EXCHANGE EFFLUX OF [H-3] PALMITATE FROM HUMAN RED-CELL GHOSTS TO BOVINE SERUM-ALBUMIN IN BUFFER - EFFECTS OF MEDIUM VOLUME AND CONCENTRATION OF BOVINE SERUM-ALBUMIN

[H-3]Palmitate, PA, exchange efflux kinetics is recorded from human erythrocyte ghosts to buffer with bovine serum albumin, BSA, at 0-degrees-C. The effects have been investigated of three medium/ghost volume ratios: 36, 80 and 500, of six BSA concentrations, [BSA]: 0.01, 0.02, 0.05, 0.2, 1 and 2% (1.5, 3.0, 7.5, 30, 150 and 300 muM) and of various nu, molar ratios of palmitate to BSA, between 0.15 and 0.94. Data are analyzed in terms of a virtually closed three-compartment model. In theory, the tracer efflux is biexponential and the rate coefficients differ at least 20 fold [1]. The efflux rate at 2% BSA is monoexponential beyond our resolution time of about 1 s, but nearly biexponential at or below 0.2% BSA with a well-defined smallest-rate coefficient beta. Beta depends strongly on [BSA] but is remarkably nu independent. The medium/ghost volume ratio has no effect on beta when [BSA] greater-than-or-equal-to 0.2%, although beta measured at 2% BSA is almost 2-fold higher than at 0.2%. This suggests the presence of an unstirred layer, USL. According to our model, the observations are understood quantitatively on basis of our previously published dissociation rate constants of the PA-BSA complex, as well as PA equilibrium bindings to ghost membranes (Bojesen, I.N. and Bojesen, E. (1991) Biochim. Biophys. Acta 1069, 297-307). Essentially, beta is theoretically a function of two terms, one comprising the membrane transport parameters and the other the medium-dependent variables. Most important is the clearance with respect to monomer concentration adjacent to the membrane. The clearance is calculated on basis of quasistationary diffusion in USL. The data are compatible with a planar USL of 6 mum depth and with the same area as a ghost but not with a spherical USL.