Bilirubin is a metabolite of heme. It serves as a means to excrete unwanted heme, which is derived from various heme-containing proteins such as hemoglobin, myoglobin, and various P450 enzymes. Bilirubin is also notable for providing the color to bile, stool, and to a lesser extent the urine. Its produced by a two-stage reaction that occurs in cells of the RES (reticuloendothelial system). The RES includes the phagocytes, mainly being the macrophages, the Kupffer cells in the liver and the cells in the spleen and bone. Heme is taken up into these cells and acted on by the enzyme heme oxygenase, liberating the chelated iron from the heme structure and releasing carbon monoxide. The carbon monoxide is excreted via the lungs. The reaction yields a green pigment known as biliverdin. Biliverdin is then acted on by the enzyme biliverdin reductase which produces bilirubin. Bilirubin consists of a yellow pigment. Bilirubin is derived from two main sources. The majority, about 80% comes from heme which is released from senescent red blood cells. The other 20% originates from other heme-containing proteins found in the liver and muscles.
Bilirubin is toxic to tissues, therefore it is transported in the blood in its unconjugated form bound to albumin. For that reason, only a small amount of the free form is present in the blood. If the free fraction increases, bilirubin with invade and cause damage to the tissues. Excess unconjugated bilirubin can cross the blood-brain barrier and cause kernicterus in neonates. The unconjugated bilirubin is taken up by hepatocytes where the albumin bond is broken. Inside the hepatocyte, the bilirubin is bound to cytoplasmic proteins ligandins and Z proteins. The primary function of these proteins is too prevent the reflux of bilirubin back into the circulatory system. Unconjugated bilirubin is lipophilic. Its conjugation with glucuronic acid renders it hydrophilic, therefore it can be eliminated utilizing bile. Conjugated bilirubin synthesis occurs in a two step reaction. First glucuronic acid is synthesized from cytosolic glucose which then attaches to uridinediphosphate (UDP) via the enzyme UDP-glucose-dehydrogenase. This forms UDP-glucuronic acid. This compound has an affinity for bilirubin for which then the glucuronic acid is transferred to the bilirubin which is catalyzed by glucuronyl transferase. Conjugation of bilirubin takes place in the endoplasmic reticulum of the hepatocytes and the end result is an ester between the glurcuronic acid and one or both of the propionic side-chains of bilirubin.
Once bilirubin is conjugated it is excreted with bile acid into the small intestine. The bile acid is reabsorbed in the terminal ileum for enterohepatic circulation, the conjugated bilirubin is not absorbed and instead passes into the colon. In the colon, the bacteria metabolize the bilirubin into urobilinogen, which can be oxidized to form urobilin, and stercobilin. Urobilin is excreted by the kidneys to give urine its yellow color and stercobilin is excreted in the feces giving stool its characteristic brown color. There can be traces levels of urobilinogen present in the blood.
Unconjugated hyperbilirubinemia in a neonate can lead to an accumulation of unconjugated bilirubin in the brain tissue. The neurological disorder is called kernicterus. The blood-brain barrier is not yet fully developed and bilirubin can freely pass into the brain interstitium. In cases of liver impairment, biliary drainage is blocked, and some of the conjugated bilirubin leaks into the urine, turning it a dark amber color. In cases of hemolytic anemia, there is increased hemolysis of red cells causing an increase in unconjugated bilirubin in the blood. In these cases, there is no problem with the livers mechanism to conjugate the bilirubin, and there will be an increase in urobilinogen in the urine. This is the difference between an increased urine bilirubin, and an increased urine urobilinogen.