title: Photoinhibition of hydroxylamine-extracted photosystem II membranes: studies of the mechanism.
abstract: The effects of photosystem II (PSII) exogenous electron donors and acceptors on the kinetics of weak light photoinhibition of NH2OH/EDTA-extracted spinach PSII membranes were examined. Under aerobic conditions, Mn2+ (approximately 1 Mn/reaction center; Km approximately 400 nM) inhibited photoinactivation and approximately 1 Mn/reaction center plus 100 microM NH2NH2 gave almost complete protection. In the absence of electron donors, strict anaerobiosis greatly inhibited photoinactivation even in the presence of an electron acceptor. Under aerobic conditions, the addition of electron acceptors (FeCN, DCIP), oxyradical scavengers, or superoxide dismutase strongly suppressed rates of photodamages. Increase in the concentrations of superoxide above those produced by illuminated NH2OH/EDTA-photosystem II membranes increased the rates of damage in the light but gave no damage in the dark. Scavengers of hydroxyl radicals and singlet oxygen did not suppress the rates of aerobic photoinhibition. These findings, along with others, lead us to conclude that photodamage of the secondary donors of the PSII reaction center occurs by two mechanisms: (1) a rapid superoxide and tyrosine YZ+ dependent process and (2) a slower process in which P680+/Chl+ catalyze the damages.
authors.affiliation: Plant Physiology, Biochemistry, and Molecular Biology Program, University of Kentucky, Lexington 40546-0091.
authors.forename: G X
authors.initials: GX
authors.lastname: Chen
authors.forename: J
authors.initials: J
authors.lastname: Kazimir
authors.forename: G M
authors.initials: GM
authors.lastname: Cheniae
mesh_terms: D016166:Free Radical Scavengers; D019811:Hydroxylamine; D006898:Hydroxylamines / Q000737:chemistry*; D007425:Intracellular Membranes / Q000528:radiation effects; D007700:Kinetics; D008345:Manganese / Q000378:metabolism; D010100:Oxygen / Q000737:chemistry; D010777:Photochemistry; D045322:Photosynthetic Reaction Center Complex Proteins / Q000528:radiation effects*; D045332:Photosystem II Protein Complex; D013379:Substrate Specificity; D013481:Superoxides / Q000737:chemistry
chemical_list: D016166:Free Radical Scavengers; D006898:Hydroxylamines; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D013481:Superoxides; D019811:Hydroxylamine; D008345:Manganese; D010100:Oxygen