Mechanisms of active transport in isolated bacterial membrane vesicles. Further studies on amino acid transport in Staphylococcus aureus membrane vesicles.

Active amino acid transport in Stajhylococcus aureus U-71 membrane vesicles is coupled to either cy-glycerol phosphate dehydrogenase or L-lactate dehydrogenase depending upon the growth conditions of the parent cells. Vesicles prepared from cells grown on gluconate as a primary carbon source exhibit an absolute specificity for a-glycerol phosphate as a physiological electron donor for transport, whereas vesicles prepared from cells grown on glucose as a primary carbon source exhibit an absolute specificity for L-lactate as an electron donor for transport. Both preparations exhibit similar dehydrogenase activities qualitatively, indicating that the coupling between these dehydrogenases and transport is altered. L-Lactate oxidation, L-lactate: dichlorophenolindophenol reductase activity, and L-lactate-dependent amino acid transport exhibit similar apparent Michaelis constants with respect to L-lactate, indicating that L-lactate oxidation per se is the rate-limiting step for amino acid transport in the appropriate membrane preparation. Amino acid transport is dependent on electron transfer, and inhibition of L-lactate oxidation by anaerobiosis, cyanide, 2-heptyl-4-hydroxyquinolineN-oxide, amytal, and oxalate is directly related to inhibition of amino acid transport. However, only anaerobiosis, cyanide, 2-heptyl-4-hydroxyquinoline-N-oxide, and amytal, each of which inhibits electron transfer after the site of energy coupling, cause efllux. Oxalate, a potent inhibitor of L-lactate dehydrogenase, does not cause efliux despite almost complete inhibition of Llactate oxidation and amino acid transport. Moreover, oxalate blocks or inhibits efliux caused by each of the other inhibitors and by 2,4-dinitrophenol. These results provide further evidence that active transport is dependent on the oxidation-reduction potential of the respiratory chain at the site of energy coupling. Cyanide-induced efflux is a saturable process with an apparent affinity constant that is approximately 500 times higher than the affinity constant for active transport. The apparent maximum velocity of efllux, on the other hand, is the same as that of active transport. These findings suggest