Albumin enhances unidirectional fluxes of fatty acid across a lipid-water interface: theory and experiments.

The effect of albumin concentration on the unidirectional fluxes of [3H]oleate across a lipid-water interface was measured by using a new model system consisting of a lipid phase (n-decane) separated from a stirred buffer phase by a planar interface. Albumin (0.15 mM) enhanced the flux of oleate from the lipid phase more than 30-fold. A similar increase in the oleate flux into the lipid was seen when the aqueous oleate and albumin concentrations were increased while keeping the unbound oleate concentration fixed. The ratio of the unidirectional fluxes was constant over the range of albumin concentrations investigated (1-100 microM). Analysis using rigorous rate theory indicated that the permeability of the interfacial surface was flux-limiting at high albumin, whereas diffusion across the unstirred water layer was flux-limiting at low albumin. The observed kinetic patterns are similar to results previously reported in cellular transport systems and attributed to specialized mechanisms for cellular uptake from the albumin-bound pool.