To clarify the relationship between NAD(P)(+)/NAD(P)H redox balances and the metabolisms of xylose or xylitol as carbon sources, we analyzed aerobic and anaerobic batch cultures of recombinant Saccharomyces cerevisiae in a complex medium containing 20 g/L xylose or 20 g/L xylitol at pH 5.0 and 30 degrees C. The TDH3p-GAL2 or gal80 Delta strain completely consumed the xylose within 24 h and aerobically consumed 92-100% of the xylitol within 96 h, but anaerobically consumed only 20% of the xylitol within 96 h. Cells of both strains grew well in aerobic culture. The addition of acetaldehyde (an effective oxidizer of NADH) increased the xylitol consumption by the anaerobically cultured strain. These results indicate that in anaerobic culture, NAD(+) generated in the NAD(P)H-dependent xylose reductase reaction was likely needed in the NAD(+)-dependent xylitol dehydrogenase reaction, whereas in aerobic culture, the NAD(+) generated by oxidation of NADH in the mitochondria is required in the xylitol dehydrogenase reaction. The role of Gal2 and Fps1 in importing xylitol into the cytosol and exporting it from the cells was analyzed by examining the xylitol consumption in aerobic culture and the export of xylitol metabolized from xylose in anaerobic culture, respectively. The xylitol consumptions of gal80 Delta gal2 Delta and gal80 Delta gal2 Delta fps1 Delta strains were reduced by 81% and 88% respectively, relative to the gal80 Delta strain. The maximum xylitol concentration accumulated by the gal80 Delta gal80 Delta gal2 Delta, and gal80 Delta gal2 Delta fps1 Delta strains was 7.25 g/L, 5.30 g/L, and 4.27 g/L respectively, indicating that Gal2 and Fps1 transport xylitol both inward and outward. (C) 2017, The Society for Biotechnology, Japan. All rights reserved.