Functions of the liver Assessment and interpretation of liver function tests

1. Functions of the liverAssessment and interpretation of liver function tests Dr. NehaKanojia University College of Medical Science & GTB Hospital, Delhi

2. WHY ?

3. Liver is the largest internal organ & largest gland in the human body. • Liver is at the epicenter of intermediary metabolism. • It performs versatile & massive biochemical pathways. • It destroys bacteria, inactivate antigens, detoxify harmful chemicals. • Thus multiple & diverse functions of liver have an impact on every tissue in the body.

4. Physiological functions of liver • Intermediary metabolism • Carbohydrate metabolism • Lipid metabolism • Bile metabolsim and entero- hepatic circulation • Protein metabolism • Coagulation • Heme metabolism • Bilirubin metabolism • Xenobiotics metabolism • Storage • Endocrine functions • Immune & inflammatory response • Blood reservoir

5. Carbohydrate metabolism • Liver is an important homeostatic regulator of blood glucose. • It can either produce glucose or store glucose • In fed state- polymerize glucose to glycogen • In unfed state- depolymerize glycogen to glucose • Glucose → hepatocytes → glycogen ↑ →glucose Lactate Glycerol aminoacids

6. Carbohydrate metabolism • Glycogen metabolism • Regulation – 2 rate limiting enzymes • Glycogen synthase- synthesis of glycogen from monomers of UDP glucose. • Glycogen phosphorylase- clevage of glycogen to glucose-1-phosphate.

7. Carbohydrate metabolism Gluconeogenesis • Liver glycogen stores depleted - hepatic gluconeogenesis to replenish blood glucose. • Substrates- - lactate - glycerol from hydrolysis of triglycerides - gluconeogenic amino acid , alanine , glutamine

9. Glycogenesis Glycogenolysis Glucose 6-PO4 + + • Insulin • Glucagon • Epinephrine +  B. Glucose  B. Glucose Hormonal regulation of carbohydrate metabolism

10. Blood glucose regulation within a narrow limit (70-100 mg/dl)  not affected in liver disease due to large reserve of hepatic function • Effects of anaesthesia on carbohydrate metabolism • Halothane •  release of insulin •  rate of glycogenolysis • Inhibition of gluconeogenic response • Isoflurane • Impaired insulin secretion

11. Lipid Metabolism • Oxidation of fatty acids • Fatty acids derived from plasma  • Enter into mitochondria  •  oxidation: fatty acids  AcetylCoA  citric acid cycle • Regulators - Glucagon - activates - Insulin - inhibits

12. Synthesis of lipoproteins • One of the major functions of the liver • Major classes • VLDL • LDL • HDL

13. VLDL • Acute or chronic liver disease – ability to produce VLDL is markedly compromised • Liver VLDLs are associated with an important class of proteins, the apo B protein • Apo B100 - important for hepatic secretion of VLDL. • Decreased in ABETALIPOPROTEINEMIA • LDLs and HDLs • Liver produces them in a small amount

14. Production of ketone bodies • Most organs except the liver- use ketone bodies as fuel • Ketone bodies – acetoacetic acid, acetone,  hydroxybutyrate • Their formation by the liver is normal and physiologically important, e.g. • Fasting  rapid depletion of glycogen stores in the liver  shortage of substrates for citric acid cycle • AcetylCoA formed from  oxidation  ketone bodies • Ketosis -  conc. of ketone bodies in blood • Starvation • DM • After high fat diet

15. Synthesis of cholesterol • Important role in cholesterol homeostasis • Liver cholesterol has both exogenous and endogenous source • Uses of hepatic cholesterol • Formation of bile acids- conjugated with other substances to form cholic acid. • Synthesis of VLDLs

16. Bile metabolism & enterohepatic circulation • Bile salts are end products of cholesterol synthesis • Daily production – 600- 800 ml/d • Functions- - activate lipase - promote micelle formation - intestinal uptake of fat soluble vitamins, cholesterol & lipids - facilitate excretion of xenobiotics, lipophillic substances, bilirubin, amphipathic steroid hormone derivative

17. Bile salts undergo enterohepatic circulation (20-30 times/day) intrahepatic bile duct ↓ common hepatic duct cystic duct CBD ↓ ↓ gall bladdersmall intestine ( terminal ileum) • Clinical implication • Opioids can induce spasm of bile duct & spinter of oddi • Reversed by – glucagon, opioid antagonists ( naloxone), smooth muscle relaxant (NTG), antimuscarinic drugs( atropine), volatile anaesthetics.

18. Protein and amino acid metabolism • Deamination of amino acids • Required before they can be used for energy or before they can be converted into carbohydrates or fats • Formation of urea for removal of ammonia from the body fluids • Production of proteins and peptides.

19. Krebs- Hanseleit cycle Major pathway for removing NH3 & other nitrogenous wastes from body Captures nitrogen in form of urea. Failing liver- BUN remain low - ammonia accumulates in liver ↓ Hepatic encephalopathy

20. Proteins & peptides Albumin • Most abundant protein • Normal plasma conc- 3 - 5 g% • Daily production -12-15 g/d • Plasma half life – 15-20 days • Functions – • maintains plasma oncotic pressure (80% by albumin) • binds ions, bilirubin, hormones & drugs • Hypoalbuminemia – Colloid oncotic pressure  edema

21. ᾳ- feto protein • Resembles albumin genetically & functionally • Formation sites- yolksac, hepatocytes, enterocytes • Fetal & neonatal life- major determinant of plasma oncotic pressure • 1 year of age- albumin largely replaces AFP • ↑ ↑ AFP- HCC

22. Fibrinogen • Synthesized exclusively by hepatocytes • Plasma fibrinogen – 100-700 mg/dl • Functions – polymerizes into long fibrin threads by the action of thrombin  formation of clot

23. Haptoglobins • Forms stable complexes with free Hb  prevents loss of iron through urinary excretion, protects kidney from damage • Ceruloplasmin – binds with copper and helps in its transport and storage • Wilson’s disease • Deficiency of ceruloplasmin  free Cu2+ in circulation  deposited in brain and liver

24. Coagulation • Synthesize most of the procoagulants except- • factor III ( tissue thromboplastin) • Factor IV ( calcium) • Factor VIII ( von Willebrand factor ) • Produce protein regulators of coagulation & fibrinolytic pathways • Protein C, protein S ( protein C – inactivate F VIIIa- Va complex) • protein Z ( degradation of Factor Xa ) • plasminogen activator inhibitor (PAI) ( inhibits tissue plasminogen activators to convert plasminogen to plasmin ) • antithrombin III

25. Liver as a Storage Organ • Vitamin A • Important role in the uptake, storage and maintenance of vitamin A levels by mobilizing its vitamin A store • Vitamin K • Vitamin K dependent factors II, VII, IX, X • Absorption of Vit K depends on normal fat absorption: any mal-absorption of lipid  vitamin K deficiency • Storage in liver- limited  hypoprothrombinemia can develop within a few weeks. • Treatment – • FFP • Antidote- parenteral vit K

26. Vitamin K cofactor & ỳ- carboxylation • Factor II, VII., IX, X , protein C & S- undergo Vit K dependent post translational modifications • Enables procoagulants to form complexes with calcium or other divalent cations for participation in the clotting cascade. • Clinical implication Warfarin inhibits vit K epoxide reductase ↓ traps Vit K in epoxide form ↓ Inhibits y- carboxylation • T/T- Enteral / parenteral Vit K. shuts

27. Storage & Homeostasis of Iron • Major site of synthesis of proteins (Transferrin, Ferritin) involved in iron transport & metabolism. • Heme metabolism Clinical implication • Porphyrias • Acute Intermittent Porphyria – commonest • Defects in the heme pathway- accumulation of porphyrinogens • Trigger substances- barbiturates, sex hormones, glucocorticoides, cigarette smoke, CYP inducers.

28. Bilirubin Metabolism

29. Bilirubin metabolism • Main source of bilirubin is heme metabolism • Daily production- 300mg • 80 % derived from senescent erythrocytes by macrophages in RE system. • Heme ↓ ( heme oxygenase + o2 ) biliverdin IX + CO + free divalent iron ↓ ( biliverdin reductase) bilirubin

30. Plasma Fragile RBCs BILIRUBIN METABOLISM RE System unconjugated bilirubin (protein – bound) Liver Urobilinogen Kidneys Liver Conjugated bilirubin Absorbed Urobilinogen Urobilin Urine Bacterial action Oxidation Urobilinogen Stercobilinogen Stercobilin Intestinal Contents Oxidation

31. CO produced has many physiological roles • Vasodilation ( regulation of vascular tone) • Platelet aggregation • Vascular myocyte proliferation • Neurotransmitter release • Cytoprotective , antiapoptotic, antioxidant effects • Biliverdin – confers protection from oxidative effects • rapidly converts to bilirubin

32. Metabolism of Drugs (Xenobiotics) • Phase-I reactions • Alter the parent drug by inserting or unmasking a polar group • Converts drugs to more polar compounds • Reactions – oxidations, reduction, hydrolysis • Cytochrome P450 – substrate binding site, located in the endoplasmic reticulum • Drugs– barbiturates, benzodiazepines, halogenated volatile anaesthetics, pethidine etc.

33. Phase-II reactions • Creates conjugates of parent compound or its metabolite with endogenous hydrophilic substrate • Reactions • Glucoronidation • Sulphation • Methylation • Acetylation • Glucoronidation • Most common type • Hepatic microsomal enzyme, UDPglucuronyl transferase mediates the transfer of glucoronic acid from UDP glucuronic acid to the functional group on the xenobiotics

34. Drug handled by phase-II – morphine, propofol, thiopentone (initially oxidized subsequently conjugated) • Phase-I reaction enzymes – more susceptible to destruction in cirrhosis • Phase-II reactions enzymes – more resistant, function even in advanced liver disease • Phase-III reactions • Involves ATP-binding cassette transport proteins (ABC) • These proteins use the energy of ATP hydrolysis to drive molecular transport • Dysfunction of ABC proteins hinders flow of bile  predisposing to drug accumulation and cholestatic liver injury

35. Microsomal enzyme induction • Anticonvulsants, rifampicin, isoniazid, glucocorticoids, chronic alcohol consumption Consequences of enzyme induction •  duration of action of drugs that are inactivated by metabolism •  intensity of action of drugs that are activated by metabolism

36. Endocrine functions • Liver can modify or amplify hormone action • Metabolic conversion of Vitamin D to form 25(OH)D • 25(OH)D  1,25(OH)2D in kidney • Peripheral conversion of T4 to T3 • Pseudocholinesterase • Hydrolysis of succinylcholine • Plasma t½ - 14 days • Severe liver disease   duration of action of succinylcholine

37. Insulin-like growth factors or somatomedins – growth hormone like action • Important role in cartilage function by promoting uptake of sulphate and synthesis of collagen • Removes circulating hormones • Insulin, glucagon, growth hormone, gastrointestinal hormones, e.g. gastrin

38. Blood reservoir • Liver is an expandable organ • 10 -15 % of total blood volume can be sequestered and quickly released after sympathetic stimulation . • Immune & inflammatory responses • kuffer cells protect against foreign intrutions, degrade toxins, process antigens, and phagocytose bacteria. • Induce & intensify inflammation by recruiting neutrophils • Release proinflammatory mediators

39. Liver Function Tests • Uses • To detect the presence of liver disease • To distinguish among different type of liver disorders • To guage the extent of known liver damage • To follow the response to treatment

40. Classification of LFTs • Tests based on detoxification and excretory functions • Serum bilirubin • Breakdown product of porphyrin ring of heme containing proteins • 2 fractions - conjugated (direct – 30%) unconjugated (indirect – 70%) • Normal total serum bilirubin 1 mg/dl •  in unconjugated fraction is rarely due to liver disease

41. Fractionate bilirubin >15% direct <15% direct Dubin Johnson syndr Rotor’s syndr Evaluation for hemolysis -ve +ve Crigler-Najjar syndr Gilbert’s syndr Hemolysis No further evaluation required

42. Urine bilirubin • Any bilirubin found in urine is conjugated, therefore bilrubinuria implies presence of liver disease • Blood ammonia • Detection of encephalopathy, monitoring hepatic synthetic function • Very poor predictor: presence/ degree of acute encephalopathy • Serum enzymes • No known function in serum • ed level-  rate of entrance into serum from damaged liver cells

43. Enzymes categories • Enzymes that reflect damage to hepatocytes • Enzymes that reflect cholestasis • Enzymes that reflect damage to hepatocytes • Aminotransferases • Aspartate aminotransferase (AST or SGOT): Liver, cardiac muscle, skeletal muscle, kidneys, brain, pancreas, etc. • Alanine aminotransferase (ALT or SGPT): 1° in liver • Sensitive indicators of liver cell injury • Normal levels <35-45 IU/L

44.  in aminotransferases • Mild - <250 IU/l • Any pathologic process that causes hepatocellular injury, e.g. hepatic steatosis, alcohol or drug induced liver disease, chronic viral hepatitis, cirrhosis, hemachromatosis • Moderate – 250-1000 IU/l • Disorder that produces hepatocellular necrosis • e.g. Acute viral hepatitis, drug induced hepatitis, exacerbation of chronic hepatitis (alcoholic) • Large - >1000 IU/l • Viral or drug induced liver damage superimposed on ALD, autoimmune hepatitis

45. Extreme - >2000 IU/l • Massive hepatic necrosis, usually from drugs (acetaminophan), halothane hepatitis, toxins, ischemic hepatitis (shock liver), acute viral hepatitis • AST/ALT ratio – DERITIS QUOTIENT • Normal - 1 or slightly > 1 • <1 – non-alcoholic steatosis or hepatitis without cirrhosis • 2-4 – ALD • >4 – Wilsonian hepatitis AST/ ALT not ↑ in purely obstructive disorder except Acute biliary obstruction caused by passage of gallstones to CBD

46. LDH • Normal level -25-100 IU/L • Massive but transient - Ischemic hepatitis • Massive, sustained - Malignant infiltration of liver • Other causes of  LDH • Hemolysis • Renal infarction • Acute stroke • Myocardial damage • Skeletal muscle injury

47. Glutathione S transferase • Relatively sensitive and specific test for detecting drug-induced hepatocellular injury • Plasma t½ 90 min, rapidly released into the circulation following hepatocellular injury • Plasma GST ( isoenzyme B ) – reveal time course of hepatocellular injury from onset to resolution • GST – located in the centrilobular region (zone 3), where hepatocytes are most susceptible to injuries from hypoxia and reactive drug metabolism

48. Bromosulphathein excretion test • BSP dye- same mechanism as bilirubin -binding -conjugation -excretion • BSP – i/v – 45 mins- levels in venous blood • Normally- <5%. • Slightly higher in old age • Sensitive test to detect mild impairement of liver

49. Enzymes reflecting cholestasis • Alkaline phosphatase- present in cells of the bile duct • Isoenzymes- bone , liver, intestine, placenta , kidney , leukocytes. • Normal levels- 42-122 IU/L - 3-13 KA units/dl

50.  in serum ALP in an apparently healthy pt. • Fractionate the ALP to identify source of isoenzyme • ALP from different tissues differ in susceptibility inactivation by heat • Measure - 5' NT, GGT