Bilirubin metabolism and jaundice

Objectives Bilirubin metabolism and pathophysiological mechanism. Bilirubin uptake and transport. Jaundice condition and types.

Pathophysiological importance of bilirubin metabolism It is the end product of heme degradation. Serum bilirubin level is an important clinical marker of hepatobiliary excretory function. Hepatic uptake, storage, conjugation and excretion of bilirubin are finely balanced. Therefore, enhancement of bilirubin throughput requires coordinated induction of multiple genes, which may be mediated by nuclear receptors.

Sources of bilirubin Erythroid Non-erythroid (80%) (20%) Normal: (80%) (20%) Normal: Senescent erythrocytes Free heme Abnormal: Hemolysis: Extravascular Intravascular Ineffective erythropoiesis Cytochromes Catalase Peroxidase Tryptophane pyrrolase Myoglobin

Opening of the heme ring and Enzyme-catalyzed formation of bilirubin

The linear structure of bilirubin: Two dipyrroles joined by a central methene bridge N H M V O CH2 OH OH O C C CH2 V CH2 M M N N CH2 H H O

Bilirubin contains several polar groups (shown in red): Yet, it is insoluble in water. O O OH OH C C CH2 CH2 V V M M CH2 CH2 M M N N N N O H H CH2 H H O

Conjugation with glucuronic acid makes bilirubin water soluble

The internal hydrogen bonds of bilirubin are disrupted by conjugation of the propionic acid carboxyl group with glucuronic acid CH2 C V M M OH CH2 O H H O N N N N O H H CH2 OH O CH2 M M CH2 V C

The internal hydrogen bonds of bilirubin are disrupted by conjugation of the propionic acid carboxyl group with glucuronic acid CH2 -GlucA C O V M M CH2 H H O N N N N O H H CH2 CH2 M M CH2 V GlucA- O C

Phototherapy changes the configuration of bilirubin making it transiently water soluble

Bilirubin throughput: schema of a hepatocyte Tight junction Liver sinusoid Fenestrated endothelium Sinusoidal surface Canalicular surface

B alb Bilirubin circulates bound to serum albumin. Albumin- binding: Keeps bilirubin soluble Prevents tissue deposi- tion. Prevents renal excretion Drugs that displace bilirubin from albumin may precipitate kernicterus: Sulfonamides Coumadin, etc. B alb

B alb Bilirubin circulates bound to serum albumin. At the sinusoidal surface of hepatocytes, it dissociates from albumin. B alb

B alb Bilirubin circulates bound to serum albumin. At the sinusoidal surface of hepatocytes, it dissociates from albumin. B alb

B alb Bilirubin circulates bound to serum albumin. At the sinusoidal surface of hepatocytes, it dissociates from albumin. alb B

B alb Bilirubin circulates bound to serum albumin. At the sinusoidal surface of hepatocytes, it dissociates from albumin. alb B

Bilirubin enters through the sinusoidal surface, probably by facilitated diffusion. Uptake is energy independent and bidirectional. Bilirubin uptake is reduced: In neonates In cirrhosis From drug effect: novobiocin In some cases of Gilbert syndrome B B

What is the mechanism of facilitated diffusion of bilirubin? Zucker has proposed that no transporter protein is needed. In a recent report, organic anion transport protein 2 (oatp2) has been implicated in bilirubin uptake. However, oatp2 transports organic anions, such as BSP, it is not sufficient for bilirubin transport.

Inside the hepatocyte, bilirubin binds to cytosolic proteins termed ligandins, which are the same as glutathione-S- transferases (GSTs). GST binding inhibits the efflux of bilirubin, thereby increasing its net uptake GSTs B B

GSTs B B

Conjugation of bilirubin with glucuronic acid is catalyzed by UGT1A1, which transfers glucuronic acid from UDP-glucuronic acid to bilirubin Conjugation with glucuronic acid makes bilirubin water-soluble and non-toxic. Glucuronidation is essential for biliary excretion of bilirubin. GSTs UDP UDPGA B B B GA UGT1A1

UDP-glucuronosyltransferases (UGTs) UDPGA UDP Substrate Glucuronide UGT UGTs are ER proteins that convert many internal and exogenous toxins to non-toxic metabolites. UGT’s are a family of enzymes concentrated in the liver. One UGT isoform, UGT1A1, conjugates bilirubin and is essential for its excretion. Inherited UGT1A1 deficiency causes jaundice.

Jaundice: It is a medical term describes the elevation of bilirubin in blood result in yellow color of skin and sclera. Other symptoms include nausea, vomiting, dark- colored urine and Types of Jaundice: fatigue. according to the cause of jaundice it is classified to three main types: Pre-hepatic jaundice Hepatic jaundice Post-hepatic (most common type)

Post-hepatic jaundice hemolytic jaundice hepatocellular jaundice obstructive jaundice Post-hepatic jaundice Hepatic jaundice Pre-hepatic jaundice Due to obstruction of bile duct which prevents passage of bilirubin into intestine. D.Bil will back to liver and then to circulation elevating its level in blood and urine. Occur in: Biliary stricture Cancer of the pancreas or gallbladder Gallstones Due to liver cell damage (cancer, cirrhosis or hepatitis) Conjugation of bilirubin decreased (ID.Bil. ). Blilirubin that is conjugated is not efficiently secreted into bile but leaks to blood (D.Bil. ) Occur in : Cirrhosis (scarring of the liver) Hepatitis Gilbert's disease Due to increase in RBCs breakdown due to hemolytic anemia. The rate of RBCs lysis and bilirubin production more than ability of liver to convert it to the conjugated form Erythroblastosis fetalis Hemolytic anemia Transfusion reaction Causes D.Bil (High) D.Bil, ID.Bil, T.Bil all (High) ID.Bil > D.Bil Type of Bil. ALP ( High) ALT, AST (High) K+ ( High) Hematology: CBC (low Hb) Conformational test

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