Population Studies of Vitamin D Binding Protein Microheterogeneity by Mass Spectrometry Lead to Characterization of its Genotype-Dependent O-Glycosylation Patterns

Mass spectrometric evidence presented here characterizes the genotype-dependent glycosylation patterns for each of the three major allele products of Vitamin D Binding Protein found in the general human population. Findings based on the analysis of over 100 individual plasma samples demonstrated that all DBP allele products, except GC*2, are modified (10-25 mol%) with a linear (NeuNAc) 1(Gal) 1(GalNAc) 1 trisaccharide and, to a much lesser extent (1-5 mol%) with a trisaccharide-independent (Gal) 1(GalNAc) 1 dissaccharide. GC*2 protein contains the disaccharide but remains completely free of the trisaccharide, even in heterozygous individuals possessing a second gene product that is modified with the trisaccharide. Thus, all allelic forms of DBP except GC*2 possess two independent O-glycosylation sites occupied by separate, yet consistently isomass oligosaccharides and, despite a consensus sequence, lack N-glycosylation.