Functional sulfhydryl groups of ferredoxin-NADP+ oxidoreductase

Chemical modification of membrane-bound ferredoxin-NADP+ oxidoreductase with oxidants of vicinal dithiols caused inactivation of NADP+ photoreduction, with no effect on the diaphorase activity. Inactivation was partially prevented by ferredoxin and reversed by dithioerythritol. N-Ethylmaleimide inhibited both activities, even though with a different kinetic pattern. Inactivation of NADP+ reduction by either N-ethylmaleimide or o-iodosobenzoate was greater in the light than in the dark. The results suggest the existence of essential sulfhydryl groups related with the ferredoxin site, in addition to those described in the soluble flavorprotein. The role of SH residues in the activity and regulation of membrane bound reductase is discussed.