Protein Binding and Conjugation of Bilirubin in the Liver Cell
For over 40 years, it has been known that following injection of “physiologic” amounts of bilirubin and a variety of other organic anions, a large proportion of the injected dose is recovered within the liver in a matter of minutes. Complete elucidation of the mechanism responsible for this rapid and seemingly selective transfer from plasma into the liver remains unknown; however, experimental studies suggest several hypotheses; (i) bilirubin is noncovalently bound to plasma albumin and enters the liver by pinocytosis as a pigment: albumin complex; (ii) “unbound” bilirubin in plasma is transferred across the plasma membrane of the liver cell by non-ionic diffusion and net uptake flux is determined by intracellular binding and/or subsequent metabolism and biliary excretion; (iii) a plasma membrane carrier system exists with relative specificity for bilirubin; (iv) net hepatic uptake of bilirubin is largely determined by hepatic blood flow and a high extraction ratio of the bile pigment, and (v) an active transport system for bilirubin and other organic anions exists in the portion of the plasma membrane of the parenchymal liver cells which faces the sinusoid.
KeywordsHepatic Uptake Glucuronyl Transferase Unconjugated Hyperbilirubinemia Bilirubin Conjugate Bilirubin Binding
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