Different Sensitivities of Chloroplasts to Uncouplers when ATP Formation is Induced by Continuous Illumination, by Brief Illumination, by Pre‐Illumination, or by Acid‐Base Transitions

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Abstract
All of the uncouplers tested are more effective in decreasing ATP formation at the beginning of the illumination period than they are during continuous illumination. The difference is quite small with the uncouplers methylamine and atebrin but very large with the uncouplers octylamine and carbonylcyanide p‐fluoromethoxyphenylhydrazone (FCCP). The illumination time required to initiate photophosphorylation in the absence of permeant ions is less than 5 ms at high light intensities and the short delay is not much affected by methylamine and atebrin. The delay on the onset of ATP formation is increased to about 40 ms by permeant ions such as K+ in the presence of valinomycin, although the same permeant ions have little or no effect on steady‐state phosphorylation [Ort and Dilley (1976) Biochim. Biophys. Acta, 449, 95–107]. Octylamine and FCCP also delay the onset of phosphorylation for about 40 ms and they do so at concentrations too low to cause significant inhibition of steady‐state phosphorylation. This may be the reason that they are much more inhibitory to phosphorylation during short illumination periods than during longer illumination periods. The delays in phosphorylation introduced by permeant ions and by low concentrations of the uncouplers octylamine and FCCP are not additive. Higher concentrations of all of the uncouplers which are capable of causing major inhibitions of steady‐state phosphorylation increase the delay in the onset of phosphorylation, whether or not permeant ions are present. The increase in delay caused by these higher concentrations is a function of the degree of uncoupling and of the extent of the delay already present at low uncoupler concentrations. For reasons which remain obscure, post‐illumination ATP formation is much more sensitive to the uncoupler methylamine than is ATP formation induced by either continuous illumination or acid‐base transitions. It is concluded that for the first 40 ms of illumination a transmembrane electric potential gradient is required for photophosphorylation and that permeant ions prevent the formation of such a potential by carrying charges across the lamellar membrane. Presumably the protonated form of octylamine, the octylammonium ion, is itself a permeant cation and therefore functions like K+ in the presence of valinomycin, whereas methylamine once protonated is probably trapped inside the lamellar vesicle, thus preserving the membrane potential. On the other hand, FCCP may destroy any developing membrane potential by carrying outside a small proportion of the hydrogen ions formed inside during electron transport. During the initial 40 ms of illumination, phosphorylation depends on the chemical potential of the hydrogen ions even though a transmembrane electric potential gradient seems also to be required.