Bilirubin is a water insoluble tetrapyrrole produced from the reduction of biliverdin in a reaction catalyzed by the enzyme bilirverdin reductase. In vivo it undergoes glucuronidation in the liver (addition of one or two glucuronic acids through a glycosidic bond) to form the water soluble mono or diglucuronide (also called bilirubin conjugate). The conjugate is excreted from the liver in bile or is converted to urobilinogen and excreted in the urine as urobilin or in the feces as stercobilin.
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