Bilirubin

Article By:

Schwartz, Morton K. Department of Clinical Chemistry, Memorial Sloan-Kettering Cancer Center, New York, New York.

Last reviewed:May 2021

DOI:https://doi.org/10.1036/1097-8542.081100

The predominant orange pigment of bile. Bilirubin (see illustration) is the major metabolic breakdown product of heme, which is the prosthetic group of hemoglobin in red blood cells, and other chromoproteins, such as myoglobin, cytochrome, and catalase. The breakdown of hemoglobin from old red blood cells takes place at a rapid rate in the reticuloendothelial cells of the liver, spleen, and bone marrow. The steps in this breakdown process include denaturation and removal of the protein globin, oxidation and opening of the tetrapyrrole ring, and removal of iron to form the green pigment biliverdin, which is then reduced to bilirubin by the addition of hydrogen. The formed bilirubin is transported to the liver, probably bound to albumin (a plasma protein produced by the liver), where it is conjugated into water-soluble monoglucuronides and diglucuronides, and to a lesser extent with sulfate. See also: Albumin; Hemoglobin; Liver; Pigmentation

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