Light-dependent rubidium transport in intact Halobacterium halobium cells

The uptake of rubidium in intact Halobacterium halobium cells was followed, and found to be light-dependent. The exchange process is slow, the steady-state rate of ⁸⁶Rb⁺/Rb⁺ exchange being given by k = 6.3 · 10^?4 min^?1. Starved cells exhibited a faster rate than unstarved cells. The influx of ⁸⁶Rb⁺ was almost completely blocked in the presence of proton conductors (CCCP, FCCP, and SF 6847), and was sensitive to the presence of the permeant cation TPMP⁺. Valinomycin very slightly increased the rate of uptake, while 1 · 10^?6 M nigericin showed significant inhibition. On the other hand, release of ⁸⁶Rb⁺ was not light-dependent, although still affected by uncouplers, TPMP⁺, and nigericin. These experimental observations may be explained in terms of a passive flux driven by an electrical potential difference, and influenced by positive isotope interaction within the membrane. In carefully matched influx-efflux studies, the extent of the positive isotope interaction was measured. Using the formal treatment of Kedem and Essig, the ratio (exchange resistance)/(resistance to net flow) for ⁸⁶Rb⁺ was found to be 1.7.