Carbohydrate structure of heterogeneity of α₁antitrypsin

Three unique tryptic glycopeptides have been isolated from human α₁antitrypsin and their amino acid sequences determined. The recognition sequences for the attachment of carbohydrate were found to be Asn-Ser-Thr, Asn-Leu-Thr and Asn-Ala-Thr. These were positioned within the total amino acid sequence of human α₁antitrypsin at Asn₄₆, Asn₈₃and Asn₂₄₇ respectively. The carbohydrate composition of human α₁antitrypsin can be explained fully in terms of the three glycosylation sites. In addition, only one other potential recognition site for glycosylation exists, an Asn-Pro-Thr at the C-terminus of αiantitrypsin and this site is not glycosylated. αiAntitrypsin exhibits microheterogeneity, a characteristic common to glycoproteins, which can be primarily explained by the existence of isoforms. These are variant forms of neither genetic nor post-secretory origin, which differ in the degree of branching of their oligosaccharide chains. This microheterogeneity is evident on isoelectric focusing as three major and several minor bands. The carbohydrate structures of the major bands were identified; band 6 (isoform I) has three biantennary oligosaccharide chains, band 4 (isoform II) has two bi- and one triantennary oligosaccharide chains and band 2 (isoform III) has one bi- and two triantennary chains. The tryptic glycopeptides were isolated from isoforms I and II. their carbohydrate compositions determined and it was shown that in isoform II only the middle attachment site (Asn₈₃) carried the triantennary oligosaccharide chain. The identity of the isoforms with the bands evident on isoelectric focusing has enabled their measurement in plasma by photometric scanning of the electrofocused gels. In healthy controls the levels of isoforms I, II and III were relatively constant and in the proportions of 5, 4 and 1 respectively. A marked change occurred during inflammation and oestrogen stress with isoforms II and III accounting for most of the increase in α₁antitrypsin. One possible consequence of the changed proportions was shown to be the increased catabolism of partially desialylated triantennary isoforms compared to that of the predominant biantennary form of the healthy individual.