Glycosylation of three molecular forms of human alpha 1-acid glycoprotein having different interactions with concanavalin A. Variations in the occurrence of di-, tri-, and tetraantennary glycans and the degree of sialylation.

Human alpha 1-acid glycoprotein (AGP) was separated into a non-bound (AGP-A; 46%), a retarded (AGP-B; 39%) and a bound fraction (AGP-C; 15%) using concanavalin A (ConA)-Sepharose chromatography. The apparent molecular masses, as determined by SDS-PAGE, of the three fractions were 43.5, 42.3 and 41.2 kDa, respectively. The occurrence of N-linked di-, tri- and tetraantennary glycans on these three molecular forms (AGP-A, -B, and -C) was studied by sequential lectin-affinity chromatography of the 14C-labelled glycopeptides. These were obtained by extensive pronase treatment followed by N-[14C]acetylation of the peptide moieties. The glycopeptides of AGP-A did not bind to ConA-Sepharose whereas for AGP-B and AGP-C 18% and 44%, respectively, of the glycopeptides were bound as diantennary structures. Glycopeptide fractions of all three forms of AGP which were not bound to ConA-Sepharose were shown to contain equal amounts of both tri- and tetraantennary glycans by chromatography with Phaseolus vulgaris leukoagglutinating lectin (L-PHA). With the assumption that each molecule contains five glycosylation sites, it could be shown that AGP-A contains no diantennary structures whereas AGP-B and AGP-C contain one and two diantennary structures, respectively. In addition each of the molecular forms contains equal amounts of tri- and tetraantennary structures on the remaining glycosylation sites. The results of this study, therefore, exclude a uniformity of glycan chains in the three molecular forms of AGP. The degree of sialylation of each of the molecular forms was investigated by chromatography on L-PHA-agarose and Ricinus communis agglutinin-I--agarose both before and after desialylation of the glycopeptides. It was shown that about 90% of the biantennary glycans of both AGP-B and AGP-C were disialylated while the remainder were monosialylated. The degree of sialylation of the tri- and tetraantennary glycans was identical for the three molecular forms. In each case, one or more terminal galactose residues occurred on at least 20% of the tri- and 65% of the tetraantennary chains. It is suggested that the decrease in the exposure of galactose residues from AGP-A to AGP-C is related to the concomittant decrease in branching of the glycans of the three molecular forms. The relevance of these findings to studies on the function of AGP during inflammatory and liver diseases is discussed.