Qualitative Test for Bile Pigments and Urobilinogen in Urine
Bile pigments are breakdown products of the blood pigment hemoglobin that are excreted in bile. Bilirubin (orange or yellow in color) and its oxidized form biliverdin (green) are two important bile pigments. Bilirubin is formed by the degradation of heme in reticuloendothelial system. After formation of unconjugated bilirubin, albumin transports it to the liver where it conjugates with glucuronic acid to form bilirubin diglucuronide. The conjugated bilirubin is excreted through bile. Unconjugated bilirubin is not water soluble, so it is unable to pass through the glomerulus and is therefore not found in urine, whereas conjugated bilirubin is water soluble and is filtered in the glomerulus and subsequently reabsorbed by the renal tubules; hence normally bile pigments are absent in urine. Bilirubin glucuronides are hydrolyzed by intestinal bacterial enzyme β-glucuronidases to liberate bilirubin which is converted to a colorless compound “urobilinogen.” A portion of the urobilinogen formed is reabsorbed by enterohepatic circulation. Urobilinogen can be converted to a yellow-colored compound “urobilin” in the kidneys and excreted. The major part of intestinal urobilinogen is reduced to stercobilin by bacterial action which imparts brown color to feces and is excreted.