Determination of the primary structures of 16 asialo-carbohydrate units derived from human plasma α1-acid glycoprotein by 360-MHz 1H NMR spectroscopy and permethylation analysis

Abstract

The present study was initiated in order to determine the primary structures of the carbohydrate units of desialyzed human plasma al-acid glycoprotein. In an earlier investigation I6 glycopeptides which originated from four of the five glycosylation sites of this protein were prepared in the homogeneous state. Since the elucidation of the structures of such a large number of carbohydrate units by the conventional chemical and enzymatic procedures would require a very long period of time and considerable amounts of material, a new method-360-MHz H NMR spectroscopy-in combination with methylation analysis and partial acetolysis was introduced i n the present study. This highly sensitive method allowed the elucidation of the structures of the carbohydrate units of the mentioned glycopeptides to be carried out in a very short period of time. The present investigation revealed that the structures of the 16 heteroglycans can be grouped into five classes, and, except for those of two classes, the structures are new. Two classes designated A and B possess the known bi- and trimtennary structures, respectively. A third class designated C proved to be tetraantennary in its structure, whereas the other classes designated BF and CF are, in comparison with the structures of classes B and C, characterized by an additional fucose residue in a hitherto unknown position and linkage. The latter monosaccharide residue is attached in an a( 1-3) bond, vicinal to Gal@(1 +4), to the GlcNAc residue 7. The present study also revealed that each glycosylation site of pooled a]-acid glycoprotein possesses carbohydrate units with different structures.