KINETICS OF BILIRUBIN TRANSFER BETWEEN SERUM-ALBUMIN AND MEMBRANE-VESICLES - INSIGHT INTO THE MECHANISM OF ORGANIC ANION DELIVERY TO THE HEPATOCYTE PLASMA-MEMBRANE

Unconjugated bilirubin is transported in the plasma bound primarily to serum albumin, from which it is taken up and metabolized by the liver . To better characterize the mechanism of bilirubin delivery to the he patocyte, stopped-flow techniques were utilized to study the kinetics of bilirubin transfer between serum albumin and both model phospholipi d and native hepatocyte plasma membrane vesicles. The transfer process was best described by a single exponential function, with rate consta nts of 0.93 +/- 0.04, 0.61 +/- 0.03, and 0.10 +/- 0.01 s(-1) (+/- S.D. ) at 25 degrees C for human, rat, and bovine serum albumins, respectiv ely. The observed variations in rate with respect to donor and accepto r concentrations provide strong evidence for the diffusional transfer of free bilirubin. Thermodynamic analysis suggests that the binding si te on bovine serum albumin demonstrates higher specificity for the bil irubin molecule than that on human or rat serum albumin, which exhibit similar binding characteristics. Kinetic analysis of bilirubin transf er from rat serum albumin to isolated rat basolateral liver plasma mem branes indicates that the delivery of albumin-bound bilirubin to the h epatocyte surface occurs via aqueous diffusion, rather than a collisio nal process, thereby mitigating against the presence of an ''albumin r eceptor.''