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.