Results

Three oligosaccharide fractions were resolved by anion exchange, a neutral component (N, 36.0"/0), and two acidic components (Al, 54.5% and A2, 9.5%). The latter eluted from the anion-exchange column with the same retention times as authentic monosialyl-and disialyl-oligosaccharides, respectively. Methylation analysis of A1 and A2 identified.?-linked galactose and 3-linked N-acetylgalactosaminitol (A I), and 3-linked galactose and 3, 6-linked N-acetylgalactosaminitol (A2). Analysis of per-trimethylsilyl glycosides by gc-ms after mild acid treatment [7] identified N-acetylneuraminic acid as the only sialic acid present in A 1 and A2.

Digestion with Arrherohucrer ureufucieris neuraminidasc converted all the oligosaccharides to neutral. Bio-Gel P4 chromatography identified two components in the neutral (N) fraction, eluting at 3.5 glucosc units (gu),(N2, 48.7'XI) and 2.5 gu (N 1. 5 I. 3'X1), while in the acidic fractions following desialylation only a 3.5 gu of glycan was observed. All the 3.5 gu oligosaccharides eluted at 2.5 gu from Bio-Gel P4 following digestion with bovine epididymal, 5-galactosidase. The naturally occurring 2.5 gu saccharide migrated on borate high-voltage electrophoresis to a position consistent with N-acctylgalactosaminitol. Based on thcse data, the structures in Table 1 are proposed for the 0-linkcd oligosaccharides of human serum IgA 1. The molar incidence of each saccharide was calculated from quantification of the radiolabcl in the fractions obtained by anion-exchange chromatography and of thc incidcnce of the 2.5 and 3.5 gu glycans resolved on Bio-Gel P4 obtaincd from the naturally neutral oligosaccharides.