Bilirubin is a water insoluble tetrapyrrole produced from the reduction of biliverdin in a reaction catalyzed by the enzyme bilirverdin reductase.  Water insoluble bilirubin (also called indirect bilirubin) in vivo undergoes glucuronidation in the liver (addition of one or two glucuronic acids through a glycosidic bond) to form the water soluble bilirubin mono or diglucuronide (also called bilirubin conjugate or direct bilirubin).  Bilirubin 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.