Xylose metabolism in Pachysolen tannophilus: purification and properties of xylose reductase

Abstract

Xylose reductase (xylitol: NADP oxidoreductase, EC 1.1.1.139) has been purified from D-xylose grown cells of the yeast Pachysolen tannophilus by application of DEAE-cellulose ion exchange chromatography, 2′,5′-ADP-Sepharose affinity chromatography, Biogel P200 gel filtration, and dextran blue Sepharose chromatography to approximately 95% homogeneity. It consists of a single polypeptide chain with a relative molecular weight of 35 000–40 000 and an isoelectric point of pH 4.9. The enzyme has a broad substrate specificity similar to that of aldose (or aldehyde) reductases from mammalian tissues. It exhibits Michaelis–Menten type kinetics (Km D-xylose, 162 mM; Km D-xylitol, 212 mM; Km NADPH, 0.059 mM; [Formula: see text], 0.071 mM). The enzyme is specific for NADPH; activity with NADH is below 0.5% of Vmax observed with NADPH. The reduction of xylose is inhibited by NADP, the anabolic reduction charge (NADPH/NADP + NADPH), and also in a complex manner by ATP. At physiological pH values the equilibrium is Keq = 10−10. The importance of these findings for the physiology of xylose fermentation by this yeast is discussed.