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Evaluating Liver Test Abnormalities

Understanding the Pathophysiology of Liver Disease

for Health Care Providers

Bilirubin Metabolism

Graphic depicting the numbered areas described by the list below.
Zoomed in graphic depicting the number 4 and 5 area with the hepatocyte and conjugated bilirubin labeled.

  1. Heme, the substrate of bilirubin, is derived from red blood cells that have died.
  2. Heme is degraded to biliverdin by heme oxygenase in the mononuclear phagocytes.
  3. Biliverdin is subsequently reduced to bilirubin by biliverdin reductase.
  4. Circulating bilirubin (insoluble) is bound to albumin and subsequently taken up by the hepatocytes.
  5. To make it soluble, bilirubin undergoes conjugation, a reaction catalyzed by bilirubin UDPglucuronyl transferase (UDPG).
  6. Conjugated bilirubin (soluble) is excreted into bile and reaches the bowel.
  7. Bilirubin glucuronides are deconjugated by colonic bacteria and eliminated in the feces.
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Glossary

bilirubin UDP
Product of heme degradation.
glucuronyl transferase (UDPG)
A hepatic enzyme catalyzing conjugation of glucorunide to bilirubin. Deficiency of this enzyme leads to direct hyperbilirubinemia