CYTOCHROME-P-450 REDUCTASE IS RESPONSIBLE FOR THE FERRIREDUCTASE ACTIVITY ASSOCIATED WITH ISOLATED PLASMA-MEMBRANES OF SACCHAROMYCES-CEREVISIAE

Cytochrome P-450 reductase (encoded by the NCP1 gene) was found to cat alyse all the NADPH-dependent ferrireductase activities associated wit h isolated plasma membranes of the yeast Saccharomyces cerevisiae. We therefore examined the contribution of this enzyme to the ferrireducta se activity of cells in vivo. Cytochrome P-450 reductase was shown to be not essential for the cell ferrireductase activity, but it influenc ed this activity, with different effects on the Fre1- and the Fre2-dep endent reductase systems. Overexpression of FRE1 did not lead to an in creased ferrireductase activity of the cells when NCP1 was repressed. In contrast, cells that overexpressed FRE2 had maximal ferrireductase activity when NCP1 was repressed. The degree of NCP1 expression also a ffected the amount of iron and copper accumulated by the cells during growth. The biochemical implications and the physiological significanc e of these observations are discussed.