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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

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



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.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


C33H36N4O6 An orange, crystalline pigment occurring in bile; the major metabolic breakdown product of heme.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
We detected a relatively high correlation between both hand and foot ESC levels with total and unconjugated bilirubin levels by using multiple linear regression analysis.
Di Padova, "Effect of ethanol on biliary unconjugated bilirubin and its implication in pigment gallstone pathogenesis in humans," Digestion, vol.
These results demonstrated that the high level of unconjugated bilirubin was directly involved in the impairment of tubular function in newborns with hyperbilirubinemia.
To prevent unconjugated bilirubin being deposited in tissues it is bound to albumin (a protein) and then transported in the blood to the liver where it is converted to its conjugated form by an enzyme called glucuronyltransferase (England, 2010; McIntosh and Stenson, 2008).
Indirect bilirubin is calculated by subtracting the direct value from the total value and provides an estimate of the unconjugated bilirubin in the serum.
There is only one case report describing high levels of unconjugated bilirubin in a pregnant patient with Crigler-Najjar syndrome as a cause of quadriplegia in the neonate.
These include concentration of unconjugated bilirubin high enough to exceed the bilirubin-binding capacity of albumin, due to severe hemolysis from Rh isoimmunization (18); bilirubin displacement from albumin by competing or affinity-altering molecules such as sulfisoxazole (19) or free fatty acids (20); and disruptions of the blood-brain barrier that allow bilirubin-protein complexes to enter the brain.
Additionally, the unconjugated bilirubin may have caused inspissated bile syndrome, which could also explain the conjugated bilirubinemia in these patients.
Albumin transports unconjugated bilirubin to the liver where it is metabolized using glucuronic acid into direct bilirubin.
The mechanism of phototherapy uses light (blue light works particularly well) to create different isomers of unconjugated bilirubin. Configurational changes of unconjugated bilirubin, specifically bilirubin IX alpha, involve the rotation of carbon-carbon bonds on the bilirubin carbon atoms designated 4 and 15.
In most provinces, requisition for a serum bilirubin level in infants does not automatically report both conjugated and unconjugated bilirubin fractions, therefore limiting the utility of the test.
Indirect bilirubin levels measure "free" or unconjugated bilirubin and direct bilirubin levels measure conjugated bilirubin (Pagana & Pagana, 2002).