Characterization of two cyano glucosyltransferases from cassava leaves

The terminal step in the biosynthesis of cyanogenic glucosides is the glucosylation of labile hydroxynitriles by UDP-glucose: hydroxynitrile-beta-D-glucosyltransferases. These enzymes have not been characterized previously because this reaction is strongly affected by the instability of the hydroxynitriles, which dissociate to carbonyls and cyanide. Dissociation constants of cyanohydrins in aqueous solutions have been determined. Using these constants we are able to calculate the equilibrium concentrations of undissociated hydroxynitriles at a specific pH and thereby to determine the K-m for several cyanohydrins of glucosyltransferases that synthesize cyanogenic glucosides. Two soluble glucosyltransferases occurring in cassava leaves were separated by chromatographic methods and their kinetic properties determined. The two glucosyltransferases could be distinguished by different K-m-values, supporting the assumption that they represent distinct isoenzymes.