Arachidonic acid transfer across the human red cell membrane by a specific transport system

The exchange efflux kinetics of [3H]arachidonate at 0 degrees C, pH 7.3 from human red cell ghosts to bovine serum albumin (BSA) in buffer is analysed in terms of a closed three-compartment model. Using albumin-free ghosts the kinetics determines the model parameters: (1) The ratio of arachidonate bound to the inner membrane leaflet to that bound to the outer leaflet (B/E), 0.30 +/- 0.03 and (2) the rate constant of unidirectional flux through the membrane from B to E (k3), 0.39 +/- 0.03 s-1. From the model parameter estimates and knowledge of apparent equilibrium constants of arachidonate binding to ghost membrane and to albumin, we estimate the dissociation rate constant of arachidonate-albumin complex (k1) to 0.21 +/- 0.02 s-1. The lowest rate coefficient (delta) of efflux kinetics from albumin-filled ghosts decreases by approximately sevenfold over a 10-fold increase in intracellular albumin. These delta-values fit fairly well with the values predicted by the corresponding model with an unstirred intracellular compartment using the parameter values obtained in the studies with ghosts without BSA. Model parameters for arachidonate efflux are completely different from those obtained for palmitate, suggesting that different transport systems determine arachidonate and palmitate membrane transfer. The data show that binding to a limited number of specific sites is functioning as the initial and obligatory step in the transport. We propose that a protein is directly or indirectly controlling the transport capacity.