THE REACTIVITY OF HYDRAZINE WITH PHOTOSYSTEM-II STRONGLY DEPENDS ON THE REDOX STATE OF THE WATER OXIDIZING SYSTEM

The decay kinetics of the redox states S2 and S3 of the water-oxidizing enzyme have been analyzed in isolated spinach thylakoids in the absence and presence of the exogenous reductant hydrazine. In control samples without NH2NH2 a biphasic decay is observed. The rapid decline of S2 and S3 with Y(D) as reductant exhibits practically the same kinetics with t1/2 = 6-7 s at pH = 7.2 and 7-degrees-C. The slow reduction (order of 5-10 min at 7-degrees-C) of S2 and S3 with endogenous electron donors other than Y(D) is about twice as fast for S2 as for S3 under these conditions. In contrast, the hydrazine-induced reductive shifts of the formal redox states S(i) (i = 0 ... 3) are characterized by a totally different kinetic pattern: (a) at 1 mM NH2NH2 and incubation on ice the decay of S2 is estimated to be at least 25 times faster (t1/2 less-than-or-equal-to 0.4 min) than the corresponding reaction of S3 (t1/2 almost-equal-to 13 min); (b) the NH2NH2-induced decay of S3 is even slower (about twice) than the transformation of S1 into the formal redox state 'S-1' (t1/2 almost-equal-to 6 min), which gives rise to the two-digit phase shift of the oxygen-yield pattern induced by a flash train in dark adapted thylakoids. (c) the NH2NH2-induced transformation S0-->'S-2' [Renger, Messinger and Hanssum (1990) in: Curr. Res. Photosynth. (Baltscheffsky, M., ed) Vol. 1, pp. 845-848, Kluwer, Dordrecht] is about three times faster (t1/2 almost-equal-to 2 min) than the reaction S1 [GRAPHICS] 'S-1'. Based on these results, the following dependence on the redox state S(i) of the reactivity towards NH2NH2 is obtained: S3 < S1 < S0 < < S2. The implications of this surprising order of reactivity are discussed.