Nicotinamide Adenine Dinucleotide Phosphate-linked Xylitol Dehydrogenase in Guinea Pig Liver Cytosol

Abstract

Abstract Guinea pig liver has previously been reported to contain a highly specific NADP-linked xylitol dehydrogenase (EC 1.1.1.10) and NAD-linked xylitol dehydrogenases (EC 1.1.1.9) of broad substrate specificity. The NADP-linked enzyme, which interconverts xylitol and L-xylulose, has been obtained from mitochondria, while NAD-linked xylitol dehydrogenases, which interconvert xylitol and D-xylulose, have been obtained from both mitochondria and cytosol. This report describes a fourth xylitol dehydrogenase activity, which occurs in the cytosol of guinea pig liver. This enzyme is similar to the mitochondrial NADP-linked xylitol dehydrogenase in its coenzyme requirement, substrate specificity, and Km for xylitol, and its activity is much higher than that of the mitochondrial enzyme. The two soluble xylitol dehydrogenases were separated from each other on the basis of differential adsorption on alumina Cγ gel. The occurrence of high activities of these enzymes in the cytosol allows the required conversion of L-xylulose to D-xylulose via xylitol in the glucoronate-xylulose pathway without involvement of the mitochondria. Questions regarding the origin and role of the mitochondrial xylitol dehydrogenases are discussed.