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

The inhibition o f the transthylakoid pH gradient, ApH, and o f photophosphorylation by the local anesthetic tetracaine was investigated with isolated chloroplasts from Spinacia oleracea L. Tetracaine strongly inhibited ApH in the presence o f low salt concentrations. In the presence o f high salt concentrations, the inhibition o f ApH was much smaller. This effect o f salt concentration was observed only when both, cation and anion were easily membrane permeable. It was concluded that the effect o f salts on ApH inhibition was excerted on the inside o f the thylakoid membrane. The rate o f photophosphorylation, Vv, driven by the PS Idependent artificial proton carrier phenazine methosulfate decreased with ApH in the presence o f both, high and low salt concentrations. In contrast, Vp driven by the endogenous proton pumps o f PS II + I-dependent linear electron flow was largely independent o f ApH changes in the presence o f 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 o f ApH. The data were discussed with respect to recent hypotheses on localized energy coupling in chloroplasts.