The document summarizes heme catabolism and bilirubin metabolism. It discusses the three stages of heme degradation: formation of bilirubin in the reticuloendothelial system, uptake and conjugation of bilirubin in the liver, and catabolism of bilirubin in the gut. It also describes different types of hyperbilirubinemia and jaundice, including physiological, genetic causes like Gilbert's syndrome, and obstructive jaundice.

1. Red blood cell destruction

2. Recommended Readings Wills’ Biochemical basis of Medicine. Chapter 25 Lippincott’s illustrated reviews biochemistry. Chapter 21

3. Learning objectives Know the main location and site of haem breakdown Outline the pathway for haem degradation Compare/contrast types of Plasma Bilirubin & types of Hyperbilirubinemia Understand the biochemical basis of different types of jaundice.

4. Content Main location and site of haem breakdown Pathway for haem degradation Formation of bilirubin in reticuloendothelial system Uptake, conjugation & secretion of bilirubin by liver Catabolism of bilirubin in the gut Types of Plasma Bilirubin Definition & types of Hyperbilirubinemia Jaundice Definition & Types & characteristics of each type Physiological Jaundice Genetic causes of Jaundice

5. Degradation of heme Red blood cells are degraded by the cells of the reticuloendothelial system particularly in the liver and spleen after 120 days in the circulation (RBC life span) Therefore, The principal sites of heme catabolism are the spleen & the liver Bilirubin is the end product of heme catabolism: 75% from hemoglobin of old RBCs, the rest from hemoglobin of immature RBCs & from cytochromes from extra-erythroid tissues 250-350 mg bilirubin /day is produced in normal adults

6. Heme catabolism is classified into 3 stages: Formation of bilirubin in reticuloendothelial system (RES) Uptake, conjugation and secretion of bilirubin by the liver Catabolism of bilirubin in the gut Pathway for haem degradation

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10. Formation of bilirubin in reticuloendothelial system (RES): In reticuloendothelial system (RES), hemoglobin is degraded into heme and globin. Globin is either degraded to amino acids or reused for synthesis of hemoglobin. Heme is converted to biliverdin by the microsomal heme oxygenase (a P450 cytochrom) using NADPH and O 2 . Ferric iron & carbon monoxide are released. Biliverdin is water soluble green pigment Biliverdin is converted to bilirubin by biliverdin reductase. Bilirubin is water insoluble yellow pigment Bilirubin is transported in blood to the liver carried by plasma albumin (it is termed indirect or unconjugated bilirubin). Certain drugs as salicylates & sulfonamides can displace bilirubin from albumin, allowing bilirubin to enter the CNS causing neural damage in infants

11. Uptake, conjugation and secretion of bilirubin by the liver: Uptake of bilirubin: Bilirubin leaves albumin & is taken by the hepatocytes Hepatic uptake of bilirubin is mediated by a carrier, & may be competitively inhibited by other organic anions Synthesis of bilirubin-diglucuronide (Conjugation): By UDP glucuronyl transferase enzyme, bilirubin is conjugated with glucuronic acid, to form bilirubin diglucuronide (it is termed direct or conjugated bilirubin & it is water soluble) UDP-glucuronate acts as donor for glucuronate group to form bilirubin monoglucuronide then bilirubin diglucuronide Conjugated bilirubin is actively secreted by hepatocyte into the biliary canaliculi (with bile) to small intestine Secreation is the rate limiting step in bilirubin metabolism

12. Catabolism of bilirubin in the gut: Conjugated bilirubin is converted by bacteria to urobilinogen : Conjugated bilirubin is deconjugated and reduced by intestinal bacteria forming urobilinogen. Most of urobilinogen is oxidized in colon to a brown pigment, urobilin (stercobilin ) , which is excreted in feces giving feces its color. Some urobilinogen (water soluble) is absorbed from gut into portal blood and re-excreted by liver in bile (enterohepatic circulation). Traces of this urobilinogen reach the systemic blood & are excreated by kidneys in urine

14. Plasma Bilirubin Normal plasma bilirubin level is < 1 mg/dL (<17 μ mol/L) Plasma bilirubin is present in 2 forms: Unconjugated bilirubin (Indirect bilirubin) Conjugated bilirubin (Direct bilirubin)

15. Unconjugated bilirubin (Indirect bilirubin) Main bilirubin present in plasma normally Water insoluble Transported in blood as Bilirubin-albumin complex Not excreted in urine Increases in blood in cases of: Hemolysis Liver disease: liver fails to uptake or conjugate it. Termed Indirect bilirubin because its estimation by van den Bergh reaction needs addition of methanol to react with the diazo reagent

16. Conjugated bilirubin (direct bilirubin) Bilirubin-diglucuronide which escapes from the liver to the systemic blood Water-soluble Excreted in urine: Bilirubinuria is due to conjugated bilirubin and is always pathological Increases in blood in cases of: Liver disease: liver fails to secrete bilirubin Obstruction in the biliary system Termed direct bilirubin because it reacts directly with the diazo reagent of van den Bergh

17. Hyperbilirubinemia Increased plasma bilirubin level more than 1mg/dL (17 μ mol/L) According to the type of bilirubin increased in plasma, hyperbilirubinemias are classified into 2 types: Unconjugated hyperbilirubinemia Conjugated hyperbilirubinemia According to the underlying defect, hyperbilirubinemias may be: Unconjugated , Conjugated Both

18. Unconjugated hyperbilirubinemia High level of unconjugated bilirubin in blood. Unconjugated bilirubin can cross the blood-brain barrier & cause severe brain damage (kernicterus) Unconjugated bilirubin is not excreted in urine Causes: Hemolysis Immaturity of the enzyme of bilirubin conjugation in neonates (physiological) Genetic defect in the enzyme of bilirubin conjugation Gilbert's syndrome Crigler-Najjar syndrome Most common causes are Hemolysis & Gilbert's syndrome

19. Conjugated hyperbilirubinemia High level of conjugated bilirubin in blood. Due to leakage of conjugated bilirubin from hepatocytes or biliary system into blood Conjugated bilirubin is water soluble, so it is excreted in urine and darkens urine color to deep orange brown Causes: Biliarry obstruction Decreased hepatic secretion of conjugated bilirubin Dubin-Johnson Syndrome Unknown cause Rotor’s Syndrome. Most common cause is biliary obstruction

20. Jaundice Jaundice is clinically seen as yellow color of skin, nail beds & sclera (due to deposition of bilirubin) when plasma bilirubin concentration exceeds 3 mg/dL (50 μmol/L) due to imbalance between bilirubin production and excretion. Types of jaundice are: Prehepatic (hemolytic) Hepatic Posthepatic (obstructive)

21. Prehepatic (hemolytic) jaundice: Due to excess production of unconjugated bilirubin after hemolysis which exceeds the capacity of liver to conjugate it Characterized by the following: High levels of indirect (unconjugated) bilirubin in plasma i.e., Unconjugated hyperbilirubinemia Dark urine caused by high levels of urobilinogen in urine Dark stool caused by high levels of Fecal urobilin

22. Posthepatic (obstructive) jaundice Due to biliary obstruction Characterized by the following: High levels of direct (conjugated) bilirubin in plasma i.e., conjugated hyperbilirubinemia Dark urine due to presence of conjugated bilirubin. Urine urobilinogen is absent Very pale stool (white, clay) due to absence of Fecal urobilin

23. Hepatic jaundice: Due to hepatocyte dysfunction that cause impaired hepatic bilirubin uptake, conjugation, or secretion: impaired uptake -> increased unconjugated bilirubin in blood), impaired conjugation -> decreased conjugated bilirubin formation in liver-> decreased urobilinogen formation in gut) impaired secretion->increased conjugated bilirubin in blood & secretion in urine) Characterized by the following: High levels of direct (conjugated) & indirect (unconjugated) bilirubin in plasma i.e., conjugated & Unconjugated hyperbilirubinemia Dark urine due to the presence of conjugated bilirubin & urobilinogen Pale stool due to decreased Fecal urobilin

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25. Neonatal (physiologic) jaundice: Transient jaundice, common in neonates (50% of normal babies) particularly in premature infants Due to immaturity of the enzymes of bilirubin conjugation leading to unconjugated hyperbilirubinemia Unconjugated bilirubin is toxic to the immature brain, it may cause kernicterus if it exceeds 20-25 mg/dl, kernicterus may result in mental retardation. Treatment: to avoid kernicterus, if plasma bilirubin is too high: Phenobarbital: inducer for UDP-glucuronyl transferase Phototherapy: converts bilirubin to more soluble derivatives that are easily excreted in bile (detoxifies bilirubin) Exchange blood transfusion to remove excess bilirubin

26. Neonatal jaundice phototherapy

27. Genetic causes of jaundice : Gilbert's syndrome Crigler-Najjar syndrome Dubin-Johnson syndrome Rotor's syndrome

28. Gilbert's syndrome : Affect up to 5% of population Due to decreased conjugation of bilirubin & decreased uptake in some cases Decreased conjugation of bilirubin is due to moderate deficiency in UDP glucuronyl transferase activity Mild intermittent unconjugated hyperbilirubinemia which is noticed after fasting or infection Harmless & asymptomatic -> normal lifespan

29. Crigler-Najjar syndrome: Type 1: Severe unconjugated hyperbilirubinemia at birth Due to absence of conjugating enzymes Fatal due to kernicterus Partial response to phototherapy, non to Phenobarbital Type 2: Severe unconjugated hyperbilirubinemia at birth Due to partial defect of conjugating enzymes Survive to adulthood Good response to phototherapy & Phenobarbital

30. Dubin-Johnson syndrome: Due to decreased hepatic secretion of conjugated bilirubin Mild intermittent conjugated hyperbilirubinemia Bilirubinuria Normal lifespan Rotor's syndrome: Cause is unknown Mild intermittent conjugated hyperbilirubinemia Bilirubinuria Normal lifespan