Bilirubin is the degradation product of -

Bilirubin metabolism Bilirubin is the end product of heme degradation. The heme is derived from - Senescent erythrocytes by the mononuclear phagocytic system in the spleen, liver and bone marrow (major source). Turnover of hemoproteins (e.g. cytochrome p.450). Heme is oxidized to biliverdin by heme oxygenase. Biliverdin is then reduced to bilirubin by biliverdin reductase. Bilirubin is transported to the liver in bound form with albumin. There is the carrier-mediated uptake of bilirubin in the liver. This bilirubin is conjugated with glucuronic acid by UDP glucuronosyl transferase (UGT1A1) to from conjugated bilirubin (bilirubin glucuronides). Conjugated bilirubin is excreted into bile. Most of the conjugated bilirubin is deconjugated and degraded to urobilinogen. The most of the urobilinogen is excreted in the faeces. Approximately 20% of the urobilinogen is reabsorbed in the ileum and colon and is returned to the liver, and promptly excreted into bile → Enterohepatic circulation. The small amount that escapes this enterohepatic circulation is excreted in urine.