Effect of a transported ligand on the binding of albumin to rat liver cells.

Organic anions destined for hepatic uptake often bind to albumin in the circulation. Because albumin binds to liver cells but is not transported, we suggest that sites on the hepatocyte surface catalyze the dissociation of albumin-anion complexes, thus making more free anion available for transport than would otherwise occur. To learn whether liver cells distinguish between free albumin and albumin-anion complexes, we measured the binding of 125I-albumin to isolated rat hepatocytes in the presence and absence of rose bengal, a transported anion that binds extensively to albumin. Albumin binding to hepatocytes is reported as the albumin space corrected for extracellular fluid (14C-inulin space). Corrected albumin spaces are 2.95 and 2.83 microliter/mg cell protein with and without rose bengal, respectively. The mean difference and its 95% confidence interval computed from four comparisons in each of six rats is 0.12 +/- 0.67 microliter/mg cell protein. Inulin space is 32% of the uncorrected albumin space. Thus the affinities of albumin-rose bengal complexes and of free albumin for the hepatocyte surface differ by at most 28%. Accordingly, free albumin can compete with albumin-rose bengal complexes for cell surface sites, impairing the surface-mediated generation of free rose bengal for uptake. This finding explains the otherwise paradoxical observation that adding albumin to liver perfusate inhibits the uptake of rose bengal even when sufficient albumin is already present to bind 99.9% of this dye.