THE SACCHAROMYCES-CEREVISIAE NDE1 AND NDE2 GENES ENCODE SEPARATE MITOCHONDRIAL NADH DEHYDROGENASES CATALYZING THE OXIDATION OF CYTOSOLIC NADH

In Saccharomyces cerevisiae, the NDI1 gene encodes a mitochondrial NAD H dehydrogenase, the catalytic side of which projects to the matrix si de of the inner mitochondrial membrane, In addition to this NADH dehyd rogenase, S. cerevisiae exhibits another mitochondrial NADH-dehydrogen ase activity, which oxidizes NADH at the cytosolic side of the inner m embrane. To investigate whether open reading frames YMR145c/NDE1 and Y DL 085w/NDE2, which exhibit sequence similarity with NDI1, encode the latter enzyme, NADH-dependent mitochondrial respiration was assayed in wild-type S. cerevisiae and nde deletion mutants. Mitochondria were i solated from aerobic, glucose-limited chemostat cultures grown at a di lution rate (D) of 0.10 h(-1), in which reoxidation of cytosolic NADH by wild-type cells occurred exclusively by respiration. Compared with the wild type, rates of mitochondrial NADH oxidation were about 3-fold reduced in an nde1 Delta mutant and unaffected in an nde2 mutant. NAD H-dependent mitochondrial respiration was completely abolished in an n de1 Delta nde2 Delta double mutant. Mitochondrial respiration of subst rates other than NADH was not affected in nde mutants, In shake flasks , an nde1 Delta nde2 Delta mutant exhibited reduced specific growth ra tes on ethanol and galactose but not on glucose, Glucose metabolism in aerobic, glucose-limited chemostat cultures (D = 0.10 h(-1)) of an nd e1 Delta nde2 Delta mutant was essentially respiratory. Apparently, un der these conditions alternative systems for reoxidation of cytosolic NADH could replace the role of Nde1p and Nde2p in S. cerevisiae,