PLASMA VITAMIN-D-BINDING PROTEIN (GC-GLOBULIN) - MULTIPLE TASKS

The transporter of vitamin D and its metabolites in blood has received increasing attention in recent years, and is recognized to be a membe r of a gene family that includes albumin and a-fetoprotein. Identical to the group specific component (Gc-globulin) of serum, the protein is a single-chain polypeptide constitutively synthesized in liver that c irculates in amounts in far excess of normal vitamin D metabolite conc entrations in blood. It plays the major role in the egress of endogeno usly synthesized vitamin D, from skin and appears to restrain D-sterol s from too rapid/excessive cell entry. Along with plasma gelsolin, it comprises the plasma actin-scavenger system that facilitates removal o f actin, liberated from lysed cells, by depolymerization and preventio n of polymerization. Recently, the protein has been shown to behave as a co-chemotaxin specific for the complement peptide C5a, and its sial ic acid-free form has been reported to play a role in macrophage activ ation. The latter functions strongly implicate its participation in in flammation responses. A unifying hypothesis might also suggest the pro tein to provide focal D-sterol delivery to cells that are important to the resolution of tissue injuries.