Cryptococcal UDP-glucose dehydrogenase: enzymic control of capsular biosynthesis.

UDP-Glucuronate, formed through dehydrogenation of UDP-glucose and itself decarboxylated to make UDP-xylose, is the presumed donor for the glucuronyl side-groups in biosynthesis of the capsular polysaccharide in Cryptococcus neoformans. A specific radiochromatographic assay for UDP-glucose dehydrogenase shows that the enzyme is present in the cytosol. The enzyme is very labile but is stabilized by 25% glycerol. The enzyme is inhibited strongly by NADH (Ki,NADH = Km,NAD = 0.1 to 0.2 mM). It is also inhibited competitively by UDP-xylose (Ki = 0.3 mM). These results suggest that the rate of production of the capsular precursor, UDP-glucuronate, is controlled by the intracellular concentrations of an end product, UDP-xylose, and of a direct product, NADH. Capsule mutants have been screened for activity of this enzyme but none is clearly deficient.