Bilirubin Metabolism: An Overview

Bilirubin is a yellow pigment produced from the breakdown of hemoglobin in red blood cells. It undergoes a complex process of metabolism, transport, and excretion.

Production of Bilirubin

When red blood cells reach the end of their lifespan, hemoglobin is broken down into heme, which is converted into biliverdin. Biliverdin is then reduced to bilirubin.

Transport of Bilirubin

Unconjugated bilirubin binds to albumin and is transported to the liver. Since it is water-insoluble, it requires conjugation for proper excretion.

Conjugation in the Liver

In the liver, unconjugated bilirubin undergoes conjugation with glucuronic acid by the enzyme UDP-glucuronosyltransferase, making it water-soluble.

Excretion of Bilirubin

Conjugated bilirubin is excreted into bile and ultimately eliminated via feces. Some bilirubin is converted into urobilinogen, which is partially reabsorbed or excreted via urine.

Jaundice Etiology and Pathogenesis

• Pre-hepatic jaundice: Excessive hemolysis leads to an increased bilirubin load.
• Hepatic jaundice: Dysfunctional liver metabolism causes impaired bilirubin conjugation or transport.
• Post-hepatic jaundice: Obstruction in bile flow results in accumulation of conjugated bilirubin.

Differentiating Conjugated and Unconjugated Hyperbilirubinemia

Conjugated bilirubin is water-soluble and accumulates in cholestatic conditions, whereas unconjugated bilirubin is seen in hemolytic disorders or liver enzyme deficiencies.