Roles of Na+ and K+ in alpha-aminoisobutyric acid transport by the marine bacterium Vibrio alginolyticus.

Effects of monovalent cations on alpha-aminoisobutyric acid (AIB) transport were examined in the marine bacterium Vibrio alginolyticus. In K+-containing cells, AIB was actively accumulated only in the presence of Na+, and the addition of K+ had essentially no effect. On the other hand, K+-depleted and Na+-loaded cells required K+ as well as Na+ for the accumulation of AIB against its concentration gradient. The characterization of the roles of Na+ and K+ in AIB transport was performed by manipulation of intra- and extracellular cation compositions. K+ concentration gradient (K+in greater than K+out) was not essential for the Na+-dependent AIB uptake. Na+ extrusion against its concentration gradient in Na+-loaded cells occurred only in the presence of K+(Rb+). Half-maximal stimulations of the Na+ extrusion and AIB uptake by K+ were observed at K+ concentration near apparent Km for K+ transport. Finally, in the presence of the Na+ electrochemical gradient (toward the inside), K+ was not necessary for AIB uptake. From these results, it was concluded that the Na+-dependent AIB uptake is driven by the Na+ electrochemical gradient across the membrane and that K+ is required for AIB uptake only for the generation of the Na+ electrochemical gradient.