The activity of key enzymes in xylose-assimilating yeasts at different rates of oxygen transfer to the fermentation medium

The activities of xylitol dehydrogenase and xylose reductase in the yeasts Candida shehatae, C. didensiae, C intermediae, C tropicalis, Kluyveromyces marxianus, Pichia stipitis, P. guillermondii, Pachysolen tannophilus, and Torulopsis molishiama were studied at different oxygen transfer rates (OTRs) to the fermentation medium (0, 5, and 140 mmol O-2/(1 h)). The activities of these enzymes were maximum in the yeasts R stipitis and C shehatae. The xylitol dehydrogenase of all the yeasts was NAD(+)-dependent, irrespective of the intensity of aeration. The xylose reductase of the yeasts C didensiae, C intermediae, C tropicalis, Kl. marxianus, P guillermondii, and T molishiama was NADPH-dependent, whereas the xylose reductase of P. stipitis, C shehatae, and Pa. tannophilus was specific for both NADPH and NADH. The effect of OTR on the activities of the different forms of xylitol dehydrogenase and xylose reductase in xylose-assimilating yeasts is discussed.