Pichia stipitis (CBS 5773) most efficiently ferments the sugar mixture from acid hydrolysed spent brewer's grains, whereby D-xylose and D-galactose (but not L-arabinose) are consumed simultaneously, after the D-glucose had been used up from the mixture. The key enzymes, xylose reductase (Xyl-R) and xylitol dehydrogenase (Xit-DH) are inducible and cannot be found in glucose-grown cells. Purified Xyl-R was shown to use both NADH and NADPH as coenzyme, and model simulations showed that under aerobic physiological conditions NADPH is preferred (95%), whereas under oxygen deficiency 85% of xylose are reduced by NADH. Xit-DH seems to use only NAD + as coenzyme, and the reaction is competitively inhibited by NADH. According to the equilibrium constants for Xyl-R (5,5·10 9 1·Mol -1 ) and for Xit-DH (6,9·10 -11 Mol·1 -1 ) the equlibria of both reactions are distinctly shifted to xylitol, which may be excreted into the medium, especially when in the absence of oxygen a surplus of NADH cannot be dehydrogenated.

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