An Oxidoreductive Pathway for D-Xylose Assimilation by Rhodosporidium toruloides

Abstract

Extracts of Rhodosporidium toruloides 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 K m 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 R. toruloides grown on xylose has the necessary enzymes to convert xylose to xylulose by the oxidoreductive pathway.