Oligosaccharide mapping reveals hormone-specific glycosylation patterns on equine gonadotropin alpha-subunit Asn56

Equine gonadotropin alpha-subunit glycosylation was examined by releasing oligosaccharides using a sequential enzymatic deglycosylation protocol and comparing the released oligosaccharide populations using a high resolution oligosaccharide mapping technique. Digestion of native alpha-subunit preparations with peptide-N-glycosidase altered their mobilities during SDS-PAGE under reducing conditions to positions intermediate between the corresponding native alpha-subunit and completely deglycosylated alpha-subunit bands. Complete alpha-subunit deglycosylation required reduction of disulfide bonds. Results of solid-phase Edman degradation demonstrated that partial deglycosylation occurred exclusively at Asn56. Oligosaccharide mapping of total oligosaccharides obtained by enzymatic deglycosylation of reduced, carboxymethylated alpha-subunit preparations revealed hormone-specific patterns of glycosylation in eLH alpha and eCG alpha. Oligosaccharide mapping of individual glycosylation sites revealed that hormone-specific glycosylation was primarily restricted to Asn56 of both subunit preparations and revealed a hormone-specific pattern of Asn56 glycosylation in eFSH alpha that was obscured in the total oligosaccharide map. eLH alpha Asn56 oligosaccharides appeared to be primarily seven variants of a monoantennary structure. eCG alpha Asn56 oligosaccharides consisted of one of two forms, either a sialylated biantennary oligosaccharide that appeared identical to a commercial carbohydrate standard or a lactosamine variant of that structure.