Characterization of the oligosaccharides of liver Z variant α₁-antitrypsin

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., et al. (1976) Hodges, L.C., Laine, R. & Chan, S.K. (1979). 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., et al. (1978). 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 C(II) (endo C(II)) 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.