Association of the Macrophage Activating Factor (MAF) Precursor Activity with Polymorphism in Vitamin D-binding Protein

Abstract

Background: Serum vitamin D-binding protein (Gc protein or DBP) is a highly expressed polymorphic protein, which is a precursor of the inflammation-primed macrophage activating factor, GcMAF, by a cascade of carbohydrate processing reactions. In order to elucidate the relationship between Gc polymorphism and GcMAF precursor activity, we estimated the phagocytic ability of three homotypes of Gc protein, Gc1F-1F, Gc1S-1S and Gc2-2, through processing of their carbohydrate moiety. Materials and Methods: We performed Gc typing of human serum samples by isoelectric focusing (IEF). Gc protein from human serum was purified by affinity chromatography with 25-hydroxyvitamin D₃-sepharose. A phagocytosis assay of Gc proteins, modified using β-glycosidase and sialidase, was carried out. Results: The Gc1F-1F phenotype was revealed to possess Galβ1-4GalNAc linkage by the analysis of GcMAF precursor activity using β1-4 linkage-specific galactosidase from jack bean. The GcMAF precursor activity of the Gc1F-1F phenotype was highest among three Gc homotypes. Conclusion: The Gc polymorphism and carbohydrate diversity of Gc protein are significant for its pleiotropic effects.