Factors influencing hydroxylamine inactivation of photosynthetic water oxidation

The kinetics of Mn release during NH2OH inactivation of the water oxidizing reaction is largely insensitive to the S-state present during addition of NH2OH. This appears to reflect reduction by NH2OH of higher S-states to a common more reduced state (S0 or S-1) which alone is susceptible to NH2OH inactivation. Sequences of saturating flashes with dark intervals in the range 0.2--5 S-1 effectively prevent NH2OH inactivation and the associated liberation of manganese. This light-induced protection disappears rapidly when the dark interval is longer than about 5 S. Under continuous illumination, protection against NH2OH inactivation is maximally effective at intensities in the range 10(3)--10(4) erg . cm-2 . S-1. This behavior differs from that of NH2OH-induced Mn release, which is strongly inhibited at all intensities greater than 10(3) erg . cm-2 . S-1. This indicates that two distinct processes are responsible for inactivation of water oxidation at high and low intensities. Higher S-states appear to be immune to the reaction by which NH2OH liberates manganese, although the overall process of water oxidation is inactivated by NH2OH in the presence of intense light. The light-induced protection phenomenon is abolished by 50 microM DCMU, but not by high concentrations of carbonyl cyanide m-chlorophenylhydrazone, which accelerates inactivation reactions of the water-splitting enzyme, Y (an ADRY reagent). The latter compound accelerates both inactivation of water oxidation and manganese extraction in the dark.