FIGURE 1.

Scheme illustrating the reactions that produce cytosolic and mitochondrial NADPH in S. cerevisiae. In the cytosol, Zwf1p, Ald6p, and Idp2p are considered to be significant for the generation of NADPH from NADP⁺ (35, 36). Cytosolic NADP⁺ is mainly produced by Utr1p, but not by Yef1p (7, 10). Cytosolic NADH kinase reactions are not depicted due to the negligible contribution of the NADH kinase activity of Utr1p or Yef1p to the cytosolic production of NADPH from NADH (5). In mitochondria, the NADH kinase reaction catalyzed by Pos5p has been proposed to be important (6). In this study, we demonstrated that, in addition to the NADH kinase reaction, the NAD kinase reaction catalyzed by Pos5p followed by dehydrogenase reactions catalyzed by Ald4/Ald5p (particularly Ald4p), but not Idp1p and Mae1p, is also critical to the supply of mitochondrial NADPH. The viability of the utr1yef1 double mutant implies that cytosolic NADP⁺ can be supplied by Pos5p (5, 7).