Facilitated diffusion and membrane permeation of fatty acid in albumin solutions

Facilitated transport is characteristic of most living systems, and usually involves a series of consecutive adjacent transfer regions, each having di fferent transport properties. As a first step in the analysis of the multir egional problem, we consider in a single unstirred layer the facilitated di ffusion of fatty acid (F) in albumin (A) solution under conditions of slow versus rapid association-dissociation, accounting for differing diffusiviti es of the albumin-fatty acid complex (AF). Diffusion gradients become estab lished in an unstirred layer between a source of constant concentration of A, AF, and F in equilibrium, and a membrane permeable to F. The posited sys tem does not reduce to a thin- or thick-layer approximation. The transient state is prolonged by slower on/off binding rates and by increasing the thi ckness of the unstirred layer. Solutions to transient and steady state depe nd upon the choice of boundary conditions, especially for thin regions. Whe n there are two regions (each with its specific binding protein) separated by a permeable membrane, the steady-state fluxes and concentration profiles depend on the rates of association and dissociation reactions, on the diff usion coefficients, local consumption rates, and on the membrane permeabili ty. Sensitivity analysis reveals the relative importance of these mechanism s. (C) 2000 Biomedical Engineering Society. [S0090-6964(00)00603-2].