Abstract
The effects of N, N'-dicyclohexylcarbodiimide (DCCD) and various ionophores on lightinduced 22Na+-transport were studied in right-side-out membrane vesicles from Halobacterium halobium R1M1. The light-induced Na+ efflux was inhibited at the same DCCD concentration (>40 nmol/mg protein) as required for inhibition of the Na+-dependent membrane potential (Δ_??_) formation. This supports our previous indication that the DCCD-sensitive, Na+-dependent transformation of pH-gradient (ΔpH) into Δ_??_ is mediated by Na+/H+-antiporter (Murakami, N. and Konishi, T. (1985) J. Biochem. 98, 897-907). FCCP or a combination of valinomycin and triphenyltin (TPT) inhibits the lightinduced Na+ efflux in accordance with the notion of protonmotive force (Δ_??_H+)-driven antiporter. However, a marked lag in initiation of the Na+ efflux occurred in the presence of valinomycin, TPMP+, or a small amount of FCCP, suggesting that a gating step is involved in the Na+ efflux. On the other hand, the ΔpH-dissipating ionophore TPT did not cause the lag. A simultaneous determination of Δ_??_, ΔpH, and Na+ efflux rate at the initial stage of illumination revealed that the antiporter is gated by Δ_??_ rather than Δ_??_H+.
