Kinetic assessment of apparent facilitation by albumin of cellular uptake of unbound ligands.

Previous studies of the effect of albumin on initial uptake of ligands by isolated cell suspensions or cultures found that the apparent uptake for unbound ligand appeared larger in the presence of binding to the albumin than when albumin was absent. Furthermore, when ligand and albumin were increased in a fixed molar ratio, uptake appeared to be competitively inhibited by the excess albumin. We examined the kinetics underlying this apparent facilitation phenomenon by incorporating unbound fraction of ligand in the medium (fu1) into the general model for diffusion between two compartments. The analysis showed that even in the absence of facilitation by albumin, the apparent rate constant for uptake of unbound ligand (k/fu1) increases as albumin concentration increases but the uptake clearance of unbound ligand remains constant. This theoretical analysis was verified experimentally by measuring the effect of albumin on uptake rates of 14C-taurocholate (12, 24, 48, 60, and 96 microM, with and without 0.87 mM albumin) in a nonphysiological system consisting of two solutions separated by a cellulose membrane. Moreover, when the taurocholate and albumin concentrations were increased in a fixed molar ratio of 0.06 (taurocholate 12-96 microM, albumin 0.2-1.6 mM), the initial uptake rate exhibited the same nonlinear pattern as the previous studies that used living cells. This pattern was due not to saturation of a putative albumin receptor but simply to the concomitant decrease in fu1 which tended to offset the increase in uptake rate due to the increasing total taurocholate concentration. The model was also used to evaluate published data describing the effect of albumin on the uptake of iopanoic acid by cultured hepatocytes. In accordance with the model, k1/fu1 increased as albumin concentration increased, but uptake clearance was independent of albumin concentration. Therefore, the kinetic pattern found in this and other studies with isolated cell suspensions or cultures argues against a special role for albumin in facilitating cellular ligand uptake.