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5.o Heme is further broken down into biliverdin by the enzyme heme oxygenase and then converted to unconjugated (indirect) bilirubin by biliverdin reductase.o In the liver, the enzyme UDP-glucuronosyltransferase (UGT) conjugates bilirubin with glucuronic acid, making it water-soluble (conjugated bilirubin).o Conjugated bilirubin is excreted into bile and transported to the intestine, where gut bacteria convert it into urobilinogen.

5. Pathway of Bilirubin Formation and Conjugation:
   • Bilirubin is formed from the breakdown of hemoglobin in aged red blood cells (RBCs). Macrophages in the spleen, liver, and bone marrow break down hemoglobin into heme and globin.
   • Heme is further broken down into biliverdin by the enzyme heme oxygenase and then converted to unconjugated (indirect) bilirubin by biliverdin reductase.
   • Unconjugated bilirubin is lipid-soluble and binds to albumin for transport to the liver.
   • In the liver, the enzyme UDP-glucuronosyltransferase (UGT) conjugates bilirubin with glucuronic acid, making it water-soluble (conjugated bilirubin).
   • Conjugated bilirubin is excreted into bile and transported to the intestine, where gut bacteria convert it into urobilinogen. Some urobilinogen is reabsorbed and excreted in urine, while the rest is converted into stercobilin, which gives stool its brown color.