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The predominant orange pigment of bile. It is the major metabolic breakdown product of heme, the prosthetic group of hemoglobin in red blood cells, and other chromoproteins such as myoglobin, cytochrome, and catalase. The breakdown of hemoglobin from the old red 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 the 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, where it is conjugated into water-soluble mono- and diglucuronides and to a lesser extent with sulfate. See Liver
In mammalian bile essentially all of the bilirubin is present as a glucuronide conjugate. Bilirubin glucuronide is passed through the liver cells into the bile caniculi and then into the intestine. The bacterial flora further reduces the bilirubin to colorless urobilinogen. Most of the urobilinogen is either reduced to stercobilinogen or oxidized to urobilin. These two compounds are then converted to stercobilin, which is excreted in the feces and gives the stool its brown color. See Hemoglobin
C33H36O6N4, a bile pigment; molecular mass 584.68. Brown crystals. Bilirubin is an intermediate product of the decomposition of hemoglobin that takes place in the macrophages of the spleen, liver, and bone marrow. It is formed by the enzymatic reduction of biliverdin. It is present in small quantities in the plasma of vertebrate animals and man (0.2–1.4 mg percent in a healthy person). When the outflow of bile is made difficult (obstruction of the bile ducts), and in some liver diseases, the bilirubin concentration increases in the blood (causing jaundice), and it appears in the urine. Hence, the presence of bilirubin in blood or urine is a diagnostic test.