Bilirubin metabolism
The pathway
- Haem (80% from senescent RBC, 20% from ineffective erythropoiesis and other haemoproteins)
- -> haem oxygenase -> biliverdin -> biliverdin reductase -> unconjugated bilirubin
- Unconjugated bilirubin is lipid-soluble, albumin-bound, water-insoluble
- -> never appears in urine; can cross the blood-brain barrier -> kernicterus
- Hepatocyte uptake (OATP)
- -> UDP-glucuronosyltransferase (UGT1A1) conjugates with glucuronic acid
- -> conjugated bilirubin - water-soluble
- -> appears in urine (dark urine); cannot cross the BBB
- -> MRP2 exports into bile
- -> colonic bacteria -> urobilinogen -> stercobilin (brown stool); a fraction reabsorbed -> urine urobilinogen
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