Abstract
The plasma α1-antitrypsin (AT) of normal individuals has been reported to have four oligosaccharides per molecule; these oligosaccharides are all of the complex type, containing only three D-mannose (Man) residues per oligosaccharide as well as several residues of galactose, sialic acid, and N-acetyl-D-glucosamine (GlcNAc) (Chan, S. K., Rees, D. C., Li, S.-C. &Li, Y.-T. (1976) J. Biol. Chem. 251, 471–476; Hodges, L. C., Laine, R. &Chan, S. K. (1979) J. Biol. Chem. 254, 8208–8212). Analysis of the carbohydrates of purified liver Z variant AT was consistent with the presence of three moles of Man-rich oligosaccharide per mole protein with an average of seven Man and two GlcNAc residues per oligosaccharide and a complete absence of galactose and sialic acid (Hercz, A., Katona, E., Cutz, E., Wilson, J. R., &Barton, M. (1978) Science 201, 1229–1232). We have now carried out a more definitive structural analysis of the oligosaccharides of liver Z variant. Glycopeptides were prepared from purified liver Z variant by digestion with pronase. Treatment of a sample of the glycopeptides with endo-β-N-acetylglycosaminidase CII (endo CII) resulted in the formation of three oligosaccharides. These were identified by gel permeation chromatography on a Biogel P-4 column, calibrated with a series of ovalbumin and Sindbis virus oligosaccharides, as GlcNAcMan5, GlcNAcMan6, and GlcNAcMan7. Treatment of a sample of the glycopeptides with endo-β-N-acetylglycosaminidase D gave rise only to a single oligosaccharide product, GlcNAcMan5. Upon incubation of the glycopeptides with UDP[U-14C]GlcNAc in the presence of purified bovine colostrum UDPGlcNAc:α-D-mannoside α-2-N-acetylglucosaminyltransferase I, AsnGlcNAc2Man5 quantitatively disappeared and a new product consistent with the structure AsnGlcNAc2Man5[14C]GlcNAc was formed. Incubation of the oligosaccharides, formed with endo CII, with α-mannosidase gave rise to a 90% homogeneous product with the probable structure of Manβ1 → 4GlcNAc. These findings suggest that the precursors of the complex oligosaccharides of the secretory glycoprotein, plasma AT, are Man-rich oligosaccharides. It appears also that the glycosidases which process the precursor to the AsnGlcNAc2Man5 stage are present and active in the rough endoplasmic reticulum of Pi (protease inhibitor) ZZ individuals; however, for an as yet undetermined reason, most of the Z variant AT appears unable to migrate at a normal rate to the smooth endoplasmic reticulum and Golgi apparatus for eventual secretion from the liver.
Publisher
Canadian Science Publishing