How is bilirubin conjugated in the liver?

Bilirubin Conjugation in the Liver

Bilirubin is transported to the liver in its insoluble unconjugated form, where it is converted into soluble conjugated bilirubin by the enzyme UDP-glucuronosyltransferase 1A1 (UGT1A1) to enable excretion into bile. 1

Bilirubin Production and Transport to the Liver

• Bilirubin is predominantly a by-product of the breakdown of the heme component of hemoglobin by the reticuloendothelial system 1
• Unconjugated bilirubin is highly hydrophobic and insoluble in water, requiring transport in the bloodstream bound to albumin 1, 2
• Transport from blood into hepatocytes occurs through:
  • Membrane transporters including organic anion transporting polypeptides (OATPs) 3, 2
  • Possible passive diffusion across the sinusoidal membrane 3

Intrahepatic Transport and Conjugation Process

1. Uptake into hepatocytes:

   • Unconjugated bilirubin enters hepatocytes via membrane transporters and/or diffusion 3, 2
   • The exact transporters for unconjugated bilirubin remain incompletely characterized 2

2. Intracellular transport:

   • Once inside the hepatocyte, bilirubin is bound to ligandin (a cytosolic protein) 3
   • Ligandin carries bilirubin to the endoplasmic reticulum (ER) of hepatocytes 3

3. Conjugation reaction:

   • In the ER, bilirubin is conjugated by bilirubin UDP-glucuronosyltransferase (UGT1A1) 3, 4
   • UGT1A1 adds glucuronic acid molecules to bilirubin, forming:
     • Bilirubin monoglucuronide (one glucuronic acid molecule)
     • Bilirubin diglucuronide (two glucuronic acid molecules) 3, 5
   • Under initial rate conditions, monoglucuronide is the predominant species (~70%) 5

4. Excretion from hepatocytes:

   • Conjugated bilirubin (both mono- and diglucuronides) is transported across the canalicular membrane into bile by multidrug resistance-associated protein 2 (MRP2) 3
   • Under conditions of impaired biliary excretion, MRP3 located in the lateral membrane can transport bilirubin glucuronides back into the blood 3

Clinical Significance of Bilirubin Conjugation

• Defects in bilirubin conjugation lead to various clinical conditions:

  • Gilbert's syndrome: Mild unconjugated hyperbilirubinemia due to reduced UGT1A1 activity, often caused by a promoter polymorphism (additional TA repeat in the TATAA element) 4
  • Crigler-Najjar syndrome: Severe unconjugated hyperbilirubinemia due to complete (type 1) or partial (type 2) inactivation of UGT1A1 4

• Impaired conjugation can result in:

  • Elevated unconjugated bilirubin levels in blood
  • Jaundice (yellowing of skin and sclera)
  • In severe cases, kernicterus (bilirubin encephalopathy) 4

Important Considerations in Bilirubin Measurement

• Most laboratories report total bilirubin, comprising both unconjugated and conjugated fractions 1
• Direct-reacting bilirubin is not identical to conjugated bilirubin, though the terms are often used interchangeably in clinical practice 1
• In adults, the majority of measurable bilirubin should be conjugated, even in individuals with significant liver disease 1
• In neonates, physiological increase in unconjugated bilirubin may occur but can become pathological if high or prolonged 1

The conjugation of bilirubin is a critical process for eliminating this potentially toxic compound from the body. Defects in this pathway can lead to various clinical disorders ranging from mild (Gilbert's syndrome) to severe (Crigler-Najjar syndrome) hyperbilirubinemia.