Energetics of sodium-dependent alpha-aminoisobutyric acid transport in the moderate halophile Vibrio costicola.

The energetics of alpha-aminoisobutyric acid transport were examined in Vibrio costicola grown in a medium containing the NaCl content (1 M) optimal for growth. Respiration rate, the membrane potential (delta psi) and alpha-aminoisobutyric acid transport had similar pH profiles, with optima at 8.5-9.0. Cells specifically required Na+ ions to transport alpha-aminoisobutyric acid and to maintain the highest delta psi (150-160 mV). Sodium was not required to sustain high rates of O2-uptake. Delta psi (and alpha-aminoisobutyric acid transport) recovered fully upon addition of Na+ to Na+-deficient cells, showing that Na+ is required in formation or maintenance of the transmembrane gradients of ions. Inhibitions by protonophores, monensin, nigericin and respiratory inhibitors revealed a close correlation between the magnitudes of delta psi and alpha-aminoisobutyric acid transport. Also, dissipation of delta psi with triphenylmethylphosphonium cation abolished alpha-aminoisobutyric acid transport without affecting respiration greatly. On the other hand, alcohols which stimulated respiration showed corresponding increases in alpha-aminoisobutyric acid transport, without affecting delta psi. Similarly, N,N'-dicyclohexylcarbodiimide (10 microM) stimulated respiration and alpha-aminoisobutyric acid transport and did ot affect delta psi, but caused a dramatic decline in intracellular ATP content. From these, and results obtained with artificially established energy sources (delta psi and Na+ chemical potential), we conclude that delta psi is obligatory for alpha-aminoisobutyric acid transport, and that for maximum rates of transport an Na+ gradient is also required.