Flash Inactivation of Oxygen Evolution

Brief saturating light flashes were used to probe the mechanism of inactivation of O(2) evolution by Tris in chloroplasts. Maximum inactivation with a single flash and an oscillation with period of four on subsequent flashes was observed. Analyses of the oscillations suggested that only the charge-collecting O(2)-evolving catalyst of photosystem II (S(2)-state) was a target of inactivation by Tris. This conclusion was supported by the following observations: (a) hydroxylamine preequilibration caused a three-flash delay in the inactivation pattern; (b) the lifetimes of the Tris-inactivable and S(2)-states were similar; and (c) reagents accelerating S(2) deactivation decreased the lifetime of the inactivable state. Inactivation proved to be moderated by F, the precursor of Signal II(s), as shown by a one flash delay with chloroplasts having high abundance of F. Evidence was obtained for cooperativity effects in inactivation and NH(3) was shown to be a competitive inhibitor of the Tris-induced inactivation. S(2)-dependent inactivation was inhibited by glutaraldehyde fixation of chloroplasts, possibly suggesting that inactivation proceeds via conformational changes of the S(2)-state.