ISOLATION AND CHARACTERIZATION OF 2 XYLITOL DEHYDROGENASES FROM ASPERGILLUS-NIGER

An NADPH-dependent L-xylulose reductase [xylitol:NADP(+) 4-oxidoreductase; EC 1.1.1.10 (LXDH)] from Aspergillus niger was purified and characterized. It is an octamer with a monomeric molecular mass of 32 kDa, showing high specificity for L-xylulose, xylitol and NADP(H). The K-m values for L-xylulose and xylitol are relatively high (16.5 and 925 mM, respectively). An NAD(+)-dependent xylitol dehydrogenase [xylitol:NAD(+) 2-oxidoreductase; EC 1.1.1.9(DXDH)] was partially purified from the same fungus. It has a monomeric molecular mass of 40 kDa and shows a high specificity for D-xylulose, xylitol and MAD(H). The K-m values for D-xylulose and xylitol are relatively low (approximately 4 and 50 mM, respectively). The reactivity towards xylitol, the product or substrate these enzymes have in common, confirms their role in the L-arabinose catabolic pathway.