Nigericin-induced stimulation of photophosphorylation in chloroplasts

Amines have been shown recently to stimulate at low concentrations the steady-state rate of photophosphorylation by unbroken chloroplasts (Giersch, C. (1982) Z. Naturforsch. 37c, 242–250). In the present contribution it is demonstrated that not only amines but also the carboxylic ionophores nigericin and monensin at concentrations of 10 and 150 nM, respectively, stimulate the phosphorylation rate. The $\text{ATP}\text{2}\text{e}$ ratio is not decreased upon the addition of nigericin at concentrations that stimulate phosphorylation. Nigericin-induced stimulation is observed only in the presence of sufficient external potassium, indicating that the observed stimulation is unlikely to be a side-effect of the uncoupler but is related to H⁺-K⁺ exchange. The proton permeability of the thylakoid membrane is increased and the proton gradient decreased by amounts of nigericin that stimulate phosphorylation. The membrane potential is not affected in the steady state, indicating that the proton-motive force is slightly reduced upon addition of the ionophore. Data on the proton-motive force were related to maximum values of the phosphorylation potential, which was 45 000–50 000 M^?1 in the absence and 30 000–35 000 M^?1 in the presence of 10 nM nigericin. The observation that the $\text{ATP}\text{2}\text{e}$ ratio is not decreased in the presence of uncoupler-induced proton leakage is suggested to indicate that the thylakoid lumen does not represent a homogeneous phase of constant proton electrochemical potential. The results presented here are in agreement with the chemiosmotic concept as far as energetic aspects are concerned but seem to be at variance with the postulated free mobility of protons inside the thylakoids. A tentative model of uncoupler-induced stimulation of phosphorylation is presented.