Shahak, Y.

The effect of thiol oxidants on the light-activated H+-ATPase has been studied in freshly broken and intact chloroplasts. The following observations were made: (i) in chloroplasts which are osmotically shocked after light activation, ferricyanide stimulates the deactivation of the enzyme in the dark, but has little effect in the light; (ii) similarly, o-iodosobenzoate is a most efficient deactivator of the ATPase in intact chloroplasts in the dark but not in the light; (iii) the activated ATPase becomes sensitive to oxidants in the light upon the addition of an uncoupler; (iv) the oxidant-induced deactivation in the dark dominates the stabilizing effect of pyrophosphate or ADP plus Mg2+; (v) full deactivation of the ATPase by dark adaptation or by oxidants does not affect the rate of photophosphorylation under saturating conditions. A model is suggested in which two kinds of conformational changes are involved in the regulation of the ATPase: those induced by the trans-membrane-proton gradient and those by oxidation-reduction of the enzyme. These changes result in the preferential interaction with thiol reductants in the light but with thiol oxidants in the dark.