Direct oxidation of NADPH by submitochondrial particles from Saccharomyces cerevisiae

It has been accepted that in Saccharomyces cerevisiae submitochondrial particles do not oxidize the NADPH and that the NADPH:cytochrome c reductase is not a mitochondrial enzyme but rather a microsomal one. The present study provides clear evidence that in S. cerevisiae a direct oxidation of NADPH occurs through the mitochondrial electron transport system. The following results wee obtained: submitochondrial particles from S. cerevisiae are capable of oxidizing NADPH with a relatively high rate. The oxidation of NADPH is sensitive to antimycin A and NaN3 but insensitive to rotenone as is known for NADPH oxidation. Also NADPH:cytochrome c reductase activity is inhibited by antimycin A. NADPH-induced reduction of cytochromes b, c + c1, and aa3 is as fast as NADPH-induced reduction. Cytochromes are reduced to the same extent with either NADH or NADPH. The changes of the ratio of NADH/NADPH oxidation rate and the ratio of NADH K3Fe(CN)6/NADPH-K3Fe(CN)6 reductase activities at various phases of growth suggest that two distinct pyridine nucleotide dehydrogenases could be responsible for NADH and NADPH oxidation. This problem remains to be elucidated.