Inhibition by nojirimycin and 1-deoxynojirimycin of microsomal glucosidases from calf liver acting on the glycoprotein oligosaccharides Glc1-3Man9GlcNAc2.

Particulate membrane fractions from calf liver catalyze the release of glucose from GlcNAc2-Man9-Glc1-3-oligosaccharides. Maximal oligosaccharide-glucosidase activity was obtained at pH 6.2 and a detergent concentration of 0.5% Triton X-100. This activity could be distinguished from non-specific alpha-glucosidase activity on the basis of different pH-dependence and lack of activation by detergent. The relative rates for the hydrolysis of the Glc3-, Glc2-, and Glc1-oligosaccharide, estimated from the initial velocity, was 1:12:3. There is no significant difference in the enzyme activity towards free, peptide-bound, or lipid-linked oligosaccharide. Nojirimycin and 1-deoxynojirimycin were strong inhibitors of microsomal oligosaccharide-glucosidases. Hydrolysis of Glc3-oligosaccharide was inhibited by 50% at concentrations of 0.16 mM and 2 microM, respectively. Hydrolysis of the Glc2- and Glc1-oligosaccharide was inhibited to a somewhat lower extent, suggesting the presence of at least two glucosidases, one acting on Glc3- and one acting on Glc1- and Glc2-oligosaccharide.