Photoproduction of the Azidyl Radical from the Azide Anion on the Oxidizing Side of Photosystem II and Suppression of Photooxidation of Tyrosine Z by the Azidyl Radical

Azide ions inhibited O₂ evolution in PSII membranes from spinachin a time-dependent manner in the light until all activity disappeared.Illumination in the presence of azide (azide-phototreatment)irreversibly inhibited the following processes: (1) both theoxidation of water and the electron transport between the redox-activetyrosine 161 of the D1 protein (Y_Z) and the secondary quinoneelectron acceptor (Q_B) site, to the same extent; (2) the donationof electrons to the primary quinone electron acceptor (Q_A),as measured by monitoring the maximum variable fluorescenceof Chl; and (3) the photoproduction of the Y_Z radical (Y). Thus,the primary site of inhibition appeared to lie between Y_Z andQ_A. On illumination of Tris-treated PSII membranes in the presenceof azide, production of the azidyl radical was observed by spin-trappingESR. Yield of Y in Tris-treated membranes on illumination wassuppressed by azide. Electron transport from Y_Z to Q_B in Tris-treatedmembranes was inhibited only when the azidyl radical was photoproduced,and it was inhibited more rapidly than it was in the oxygenicPSII membranes. These results indicate that the azidyl radicalwas produced via a univalent oxidation of azide by Y and thatit irreversibly inhibited the electron transport from Y_Z toQ_A in Tris-treated membranes. Although the azidyl radical wasundetectable in the oxygenic PSII membranes, probably due tosteric interference by the peripheral proteins of water-oxidizingcomplex with the access of the spin-trapping reagent to theproduction site of the radical, the participation of the azidylradical in the inhibition of the oxygenic PSII membranes issuggested since simultaneous occurrence of both electron transportand azide was required for the inhibition. Possible inhibitorymechanisms and the target sites of azidyl radical are discussed. (Received April 21, 1995; Accepted July 3, 1995)