Metabolism - Uptake
Bilirubin metabolism
Hepatic uptake
Upon reaching the liver, bilirubin dissociates from albumin and enters the hepatocyte, probably by carrier-mediated diffusion. Two cytoplasmic proteins have high bilirubin-binding capacity. The most important is ligandin or Y-protein. It can efficiently bind bilirubin, as well as corticosteroids and other organic anions. A second cytoplasmic protein called Z-protein also binds bilirubin, but with less affinity than ligandin.There is a significant amount of evidence indicating that bilirubin movement across the hepatocyte membrane is bidirectional. It has been estimated that up to 40% of the bilirubin taken up by the hepatocyte refluxes unchanged back into plasma. Although Y and Z proteins are probably not directly involved in bilirubin uptake, they may help retain bilirubin in the hepatocyte.Efficient hepatic uptake of bilirubin is dependent on adequate hepatic blood flow. Conditions associated with a persistent ductus venosus shunt, hyperviscosity or hypovolemia can lead to decreased hepatic perfusion, decreased hepatic bilirubin uptake and unconjugated hyperbilirubinemia. Return to top of page