| Abstract: | A class of compounds, usually referred to as ADRY reagents, destabilize intermediates in the photosynthetic water-oxidizing process. The effects of these species on the reduction kinetics of Z?, the oxidized donor to P-680, have been monitored in Tris-washed chloroplasts by following the decay of EPR Signal IIf. In the presence of ADRY reagents (e.g., sodium picrate, carbonyl cyanide m-chlorophenylhydrazone) this process follows an exponential time course, the decay half-time of which decreases as the ADRY reagent concentration increases. From this pseudo-first-order behavior, the second-order rate constants for four commonly used ADRY reagents have been extracted. The ADRY-induced acceleration in Z? reduction proceeds independently of conditions imposed on the acceptor side of Photosystem II and shows no synergism with exogenous electron donors. These observations are most easily rationalized in terms of a model which proposes direct reduction of Z? by the ADRY reagent followed by regeneration of the reduced ADRY reagent in a nonspecific reaction with membrane components such as carotenoids, chlorophyll or protein. A comparison of the second-order rate constants we obtain for ADRY reagents in their reaction with Z? in Tris-washed chloroplasts with those obtained from the literature for the ADRY- reagent induced deactivation of states S2 and S3 in oxygen-evolving chloroplasts reveals a close similarity between the two processes. From this observation, a general model for the action of ADRY reagents in destabilizing the high-potential oxidizing equivalents generated in Photosystem II is proposed. |
