Interaction of the vitamin D-binding protein (group-specific component) and its ligand 25-hydroxy-vitamin D3: binding differences of the various genetic types disclosed by isoelectric focusing

The three common variants of the vitamin D binding protein, also known as group specific component (Gc), namely types 1S, 1F and 2, as well as some rare variants were studied by thin-layer polyacrylamide gel isoelectric focusing in a pH 4.5-5.4 carrier ampholyte generated pH gradient, additionally containing N-(2-acetamido)-2-aminoethanesulfonic acid (ACES). Prior to isoelectric focusing, whole serum or purified preparations of the vitamin D binding protein were incubated with 25-hydroxycholecalciferol at various ligand/protein ratios. Binding differences were found for the anodal and cathodal isoforms of Gc 1 variants and also for various allelic types. Isoforms with higher isoelectric points generally had a lower affinity for the ligand than the variants with lower isoelectric points.