Interaction of group-specific component (vitamin D-binding protein) with immobilized Cibacron blue F3-GA

Group-specific component (vitamin D-binding protein) was purified to homogeneity from human plasma by a three-step procedure involving pseudo-ligand affinity chromatography on immobilized Cibacron blue F3-GA followed by gel filtration and ion-exchange chromatography. Upon pseudo-ligand chromatography, Gc globulin was separated into two peaks. The first, which represented approx. 4% of the total Gc globulin, was eluted together with other alpha-globulins of similar Mr and/or pI, and the second (96% of Gc globulin) was clearly retarded. Collection of the latter provided a fraction 10-fold enriched in Gc globulin, with yields higher than 90%. Incubation of plasma with trace amounts of radioactively-labeled 25-OH vitamin D3 showed that the radioactivity coeluted with the first peak. In addition, after saturation with 25-OH vitamin D3, all the Gc globulin was eluted in the first peak. This indicates that the two peaks correspond to the holo and the apo forms of the protein, respectively, and suggests that either the interaction of the apo form with the Cibacron blue dye involves the binding site for vitamin D metabolites, or that the holo-protein undergoes a conformational change as a consequence of formation of the complex.