Affinity Purification and Characterization of Glucosidase II from Pig Liver

Abstract

Glucosidase II has been purified from crude pig liver microsomes by a convenient procedure involving DEAE-Sephacel, Con A-Sepharose and affinity chromatography on N-5-carboxypentyl-1-deoxynojirimycin-AH-Sepharose. Specific binding of glucosidase II to the affinity matrix required its prior separation from glucosidase I, which was accomplished by fractional Con A-Sepharose chromatography. The three-step procedure yielded, with approximately 15% enzyme recovery, a > 190-fold enriched glucosidase II, consisting of two proteins (107 kDa and 112 kDa). Both polypeptides are N-glycosylated with probably one glycan chain, in line with their binding to Con A-Sepharose. Immunological cross-reactivity and other experimental data indicate that the 107 kDa N-glycoprotein is derived from the 112 kDa species by partial proteolysis. The occasional presence of a 60 kDa peptide co-eluting with the catalytic activity suggests that glucosidase II may be associated with other protein subunit(s) in a heteromeric membrane complex. Glucosidase II hydrolyzes the alpha1,3-glucosidic linkages in Glc(2-1)-Man9-GlcNAc2, as well as synthetic alpha-glucosides, efficiently but does not remove the distal alpha1,2-linked glucose in Glc3-Man9-GlcNAc2. The enzyme has a pH optimum close to 6.5 and is not metal ion-dependent. Catalytic activity is strongly inhibited by basic sugar analogues including 1-deoxynojirimycin (dNM; app. Ki approximately 7.0 microM), N-5-carboxypentyl-dNM (app. Ki approximately 32 microM) and castanospermine (app. Ki approximately 40 microM). Substitution of the 3-OH or 6-OH group in dNM by a fluoro group reduces the inhibitory potential drastically. We conclude from these observations that the two hydroxy groups are essential for inhibitor/substrate binding due to their ability to interfere as hydrogen bond donors. A polyclonal antibody raised against the 107 kDa polypeptide reacted specifically with two proteins from different cell types on Western blots. Their molecular masses were identical with those from pig liver microsomes, pointing to a highly conserved amino acid sequence of glucosidase II. This suggests that the variance in molecular mass for glucosidase II reported for the enzyme from other tissues and species may be due to partial proteolysis.