urobilin

Urobilin, also called urobilin, is a yellow to amber pigment formed in the breakdown of heme. It is produced from urobilinogen by oxidative processes and is one of the pigments involved in the coloration of urine.

Metabolism and formation: Heme from aged red blood cells is converted to biliverdin and then bilirubin, which is conjugated in the liver and excreted into bile. In the intestine, bacterial enzymes convert bilirubin diglucuronide into urobilinogen. A portion of urobilinogen is absorbed and reaches the liver again, and is oxidized in the kidneys to urobilin, which is excreted in urine; the remainder is converted by intestinal bacteria to stercobilin, giving feces its brown color. Urobilin, along with urochrome, contributes to the typical yellow hue of urine, though urochrome is a major determinant of color.

Clinical significance: Urinary urobilinogen and urobilin levels reflect bilirubin metabolism and biliary tract function. Increased urinary urobilinogen can occur with heightened bilirubin delivery to the intestine, such as in certain hemolytic states or liver diseases, while reduced or absent urinary urobilinogen can be seen with biliary obstruction or severe hepatocellular impairment. Analysis of urobilinogen in urine is used in conjunction with other tests to assess liver function and the etiology of jaundice.

See also: Urobilinogen, bilirubin, stercobilin, urochrome.