Extracts of grown aerobically on xylose contained xylitol dehydrogenase and D-xylose reductase activities. Extracts of cells grown on glucose contained one-tenth as much xylose reductase and no detectable xylitol dehydrogenase. The xylitol dehydrogenase was purified to near homogeneity, and is a tetramer of 45 kDa subunits. This labile enzyme, could be stabilized by glycerol (25%) and was rapidly inactivated by 10 mM-EDTA. It catalyses the reversible, NAD-dependent oxidation of xylitol to xylulose. Apparent values are 19 mM-xylitol and 0·3 mM-NAD at 30°C, pH 8·5. Partially purified preparations of xylose reductase catalysed the NADPH-dependent reduction of D-xylose to xylitol, and were 16 times as active with 33 mM-DL-glyceraldehyde as with 33 mM-D-xylose. Apparently grown on xylose has the necessary enzymes to convert xylose to xylulose by the oxidoreductive pathway.


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