Bilirubin is produced from biliverdin in catabolism of the heme of hemoglobin. These reactions, which occur in the spleen (Figure 21.31), yield bilirubin, which is quite insoluble in water, and must be removed by several organ systems. First, bilirubin complexes with serum albumin for transport to the liver. There, it is solubilized by conjugation with two molecules of glucuronic acid. The reaction is comparable to other glycosyltransferase reactions, with the substrate being UDP-glucuronate. This solubilized compound, bilirubin diglucuronide, is secreted into the bile and ultimately excreted via the intestine. Defects in metabolizing bilirubin properly give rise to jaundice.

1. Porphyrin and Chlorophyll Metabolism

2. Photosynthetic Pigments