Comparative Affinity of the Major Genetic Variants of Human Group-specific Component (vitamin D-binding Protein) for 25-(OH) Vitamin D

Binding studies with [3H] 25-(OH) D3 were performed under a variety of conditions using Gc (Vitamin D-binding protein) purified from individuals displaying different phenotypes. No significant differences in affinity of binding were found between Gc1f, Gc1s and Gc2 allelle products in either homozygous or heterozygous individuals, nor between Gc1 anodal and Gc1 cathodal isotypes. Affinity was not significantly affected by different reaction temperatures (4, 22 or 37 degrees C), the presence or absence of Ca2+ ions, and binary and ternary interactions with G-actin and G-actin-DNase complexes respectively. However, reduction of pH caused a progressive decrease in binding with virtual abolition at pH less than or equal to 5.0. The latter might promote dissociation of D3 metabolites from Gc carrier protein in acidic compartments of cells.