The induction of xylose reductase and xylitol dehydrogenase activities on mixed sugars was investigated in the yeasts Pachysolen tannophilus and Pichia stipitis. Enzyme activities induced on D-xylose served as the controls. In both yeasts, D-glucose, D-mannose, and 2-deoxyglucose inhibited enzyme induction by D-xylose to various degrees. Cellobiose, L-arabinose, and D-galactose were not inhibitory. In liquid batch culture, P. tannophilus utilized D-glucose and D-mannose rapidly and preferentially over D-xylose, while D-galactose consumption was poor and lagged behind that of the pentose sugar. In P. stipitis, all three hexoses were used preferentially over D-xylose. The results showed that the repressibility of xylose reductase and xylitol dehydrogenase may limit the potential of yeast fermentation of pentose sugars in hydrolysates of lignocellulosic substrates.

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Electrophoretic analysis of three dehydrogenase and two reductase isozymes was performed on 18 commonly used yeast strains. Gene polymorphism was noted for all five isozyme activities. Based on the homology of banding patterns, two main groups of yeast strains were identified. No two yeast strains had identical banding patterns.