Maintenance of Photophosphorylation Despite Inhibition of the Transthylakoid pH Gradient by Tetracaine

Abstract

The inhibition of the transthylakoid pH gradient, ΔpH, and of photophosphorylation by the local anesthetic tetracaine was investigated with isolated chloroplasts from Spinacia oleracea L. Tetracaine strongly inhibited ΔpH in the presence of low salt concentrations. In the presence of high salt concentrations, the inhibition of ΔpH was much smaller. This effect of salt concentration was observed only when both, cation and anion were easily membrane permeable. It was concluded that the effect of salts on ΔpH inhibition was excerted on the inside of the thylakoid membrane. The rate of photophosphorylation, Vp, driven by the PS Idependent artificial proton carrier phenazine methosulfate decreased with ΔpH in the presence of both, high and low salt concentrations. In contrast, Vp driven by the endogenous proton pumps of PS II + I-dependent linear electron flow was largely independent of ΔpH changes in the presence of low salt concentration. It appeared that energy coupling during linear electron transport, in contrast to artificially produced PS I-dependent coupling, may be localized to membrane-bound proton domains which are not accessible to the employed indicators of ΔpH. The data were discussed with respect to recent hypotheses on localized energy coupling in chloroplasts.