homocystinuria and hypermethioninemia -​ Cystinuria and cystinosis and sulfite oxidase deficiency -​ Hartnup’s disorder -​ Maple syrup urine disease (MSUD) -​ Nonketotic hyperglycinemia @ glycine encephalopathy 6.​ Aminoaciduria -​ Abnormal presence of amino acids in urine Primary Secondary ​ Inherited enzyme deficiency = inborn Due to disease (liver/ renal tubular error of metabolism dysfunction) @ protein energy malnutrition ​ Defect in pathway (amino acid is metabolized) @ renal tubular system (amino acid is absorbed) 9 7.​ Newborn screening test -​ Guthrie test -​ It tests babies for serious disorders -​ Between 48 and 72 hours old -​ Early treatment can prevent mental retardation, physical disability, or death. -​ Sample collection = heel (tumit) prick & putting a few drops of blood on a special filter paper 8.​ Disease in baby Disease Explanation Amino acid disorders Unable to break down certain amino acids in the blood. Early treatment can help prevent mental retardation, seizures, organ damage and death Fatty acid oxidation disorders ​ Unable to convert fat into energy ​ Treatment with a low fat dietary supplements and avoidance of fasting ​ Early treatment may prevent low blood sugar, coma and death Organic acid disorders ​ Unable to convert amino acids into energy ​ Treatment with a low-protein diet and supplements ​ Early treatment can prevent vomiting, seizures, coma and death Phenylketonuria Unable to break down the amino acid phenylalanine ​ Early treatment may prevent mental retardation Congenital hypothyroidism ​ Lack of thyroid hormone, which can lead to poor growth and mental retardation ​ Early treatment will allow child to grow and develop normally Galactosaemia ​ Baby cannot break down the galactose part of milk sugar ​ In some cases, life threatening damage to the brain and liver can occurs as early as one week after birth ​ Treatment = a special milk - free diet Cystic Fibrosis ​ Sticky secretions in the lung and gut ​ Treatment = dietary supplements, antibiotics and physiotherapy 10  ​ Early treatment = help prevent poor growth, chest infections and shortened lifespan 9.​ Guthrie Test -​ Also known as the ‘Guthrie bacterial inhibition assay’ -​ Medical test performed on newborn infants to detect phenylketonuria - an inborn error of amino acid metabolism 10.​Procedure of Guthrie Test 1. A drop of blood is obtained by pricking the heel (tumit) of a newborn infant 2. The blood is placed on a piece of filter paper 3. A small portion of blood sample on the filter paper is punched out (small disk) 4. The small disk of the blood sample are placed on an agar gel plate containing Bacillus subtilis and B-2-thienylalanine 5. Within a day, the bacterial growth surrounding the blood sample disk indicates the (+ve) result of PKU. Instead, clear zones around the disk indicate (-ve) the result of PKU. 11.​Function of Bacillus subtilis and B-2-thienylalanine in Guthrie Test Bacillus subtilis: Role: Bacillus subtilis is a bacterium used in the Guthrie test to assist in the detection of elevated levels of phenylalanine. Mechanism: The test involves placing a blood sample from a newborn onto a filter paper containing Bacillus subtilis. The bacteria require phenylalanine as a nutrient for growth. If the newborn has elevated phenylalanine levels due to PKU, the excess phenylalanine in the blood inhibits the growth of Bacillus subtilis. β-2-Thienylalanine: Role: β-2-thienylalanine is a synthetic analog of phenylalanine. Mechanism: In the Guthrie test, a second spot on the filter paper contains β-2-thienylalanine. This compound competes with phenylalanine for uptake by the bacterial cells. If the concentration of phenylalanine is high, it inhibits the uptake of β-2-thienylalanine by the bacteria. This competitive inhibition helps to differentiate between elevated phenylalanine levels and the normal metabolism of phenylalanine. 12.​Urea/ BUN (blood urea nitrogen) -​ Protein -> Amino acid -> Ammonia -> Urea 11 13.​ BUN synthesis -​ UREA - major nitrogenous end product of protein and amino acid catabolism -​ UREA is formed by the liver and carried by the bloodstream to the kidneys to be excreted out -​ The blood urea nitrogen (BUN) is usually determined in conjunction with the creatinine level in assessing renal function 14.​Excretion -​ KIDNEYS - almost all urea is filtered out by blood by glomerular function ​ Some urea reabsorbed with water most is removed in urine -​ Increased with high dietary protein intake or increase catabolism 15.​Specimen collection for Urea -​ Fluoride or Citrate tube must be avoided - both inhibit urease -​ Urea susceptible to bacterial decomposition, so samples cannot be analyzed within few hours should be refrigerated 16.​BUN/ Blood urea nitrogen -​ BUN measures the amount of urea nitrogen -​ A measurement of BUN can be used as a test of renal function @ indicator of other abnormalities -​ Rises more rapidly than serum creatinine -​ Less sensitive indicator of a declining renal function compared to creatinine clearance test. 17.​Normal values for UREA ​ Female = 0.6 - 1.2 mg/dL ​ Male = 0.8 - 1.4 mg/dL ​ Children = 0.2 - 1.0 mg/dL 18.​Abnormal urea value -​ An elevated concentration of urea in a blood = Azotemia -​ If there is a very high plasma concentration of urea accompanied with a renal failure = uremia/ ureamic syndrome -​ If this happen, it should be treated with dialysis or transplantation of kidney immediately because it will cause fatal 19.​THREE categories of conditions that cause elevated plasma urea. ​ Pre-renal azotemia = condition where is a reduction of blood flow to the kidney and as a result less urea is filtered ​ Renal = associates with a decrease renal function and lead to increase in plasma urea concentration ​ Post renal azotemia = associated with a obstruction of the urinary flow anywhere in urinary tract by renal calculi, tumor of bladder or prostate or severe UTI 12 20.​Azotemia -​ An elevated BUN may be caused by: ​ Impaired renal function ​ Congestive heart failures as a result of poor renal perfusion ​ Dehydration ​ Shock ​ Hemorrhage into the gastrointestinal tract ​ Acute myocardial infarction ​ Stress ​ Excessive protein intake or protein catabolism 21.​Decreased Urea may be caused by= -​ Atrophy @ muscle tissue -​ Pregnancy -​ Liver failure -​ Malnutrition -​ Impaired nutrient absorption -​ Overhydration 22.​Renal function tests: -​ Blood tests ​ Urea/ BUN ​ Creatinine ​ Uric acid ​ Levels of several elements -​ Urine Tests ​ GFR (glomerular filtration rate) ​ Urine volume ​ Urine urea ​ Minerals in urine ​ Urine protein ​ Urine glucose ​ Hematuria ​ Osmolality 23.​Disease associated with the urea cycle included: -​ Citrullinemia -​ Hyperammonemia -​ Ornithine translocase deficiency -​ N-acetylglutamate synthase deficiency 13  24.​Screening test -​ Laboratory performs a screening test for citrullinemia and argininosuccinic aciduria by tandem mass spectrometry (MS/MS) -​ Elevations in the level of citrulline are specific by ASD and ALD -​ Reference range of citrulline: Classification Reference range -​ Citrulline level in newborn Between 10 and 20 micooM -​ Infants with neonatal onset ASD Greater than 1000 microM -​ Infants with neonatal onset ALD Between 100 and 300 microM 25.​Confirmatory test -​ Plasma and urine amino acid analyses are the laboratory standards for confirmation of citrullinemia, and argininosuccinic aciduria Newborn screening includes testing for the following urea cycle disorders: -​ Argininemia -​ Citrullinemia -​ Argininosuccinic aciduria Note: other urea cycle disorders, including ornithine transcarbamylase (OTC) deficiency, are not detected by newborn screening 3.3 Creatine/ Creatinine 1.​ Creatine = found in the muscles 2.​ Creatinine = breakdown product of creatine phosphate and creatine muscles. Creatine -​ Amino acid derivative -​ Derivative synthesized from arginine, glycine, and methionine in the kidneys, liver and pancreas -​ Also be obtained through diet, mainly from meat and fish 14 Creatinine -​ Breakdown product of creatine and creatinine phosphate in muscle -​ Produced depending on the muscle mass -​ Mainly filtered by the kidney -​ Little to no tubular reabsorption of creatinine -​ If the filtering of the kidney is deficient, blood levels rise -​ As a result, creatinine blood levels may be used to calculate creatinine clearance (CCr), which reflects the glomerular filtration rate (GFR) -​ The GFR is clinically important because it is a measurement of renal function -​ When kidneys are working properly, serum creatinine level is low but with renal function impaired - creatinine level increase -​ Men tend to have higher levels of creatinine because they have more skeletal muscle than women -​ Vegetarians tend to have lower creatinine levels, because meat contains creatine. Plasma creatinine -​ Endogenous production of creatinine constant = creatinine excreted in urine constant -​ Creatinine inversely proportional to creatinine clearance -​ Raised plasma creatinine = impaired renal function Specimen collection -​ Patient have to fast for 8 hours before test -​ Use heparin tube @ plain tube -​ Prolong delay of specimen to laboratory can cause ammonia to form and warming can cause falsely elevated results Creatinine clearance -​ Measure how well creatinine is removed from blood by kidneys - reflects GFR -​ Gives better information than blood creatinine test on how well kidneys working - serum creatinine may be affected by muscle bulk -​ In renal failure - GFR is deficient -> creatinine in blood rise, creatinine in urine decrease -​ Therefore, creatinine levels in blood and urine may be used to calculate the creatinine clearance (crCl), which reflect GFR Creatinine clearance Test -​ Consists of 2 components ​ 24 hour urine collection ​ Blood sample -​ Conducting both urine and blood testing allow for the comparison between of the serum creatinine level with the amount of creatinine excreted in the urine 15 Sample collection for creatinine clearance test -​ Avoid vigorous exercise -​ Instruct pt to avoid excessive intake of meat -​ Avoid coffee and tea -​ Sample: urine & serum -​ Needs to be refrigerated on ice Why do creatinine clearance samples need to be refrigerated??? 1.​ Prevention of Bacterial Growth: Urine is an ideal medium for bacterial growth. Refrigeration slows down bacterial proliferation, reducing the risk of contamination during the 24-hour collection period. Bacterial contamination could potentially affect the accuracy of creatinine measurements. 2.​ Minimization of Enzymatic Degradation: Enzymes present in urine can degrade creatinine over time. Refrigeration helps slow down enzymatic activity, preserving the integrity of creatinine in the collected sample. This is crucial for obtaining accurate measurements of creatinine concentration in both urine and blood. Creatinine clearance calculation Creatinine clearance rate = (urine creatinine x urine volume) / creatinine in serum CCr = (U x V) / P U = urine concentration of creatinine V = urine flow in mL/ min. Urine volume collected in 24h divided by 24x60 to give the volume produced per minute P = serum or plasma concentration of creatinine Example -​ Plasma creatinine = 1.5 mg/dL -​ Urine creatinine = 1.2 g/L -​ Urine volume = 1.2 L -​ Collection time = 24 hours Calculate the creatinine clearance. 16 Answer = Preference -​ Plasma creatinine is MORE PRECISE than creatinine clearance -​ Creatinine clearance - useful, effective and sensitive to measure actual execratory of creatinine of kidney -​ Plasma creatinine is PREFERRED in evaluating kidney/ renal function. 17 Reference range and clinical significance 3.4 Uric Acid -​ Uric acid = end product of protein (purines) metabolism and excreted by kidneys and bowels -​ Purines - such as adenosine and guanine - from the breakdown of ingested nucleic acids/ from tissue destruction are converted into uric acid mainly in the liver -​ Temporary increase in serum uric acid (hyperuricemia) from ingestion of food high in purine (meat, fish), strenuous exercise, or heavy alcohol ingestion will usually return to normal within 1 day. -​ Nearly all the uric acid in plasma is present as monosodium urate -​ At the pH of plasma, urate in this form is relatively insoluble, and at levels above 6.4 mg/dL, the plasma is saturated -​ As a result, urate crystal may form and precipitate in the tissues and joints Specimen collection for uric acid -​ Heparin tube, plain tube and 24 hour urine -​ serum/ plasma should be removed from the cells as quickly as possible to prevent dilution by intracellular contents 18 Purpose of uric acid test Serum Urine Heparin tube @ plain tube 24 hour urine ​ To confirm the diagnosis of gout ​ Helps detect renal calculi or those at risk for ​ Helps detect renal impairment that causes development of calculus pre-renal azotemia and renal failure ​ Also used to assess the effect of enzyme ​ Four interfering factors: deficiency or metabolic abnormality that 1.​ Starvation results in the overproduction of uric acid 2.​ Caffeine 3.​ Vitamin c ingestion 4.​ High purine diet Clinical Significant Classification of chronic kidney disease 19 Chapter 5: Fatty Acid and Lipid 1.​ Lipid - an oily organic compound -​ Insoluble in water but soluble in organic solvents -​ Essential structural component of living cells 2.​ Major lipids present in the plasma -​ Fatty acids -​ Triglycerides -​ Cholesterol -​ Phospholipids -​ Elevated plasma concentrations of lipids particularly cholesterol, are causally related to the pathogenesis of atherosclerosis Fatty acids -​ Mostly straight - chain monocarboxylic acids -​ Derived from dietary @ tissue TG -​ Saturated (majority) & unsaturated -​ Important components of cell membranes Triglycerides -​ Fatty acids esters of glycerol -​ Main lipids in diets -​ Formed by the esterification of all 3 hydroxyl groups of the alcohol glycerol with identical or different fatty acid molecules -​ Broken down to a mixture of monoglycerides, fatty acids and glycerol -​ Products are absorbed, and TG are resynthesised in mucosa cell -​ Most of exogenous TG pass into plasma as chylomicrons -​ FA and glycerol -> endogenous TG (liver) -> transported as VLDL Phospholipids -​ Complex lipids -​ Similar structure to TG -​ Mainly synthesized in liver and small intestine -​ Important constituent of cells & often present in cell membranes 20 Cholesterol -​ Steroid that is present in the diet -​ But it is mainly synthesized in the liver and small intestine -​ Major components of cell membranes, and acts as the substrate for steroid for steroid hormone formation in the adrenals and the gonads. -​ Present in plasma mainly esterified with fatty acids -​ Cholesterol and bile acids both undergo an enterohepatic circulation Lipoproteins -​ Cholesterol, triglycerides and phospholipid are all transported in plasma as lipoprotein particles -​ Fatty acids are transported bound to albumin -​ Lipoprotein particles comprise a peripheral envelope, consisting mainly of phospholipids and free cholesterol -​ Most lipoproteins are assembled in the liver or small intestine 5 main types of lipoprotein particles can be recognized 1.​ Chylomicron - carried dietary triglycerides to the tissues 2.​ VLDL (very low density lipoprotein) - triglycerides-rich particles that form the major route where endogenous triglycerides are carried to the tissue from the liver and small intestine 3.​ Intermediate density lipoproteins (IDL or VLDL remnants) - formed during removal of triglycerides from VLDL during the transition from VLDLS to LDLs 4.​ LDL (Low density lipoprotein) - formed from IDLs by the removal of more triglycerides and apolipoprotein. Increased plasma (LDL cholesterol) and hence plasma (total cholesterol), is positively correlated with the incidence of ischaemic heart disease. 5.​ HDLs (high density lipoprotein) - transports cholesterol from peripheral cells to the liver for excretion. Increased plasma (HDL cholesterol) is negatively associated with the incidence of ischaemic heart disease. Lipoprotein Metabolism 21 -​ Although the term “lipid” includes several types of molecules, lipid metabolism usually refers to the breakdown and synthesis of fats. Fats are triglycerides, they are esters of glycerol and three fatty acids. -​ Fats can come from the diet, from stores in adipose tissue, or can be synthesized from excess dietary carbohydrates in the liver. Dietary fats are digested mainly in the small intestine, by the action of bile salts and pancreatic lipase. -​ Bile salts emulsify fats. They act as a detergent, breaking large globules of fat into smaller micelles, making them more accessible to lipase. -​ Pancreatic lipase then converts triglycerides into monoglycerides, free fatty acids, and glycerol. These products move into the cells of intestinal epithelium - the enterocytes, inside which they're-combine again to form triglycerides. Triglycerides are packaged along with cholesterol into large lipoprotein particles called chylomicrons. Lipoproteins enable transport of water-insoluble fats within aqueous environments. -​ Chylomicrons leave the enterocytes, enter lymphatic capillaries, and eventually pass into the bloodstream, delivering fats to tissues. The walls of blood capillaries have a surface enzyme called lipoprotein lipase. This enzyme hydrolyzes triglycerides into fatty acids and glycerol, enabling them to pass through the capillary wall into tissues, where they are oxidized for energy, or re-esterized for storage. -​ Fats that are synthesized endogenously in the liver are packed into another type of lipoprotein, the VLDL, to be transported to tissues, where triglycerides are extracted in the same way. When required, fat stores in adipose tissue are mobilized for energy production, by the action of hormone-sensitive lipase, which responds to hormones such as epinephrine. -​ Lipid metabolism pathways are closely connected to those of carbohydrate metabolism. Glycerol is converted to a glycolysis intermediate, while fatty acids undergo beta-oxidation to generate acetyl-CoA. Each round of beta-oxidation removes 2 carbons from the fatty acid chain, releasing one acetyl-CoA, which can then be oxidized in the citric acid cycle. Beta-oxidation also produces several high-energy molecules which are fed directly to the electron transport system. Fats yield more energy per unit mass than carbohydrates. When acetyl-CoA is produced in excess, it is diverted to create ketone bodies. -​ During glucose starvation, ketone bodies are an important source of fuel, especially for the brain. However, ketone bodies are acidic, and when produced in excess, can overwhelm the buffering capacity of blood plasma, resulting in metabolic acidosis, which can lead to coma and death. -​ Ketoacidosis is a serious complication of diabetes, in which cells must oxidize fats for fuel as they cannot utilize glucose. Extreme diets that are excessively low in carbohydrates and high in fat may also result in ketoacidosis. On the other hand, diets that are high in carbohydrates generate excess acetyl-CoA that can be converted into fatty acids. -​ Synthesis of fatty acids from acetyl-CoA is stimulated by citrate, a marker of energy abundance, and inhibited by excess fatty acids. Fatty acids can be converted into triglycerides, for storage or synthesis of other lipids, by combining with glycerol derived from a glycolysis intermediate. 22 The 4 main lipoproteins and their functions Lipoprotein Main apolipoproteins Function Chylomicrons B46, A-I, C-II, E Largest lipoprotein. Synthesized by gut after a meal. Not present in normal fasting plasma. Main carrier of dietary triglycerides VLDL B100, C-II, E Synthesized in the liver. Main carrier of endogenously produced triglycerides. LDL B100 Generated from the VLDL in the circulation. Main carrier of cholesterol. HDL A-I, A-II Smallest lipoprotein. Protective function. Takes cholesterol from extrahepatic tissues to the liver for excretion. 2 cycles in lipoprotein metabolism: -​ Exogenous -​ Endogenous ​ Both centered on the liver ​ Interconnected 2 key enzyme systems are involved in lipoprotein metabolism: 1.​ Lipoprotein Lipase (LPL) = releases free fatty acids & glycerol from chylomicrons & VLDL into the tissues 2.​ Lecithin cholesterol acyl transferase (LCAT) = forms cholesterol from free cholesterol & fatty acids 23 The exogenous lipid cycle -​ Dietary lipid is absorbed in the SMALL INTESTINE and incorporated into CHYLOMICRONS that are secreted into the lymphatics & reach the bloodstream via the thoracic duct -​ In the circulation, TG is gradually removed from these lipoproteins by the action of lipoprotein lipase -​ As it loses, TG, the chylomicron becomes smaller & deflated, with folds of redundant surface material -​ These remnants are removed by the liver -​ The cholesterol may be utilized by the liver to form cell membrane components or bile acids, or may be excreted in the bile -​ Cholesterol can only be excreted from the body by way of the liver The endogenous lipid cycle -​ The liver synthesis VLDL particles that undergo the same form of delipidation as chylomicrons by the action of lipoprotein lipase -​ This result in the formation of an intermediate density lipoprotein (IDL), which becomes low density lipoprotein (LDL) when further dilapidated -​ LDL may be removed from the circulation by the high affinity LDL receptor or by other scavenger routes that are thought to be important at high LDL levels & the main way in which cholesterol is incorporated into atheromatous plaques -​ HDL particles are derived from both liver & gut. They act as cholesteryl ester shuttles, removing the sterol from the peripheral tissues & returning it into the liver. -​ The HDL is taken up either directly by the liver, or indirectly by being transferred to other circulating lipoproteins, which then return to the liver Chylomicrons -​ Large, TG-rich complexes -​ Formed from dietary lipids and intestinal apolipoproteins in the microvilli of the small intestine & released into the lacteals of the lymphatic system, reaching the systemic circulation via the thoracic duct -​ Being the large in size, they have the lowest density -​ Transport exogenous lipid (dietary TG) from the intestine to all cells (used as energy source or stored) Very Low Density Lipoproteins (VLDL) -​ Moderately large particles, with triglycerides as the main component, and apolipoprotein B-100, the main apolipoprotein -​ Formed mainly in the liver and intestinal mucosa -​ Main function = transport endogenously derived lipid from the liver to cells for storage or energy utilization 24 Low Density Lipoproteins (BAD) -​ Relatively smaller cholesterol rich particles -​ Mainly derived from intravascular catabolism of VLDL. However, direct hepatic secretion of LDL has been demonstrated in certain patients with hypercholesterolaemia & Type II Hyperlipidemia -​ LDL serves as the major cholesterol carrier to peripheral tissue High Density Lipoprotein (GOOD) -​ HDL are the smallest of the lipoproteins & the most dense -​ Involved in mediating the transport of cholesterol from cells to the liver for excretion from the body Certain general principles and common properties apply to lipoprotein classes as whole -​ The TG-rich lipoprotein complexes are larger & less dense, have lower protein content -​ Because of their large size, chylomicrons and VLDL particles scatter light -​ This account for the turbid/ milky appearance of plasma specimens containing high levels of these lipoprotein -​ Fasting samples from normal individuals do not contain chylomicrons Clinical Significance. Disorders of Lipid Metabolism 1.​ Pathogenesis of atherosclerosis: Role of plasma lipids & lipoproteins ​ Atherosclerosis = the combination of changes in the intima of arteries resulting in the formation & progressive enlargement of one or more fibrous plaques which protrude into the vascular lumen, causing partial, and sometimes complete, obstruction to blood flow. ​ The major components of the advanced lesion accounting for stenosis are: 1.​ Focal accumulation of lipids 2.​ Fibrous tissue formation 3.​ Calcification 4.​ Hyperlipidaemia & in particular hypercholesterolaemia is the best established cause of atherosclerosis 5.​ LDL is regarded as the main culprit. However, the triglyceride-rich VLDL & chylomicrons also play a role 25 Friedewald formula LDL cholesterol = total cholesterol - total cholesterol - (triglycerides/ 5) Cholesterol reference range ​ 240 mg/dL - high cholesterol TG reference range ​