Microsomal Glucosidases of Rat Liver. Partial Purification and Inhibition by Disaccharides

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Abstract
Further work on microsomal glucosidases of rat liver has confirmed that at least 2 enzymes are involved in the removal of glucose from the glucose-containing oligosaccharide. One acts on the oligosaccharide containing 3 glucose residues and another on the oligosaccharide which has 1 or 2 glucoses. The glucosidase which acts on (Glc)2(Man)9(GlcNAc)2 could be purified with a Concanavalin-A-Sepharose column followed by electrofocusing. This purified preparation was active on the oligosaccharide containing 1 or 2 glucoses. Heat inactivation and inhibition by disaccharides were parallel for both activities. Inhibition of the glucosidase active on (Glc)3(Man)9(GlcNAc)2 was obtained with kojibiose which has an .alpha.1-2 linkage, while the glucosidase acting on (Glc)1-2(Man)9-(GlcNAc)2 was inhibited by nigerose (.alpha.1-3 linkage), maltose (.alpha.1-4 linkage) and glucose at a higher concentration. None of the .beta.-anomers inhibited glucosidase. These results are consistent with an .alpha. configuration of the 3 glucoses of the dolichyl-diphosphate-linked oligosaccharide. Kojibiose inhibited glucosidase action not only on the free but also on the protein-bound oligosaccharide.