A mathematical model for membrane transport of amino acid and Na+ in vesicles

A model with a carrier having sites for both amino acid and Na+ can account for AIB (alpha-aminoisobutyric acid) transport kinetics observed in membrane vesicles from SV3T3 (simian virus 40-tranformed Balb/c33 cells) and 3T3 (the parent cell line). The main feature of this cotransport model is that Na+ binding to carrier decreases the effective Km for AIB transport. Na+ transport kinetics observed in both vesicle systems can be described by passive (possibly facilitated) diffusion. The lag of Na+ transport across the membrane compared to that for AIB, coupled to the Na+-dependent decrease in the Km for AIB, accounts for the overshoot in intravesicular AIB observed for SV3T3 in the presence of an initial Na+ gradient. Extra-vesicular Na+ maintains a decrease in the Km for AIB influx before intra-vesicular Na+ has accumulated to balance it with a comparable decrease in the Km for AIB efflux. 3T3 vesicles display little overshoot, and this finding can be explained mostly by a lower carrier affinity for Na+.