The mitochondrial alcohol dehydrogenase adh3p is involved in a redox shuttle in Saccharomyces cerevisiae

NDI1 is the unique gene encoding the internal mitochondrial NADH dehydrogen ase of Saccharomyces cerevisiae, The enzyme catalyzes the transfer of elect rons from intramitochondrial NADH to ubiquinone. Surprisingly, NDI1 is not essential for respiratory growth, Here we demonstrate that this is due to i n vive activity of an ethanol-acetaldehyde redox shuttle, which transfers t he redox equivalents from the mitochondria to the cytosol, Cytosolic NADH c an be oxidized by the external NADH dehydrogenases. Deletion of ADH3, encod ing mitochondrial alcohol dehydrogenase, did not affect respiratory growth in aerobic, glucose-limited chemostat cultures. Also, an ndil Delta mutant was capable of respiratory growth under these conditions. However, when bot h ADH3 and NDI1 were deleted, metabolism became respirofermentative, indica ting that the ethanol-acetaldehyde shuttle is essential for respiratory gro wth of the ndil Delta. mutant. In anaerobic batch cultures, the maximum spe cific growth rate of the adh3 Delta mutant (0.22 h(-1)) was substantially r educed compared to that of the wild-type strain (0.33 h(-1)). This is consi stent with the hypothesis that the ethanol-acetaldehyde shuttle is also inv olved in maintenance of the mitochondrial redox balance under anaerobic con ditions. Finally, it is shown that another mitochondrial alcohol dehydrogen ase is active in the adh3 Delta ndil Delta mutant, contributing to residual redox-shuttle activity in this strain.