Bilirubin is synthesized from heme by a two-stage reaction that takes place in phagocytic cells of the reticuloendothelial system, including Kupffer cells and cells in the spleen (
Fig. 32.13 ). The enzyme heme oxygenase that is present in these cells liberates iron from the heme molecule and produces the green pigment biliverdin. This in turn can be reduced to form yellow bilirubin. Because bilirubin is essentially insoluble in aqueous solutions at neutral pH, it is transported through the bloodstream bound to albumin. When this complex reaches the liver, it enters the space of Disse, where bilirubin is selectively taken up across the basolateral membrane of hepatocytes via an OATP transporter (see
Table 32.1 ). In the microsomal compartment, bilirubin is then conjugated with one or two molecules of glucuronic acid to enhance its aqueous solubility. The reaction is catalyzed by UDP glucuronyl transferase (UGT). This enzyme is synthesized only slowly after birth, which explains why mild jaundice is relatively common in •Fig. 32.13 Conversionofhemetobilirubin.Thereactionsinsidethedashedboxoccurincellsofthereticuloendothelialsystem.NADP+,oxidizedformofnicotinamideadeninedinucleotidephosphate;NADPH,reducedformofnicotinamideadeninedinucleotidephosphate.
newborn infants. The bilirubin conjugates are then secreted into bile by a multidrug resistance–associated protein (MRP2) located in the canalicular membrane. Notably the conjugated forms of bilirubin cannot be reabsorbed from the intestine, thereby ensuring they can be excreted. However, transport of bilirubin across the hepatocyte (and indeed its initial uptake from the bloodstream) is relatively inefficient, so some conjugated and unconjugated bilirubin is present in plasma even under normal conditions. Both circulate bound to albumin, but the conjugated form is bound more loosely and thus can enter the urine.
In the colon, bilirubin conjugates are deconjugated by bacterial enzymes, whereupon the bilirubin liberated is metabolized by bacteria to yield urobilinogen, which is reabsorbed, and urobilins and stercobilins, which are excreted. Absorbed urobilinogen in turn can be taken up by hepatocytes and reconjugated, thus giving the molecule yet another chance to be excreted.
Measurement of bilirubin in plasma, as well as assessment of whether it is unconjugated or conjugated, is an important tool in the evaluation of liver disease. The presence of unconjugated bilirubin, which is essentially fully albumin bound and cannot be excreted in urine, reflects either loss of UGT (or a normal temporary delay in its maturation in infants) or a sudden oversupply of heme that overwhelms the conjugation mechanism (such as occurs in transfusion reactions or in Rhesus-incompatible newborns). Conjugated bilirubinemia on the other hand is characterized by the presence of bilirubin in urine, to which it imparts a dark coloration. This is indicative of genetic defects in the transporter that mediates bilirubin glucuronide/diglucuronide secretion into the canaliculus, or it may be due to blockage to the flow of bile, perhaps
CHAPTER 32 Transport and Metabolic Functions of the Liver caused by an obstructing gallstone. In both cases, bilirubin conjugates are formed in the liver, but with no means of exit they regurgitate back into plasma for urinary excretion.