ION TRANSLOCATION IN ISOLATED CHLOROPLASTS . UNCOUPLING OF PHOTOPHOSPHORYLATION AND TRANSLOCATION OF K+ AND H+ IONS INDUCED BY NIGERICIN

The effect of a novel type of uncoupling agent in chloroplasts is described. In the presence of K+, nigericin uncoupled adenosine triphosphate (ATP) formation from electron transport, inhibited ATP hydrolysis, ATP-P; exchange, and the light-induced proton uptake in spinach chloroplasts. Inhibition (50%) of these reactions was obtained at a concentration of about 5 × 10-8 m nigericin. The selectivity for K+ among the alkali metal cations required for inhibition of these reactions was demonstrated. Nigericin induced, in the dark, a specific K+-H+-exchange reaction with chloroplasts suspended in the absence of external salts. The stoichiometry of this exchange was 1.2-1.5 K+ effluxed/H+ taken up. Under conditions that brought about uncoupling, nigericin was found to induce a light-dependent decrease in light-scattering intensity of chloroplast suspensions. This change in light-scattering intensity corresponds to swelling of the chloroplasts as corroborated by electron microscopy. The effect of nigericin, gramicidin, and valinomycin on H+ uptake, K+-H+ exchange, and the light-induced absorbance changes were compared. From these results it is suggested that nigericin acts in chloroplasts at the level of the ion translocation mechanism in the membrane. © 1968, American Chemical Society. All rights reserved.