Rapid dissociation of organic anions from plasma albumin maximizes the presentation of free ligand to the cell surface and thus favors its efficient hepatic extraction. Even assuming these optimal conditions, however, taurocholate and rose bengal have hepatic extraction fractions that are higher than can be accounted for by spontaneous dissociation of their albumin-ligand complexes. In this study we developed a transport model that attributes this behavior to sites on the hepatocyte plasma membrane that bind the albumin-ligand complexes, promoting the transport of ligand into the hepatocyte. Fitting this model to rose bengal removal rates measured over a wide range of albumin concentrations yields estimates of the number of cell surface sites and their affinity for albumin. These estimates are in good agreement with those reported by Weisiger, Gollan, and Ockner for the binding of ligand-free albumin to isolated hepatocytes. We conclude that both experiments measure the same phenomenon and, accordingly, that the binding of albumin to the cell surface is the functional equivalent of albumin-mediated transport.
E L Forker, B A Luxon