THE PHOTOPRODUCTION OF SUPEROXIDE RADICALS AND THE SUPEROXIDE-DISMUTASE ACTIVITY OF PHOTOSYSTEM-II - THE POSSIBLE INVOLVEMENT OF CYTOCHROMEB559

In the present study the light induced formation of superoxide and int rinsic superoxide dismutase (SOD) activity in PS II membrane fragments and D1/D2/Cytb559-complexes from spinach have been analyzed by the us e of ferricytochrome c (cyt c(III)) reduction and xanthine/xanthine ox idase as assay systems. The following results were obtained: 1.) Photo reduction of Cyt c (III) by PS II membrane fragments is induced by add ition of sodium azide, tetracyane ethylene (TCNE) or carbonylcyanide-p -trifluoromethoxy-phenylhydrazone (FCCP) and after removal of the extr insic polypeptides by a 1M CaCl2-treatment. This activity which is abs ent in control samples becomes completely inhibited by the addition of exogenous SOD. 2.) The TCNE induced cyt c(III) photoreduction by PS I I membrane fragments was found to be characterized by a half maximal c oncentration of c(1/2)=10 mu M TCNE. Simultaneously, TCNE inhibits the oxygen evolution rate of PS II membrane fragments with c(1/2)approxim ate to 3 mu M. 3.) The photoproduction of O-2(-) is coupled with H+-up take. This effect is diminished by the addition of the O-2(-)-trap cyt c(III). 4.) D1/D2/Cytb559-complexes and PS II membrane fragments depr ived of the extrinsic proteins and manganese exhibit no SOD-activity b ut are capable of producing O-2(-) in the light if a PS II electron do nor is added. Based on these results the site(s) of light induced supe roxide formation in PS II is (are) inferred to be located at the accep tor side. A part of the PS II donor side and Cyt b559 in its HP-form a re proposed to provide an intrinsic superoxide dismutase (SOD) activit y.