NO INTERACTS WITH THE TYROSINE RADICAL Y-D(CENTER-DOT) OF PHOTOSYSTEM-II TO FORM AN IMINOXYL RADICAL

Incubation of photosystem II, PSII, membranes with NO for a few minute s results in the reversible elimination of the electron paramagnetic r esonance (EPR) signal II from the oxidized Tyr Y-D ., presumably due t o the formation of a weak Tyr Y-D .-NO complex [Petrouleas, V., & Dine r, B. A. (1990) Biochim. Biophys. Acta 1015, 131-140]. Illumination of such a sample at ambient or cryogenic temperatures produces no new EP R signals. If, however, the incubation with NO is extended to the hour s time range, illumination induces an EPR signal with resolved hyperfi ne structure in the g=2 region. The signal shows the typical features of an immobilized iminoxyl radical (>C=NO .) with hyperfine values A(p arallel to)=44 G, A(perpendicular to)=22 G, and A(iso)=29.3 G. The fol lowing observations suggest that the iminoxyl signal is associated wit h PSII: (a) the signal results from an immobilized species at room tem perature probably associated with a membrane-bound component, (b) the abundance of the signal is (sub)-stoichiometric to PSII, (c) the signa l is light-induced, (d) some of the treatments that affect PSII (Tris, Ca2+ depletion, high-salt wash) severely diminish the size of the sig nal, and (e) the development of the signal correlates with the release of Mn. In addition, the following observations suggest that the imino xyl signal results from an interaction of Y-D . with NO: (a) the evolu tion of the signal correlates with the loss in reversibility of the Ty r Y-D .-NO interaction and (b) the size of the signal correlates with the initial amount of oxidized Tyr Y-D. It is accordingly proposed tha t during the incubation with NO, a weak Tyr Y-D .-NO complex is rapidl y formed and is then slowly converted to a tyrosine-nitroso adduct. Li ght induced oxidation of the latter produces the iminoxyl radical. The nitrosotyrosine is expected to have an oxidation potential significan tly lower than the parent tyrosine and can act as an efficient electro n donor in PSII even at cryogenic temperatures, It is probably this lo wered redox potential of the tyrosine Y-D that explains the release of Mn concomitant with the formation of the nitroso species.