A cytosolic aldo-keto reductase was purified from Saccharomyces cerevisiae ATCC 26602 to homogeneity by affinity chromatography, chromatofocusing, and hydroxylapatite chromatography. The relative molecular weights of the aldo-keto reductase as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and size exclusion chromatography were 36,800 and 35,000, respectively, indicating that the enzyme is monomeric. Amino acid composition and N-terminal sequence analysis revealed that the enzyme is closely related to the aldose reductases of xylose-fermenting yeasts and mammalian tissues. The enzyme was apparently immunologically unrelated to the aldose reductases of other xylose-fermenting yeasts. The aldo-keto reductase is NADPH specific and catalyzes the reduction of a variety of aldehydes. The best substrate for the enzyme is the aromatic aldehyde p-nitrobenzaldehyde (K m ‫؍‬ 46 M; k cat /K m ‫؍‬ 52,100 s ؊1 M ؊1), whereas among the aldoses, DL-glyceraldehyde was the preferred substrate (K m ‫؍‬ 1.44 mM; k cat /K m ‫؍‬ 1,790 s ؊1 M ؊1). The enzyme failed to catalyze the reduction of menadione and p-benzoquinone, substrates for carbonyl reductase. The enzyme was inhibited only slightly by 2 mM sodium valproate and was activated by pyridoxal 5-phosphate. The optimum pH of the enzyme is 5. These data indicate that the S. cerevisiae aldo-keto reductase is a monomeric NADPH-specific reductase with strong similarities to the aldose reductases.

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Although it is generally accepted that Saccharomyces cerevisiae is unable to assimilate D-XylOSe, four strains were found to utilize xylose aerobically at different efficiencies in the presence of a mixture of substrates. The degree of D-xylose utilization by S. cerevisiae ATCC 26602 depended upon the presence of other substrates or yeast extract. The greatest amount of xylose (up to 69% over 7 d) was utilized when sugar substrates such as D-ribose were co-metabolized. Much lower degrees of utilization occurred with co-metabolism of organic acids, polyols or ethanol. A mixture of D-glucose, D-ri bose, D-raffinose, glycerol and D-xylose resulted in greater xylose utilization than the presence of a single substrate and xylose. The absence of growth on a co-substrate alone did not prevent the utilization of xylose in its presence. Xylose was co-metabolized with ribose under anaerobic conditions but at a much slower rate than under aerobic conditions. When [ 14C]xylose was utilized in the presence of ribose under anaerobic conditions, the radioactive label was detected mainly in xylitol and not in the small amounts of ethanol produced. Under aerobic conditions the radioactive label was distributed between xylitol(91.3 0-8%), COz (2.6 & 2.3%) and biomass (1.7 & 0.6%). No other metabolic products were detected. Whereas most xylose was dissimilated rather than assimilated by S. cerevisiae, the organism apparently possesses a pathway which completely oxidizes xylose in the presence of another substrate.

The influence of D-ribose as a cosubstrate on the uptake and metabolism of the non-growth substrate D-xylose by Saccharomyces cerevisiae ATCC 26602 was investigated. Xylose was taken up by means of low-and high-affinity glucose transport systems. In cells exposed for 2 days to a mixture of xylose and ribose, only the high-affinity system could be detected. Glucose strongly inhibited the transport of xylose by both systems. Starvation or exposure to either xylose or ribose resulted in inactivation of xylose transport, which did not occur in the presence of a mixture of ribose and xylose. A constitutive non-glucose-repressible NADPH2dependent xylose reductase with a specific activity of ca. 5 mU/mg of protein that converted xylose to xylitol was present in a glucose-grown culture. No activity converting xylitol to xylulose or vice versa was found in crude extracts. Both xylose and ribose were converted to their corresponding polyols, xylitol and ribitol, as indicated by '3C nuclear magnetic resonance spectroscopy. Furthermore, ethanol was detected, and this implied that pathways for the complete catabolism of xylose and ribose exist. However, the NADPH2 required for the conversion of xylose to xylitol is apparently not supplied by the pentose phosphate pathway since the ethanol produced from D-[1-13C]xylose was labelled only in the C-2 position. Acetic acid was produced from ribose and may assist in the conversion of xylose to xylitol by cycling NADPH2.