PALMITATE BINDING TO AND EFFLUX KINETICS FROM HUMAN ERYTHROCYTE GHOST

At 0-degrees-C, pH 7.3, palmitate (PA) binds to human erythrocyte ghosts suspended in 0.2% bovine serum albumin (BSA) solution with molar ratios of PA to BSA, nu, between 0.2 and 1.3. The binding depends on the water phase PA concentration, measured in equilibrium experiments, using BSA-filled ghosts as semipermeable bags. The saturable binding has a capacity of 19.4 +/- 7.5 nmol g-1 packed ghosts (7.2.10(9) cells) and K(d) = 13.5 +/- 5 nM. PA exchange efflux kinetics to 0.2% BSA is recorded from ghosts without and with 0.2% BSA with a resolution time of about 1 s. Data are analyzed in terms of compartmental models. Using BSA-free ghosts the kinetics is essentially monoexponential. The rate constant is 0.0287 +/- 0.0022 s-1. Using ghosts with BSA, the kinetics is biexponential with widely different rate constants. Extrapolated zero-time values reflect, according to additional investigations, 'instantaneous' release of PA from the outer surface of the ghosts. Analyses of the biexponential curve up to about 55% tracer efflux assign unequivocally values to three model parameters. (1) kappa-1 the dissociation rate constant of the PA-BSA complex is (1.47 +/- 0.03). 10(-3) s-1 and (2.56 +/- 0.08). 10(-3) s-1 and (4.08 +/- 0.13).10(-3) s-1 at nu = 0.2, 0.6 and 1.4, respectively. (2) k3*, the overall rate constant of PA transport from the inside of the ghost membrane to the medium is 0.0269 +/- 0.0020 s-1 independent of nu. (3) Q(kin), the ratio of PA on the inside of the membrane to PA on BSA within the ghosts is nu dependent and smaller than a corresponding ratio Q(eq) measured in equilibrium by a value corresponding to PA on the outer surface. This fraction is released with a rate constant, k5, which is of the order of 1 s-1. The data suggest a maximum PA transport capacity, J(max), of 2 pmol min-1 cm-2, 0-degrees-C, pH 7.3.