Isolation of a homogeneous glucosidase II from pig kidney microsomes.

The processing of the oligosaccharide precursor chain, (GlcNAc)2(Man)9(Glc)3, of N-glycosylated glycoproteins starts with the action of glucosidase I which excises the terminal (alpha 1-2)-linked glucose residue. Glucosidase II removes the two inner (alpha 1-3)-linked glucose residues. We have purified glucosidase II to homogeneity from pig kidney microsomes. The enzyme is a glycoprotein and contains a single type of subunit of molecular mass approximately equal to 100 kDa. The native enzyme is probably a tetramer. It cleaves glucosidic alpha 1-3 and alpha 1-4, but not alpha 1-1, alpha 1-2 or alpha 1-6 bonds and lacks alpha-mannosidase and glucosidase I activity. The pH optimum is between 6.0 and 7.5. Specific antibodies against the native enzyme and the denatured subunit were prepared. By activity measurements and immune blotting, a similar enzyme was found in rat liver. In the fractionated rat liver, the enzyme was localized in the lumen of the endoplasmic reticulum, probably loosely bound to the inner face of the membrane. Purified Golgi fractions contained only low levels of the enzyme.