Redox regulation of yeast flavin-containing monooxygenase

The flavin-dependent monooxygenase from yeast (yFMO) oxidizes biological th iols such as cysteine, cysteamine, and glutathione. The enzyme makes a majo r contribution to the pools of oxidized thiols that, together with reduced glutathione from glutathione reductase, create the optimum cellular redox e nvironment. We show that the activity of yFMO, as a soluble enzyme or in as sociation with the ER membrane of microsomal fractions, is correlated with the redox potential. The enzyme is active under conditions normally found i n the cytoplasm, but is inhibited as GSSG accumulates to give a redox poten tial similar to that found in the lumen of the ER. Site-directed mutations show that Cys 353 and Cys 339 participate in the redox regulation. Cys 353 is the principal residue in the redox-sensitive switch. We hypothesize that it may initiate formation of a mixed disulfide that is partially inhibitor y to yFMO. The mixed disulfide may exchange with Cys 339 to form an intramo lecular disulfide bond that is fully inhibitory, (C) 2000 Academic Press.