Dr Vivek Joshi, MD

Heme catabolism  Commonly occurs in liver and spleen  Done by reticuloendothelial cells  Most of the heme for degradation comes from senescent/old RBCs  Rest comes from immature RBC & cytochromes from extraerythroid tissues Hemoglobin in RBCs GlobinHeme Amino acids IronPorphyrin May be reused Degraded

Steps of Heme Catabolic pathway 3  Formation of Indirect bilirubin  Release of Indirect Bilirubin in the Blood  Uptake of Indirect bilirubin by the liver parenchymal cells  Conjugation of bilirubin in the liver forming bilirubin diglucuronide  Secretion of conjugated bilirubin into the bile  Formation of urobilinogen and stercobilinogen in the intestine

 Reticuloendothelial system  Carried out by the heme oxygenase system  Substrate inducible  Located near the electron transport chain  Requires oxygen and NADPH  Produces biliverdin, CO, iron, NADP+ Green + + Red purple Formation of Indirect bilirubin

 Biliverdin reductase acts on biliverdin(green color) converting it into bilirubin(yellowish orange color)  Changing color of the bruise- intermediates.  This bilirubin formed is unconjugated- indirect bilirubin Yellow orange Formation of Indirect bilirubin

 Requires Albumin for transport  Transports bilirubin to the liver from RES  Has 2 binding sites for bilirubin  High-affinity site Can accommodate about 25 mg of bilirubin per 100 ml plasma  Low-affinity site Binds bilirubin loosely when exceeds 25 mg/100 ml in plasma Bilirubin can easily be displaced by anionic drugs(Salicylates and sulfonamides) and diffuse into the CNS- Kernicterus in the newborn Release of Indirect Bilirubin in the Blood

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Uptake of Indirect bilirubin by liver parenchymal cells  Occurs at the sinusoidal surface of the hepatocytes  Occurs via a facilitated transport system with a very large capacity  Allows the equilibrium of bilirubin across the sinusoidal membrane of the hepatocyte  Dependent upon removal of bilirubin by subsequent metabolic pathways Gilberts syndrome # Inherited Unconjugated hyperbilirubinemia-AD # Inherited Unconjugated hyperbilirubinemia-AD # Defective uptake of bilirubin -No Conjugation of bilirubin # Defective uptake of bilirubin -No Conjugation of bilirubin # Mild jaundice # Mild jaundice

 Addition of glucuronic acid to bilirubin converts the non-polar bilirubin to more water-soluble derivatives  Catalyzed by microsomal bilirubin glucuronyltransferase using UDP- glucuronic acid as glucuronate donor  Bilirubin glucuronyltransferase (Can be induced by phenobarbital) Conjugation of bilirubin in the liver forming bilirubin diglucuronide

Crigler Najjar- ICrigler Najjar -II Complete UDP Glucuronyl transferase deficiency Partial UDP Glucuronyl transferase deficiency Poor survivalCompatible Inherited Unconjugated hyperbilirubinemia

Secretion into the bile  Occurs by an active transport mechanism  Rate-limiting for the entire process of hepatic bilirubin metabolism  Inducible by the same drugs that are capable of inducing the conjugation of bilirubin  Conjugation and excretion system for bilirubin behave as a functional unit

Dubin –Johnson syndrome Rotors syndrome Defect in the excretion of bilirubin in the bile Black pigmented liverLiver-No pigmentation Inherited Conjugated hyperbilirubinemia

Bilirubin diglucuronide β glucuronidases Glucuronic acid Bilirubin Urobilinogen Oxidized to stercobilin Excreted in feces (250 mg/day ;Brown color to the stools) Reabsorbed and reexcreted to the liver( Enterohepatic urobilinogen cycle ) Excreted in urine as urobilin (1 mg/day) Formation of urobilinogen and stercobilinogen in the intestine Bilirubin is hydrolyzed and reduced by bacteria to yield urobilinogen.

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Bilirubin Conjugated Bilirubin Unconjugated Bilirubin Water solubleSoluble in methanol Can be Present in urineAbsent in urine Excreted in bileNot excreted in bile mg/dl mg/dl Total Bilirubin:, 1.0 mg/dl

Hyperbilirubinemia  When bilirubin in blood exceeds 1 mg/dL (17.1 µmol/L)- Hyperbilirubinemia  May be due to :  Production of more bilirubin than the normal liver can handle  Failure of a damaged liver to conjugate and excrete bilirubin produced in normal amounts  Obstruction to the excretory ducts of the liver  When Serum Bilirubin >2 mg/dl - Jaundice (Icterus)

Jaundice  Yellowish discoloration of skin,nail beds and sclera due to deposition of bilirubin  High Serum bilirubin levels measured by Van den Bergh reaction:  Conj.Bil combines with diazotized sulfanilic acid-Red azobilirubin (Direct positive-Conjugated Bil)  Reaction carried out with methanol- Total bilirubin  Unconjugated Bil –Obtained by substracting total from the conjugated

Hemolytic /Pre Hepatic Jaundice # Normal Capacity of the Liver to conjugate and excrete bilirubin (3000 mg/day) # Normal bilirubin production (300 mg/day) # Massive lysis of the red blood cells-Bilirubin produced faster than conjugated

Causes for unconjugated hyperbilirubinimia  # Massive hemolysis seen in:  Malaria  Sickle cell anemia  G6PD Deficiency  Pyruvate kinase deficiency  Hereditary spherocytosis  # Biochemical findings  Increased serum unconjugated bilirubin  Increased urine urobilinogen  Bilirubin absent in urine

 Damage to the liver cells leading to Decreased conjugation of bilirubin in the liver/Intra hepatic obstruction- Rise in unconjugated bilirubin Seen in Cirrhosis / Hepatitis Patient has anorexia and vomiting  Bilirubin when conjugated is not efficiently secreted (Intra hepatic obstruction) into the bile- Rise in conjugated bilirubin  Biochemical findings:  Increased levels of Unconjugated /conjugated bilirubin  Normal or Decreased urine urobilinogen  Bilirubin in urine - Dark colored urine (Intra hepatic obstruction)  Plasma levels of ALT and AST are elevated Hepatocellular Jaundice

Obstructive jaundice  Results from the obstruction of the bile duct-Blocks the passage of bilirubin into the intestine of bilirubin into the intestine  Seen in case of Hepatic tumor/Bile stones or stricture  Biochemical findings  Increased conjugated bilirubin in the blood  Complete obstruction of the bile duct -No urobilinogen in urine and stercobilinogen in faeces (Pale, clay colored Stools)  Bilirubin present in the urine (Liver “regurgitates” conjugated bilirubin into the blood which is excreted into the urine  Prolonged obstruction of bile duct can lead to liver damage and a subsequent rise in unconjugated bilirubin damage and a subsequent rise in unconjugated bilirubin

 Physiological Jaundice  2 nd -3 rd day of birth  Unconjugated hyperbilirubinemia seen in premature babies  Transient condition  Results from an accelerated hemolysis and an immature hepatic system for the uptake, conjugation, and secretion of bilirubin  UDP-glucuronyl transferase activity is low in newborns and specially in premature babies Jaundice of the newborn

 Pathological Jaundice  Rise in Unconjugated bilirubin concentration in plasma (>25 mg/dl)  Bilirubin is bound to low affinity binding site of albumin- Easily removed  Can penetrate the blood-brain barrier and cause kernicterus (Deposition of bilirubin in the brain leading to neurologic sequelae)  Seen in cases of Rh /ABO incompatibility/Hereditary spherocytosis Jaundice of the newborn

Treatment- Blue fluorescent light (phototherapy) which converts bilirubin to more polar, water-soluble isomers which can be excreted without conjugation to glucuronic acid

Jaundice of the newborn

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