Displacement Potency of Vitamin D2 Analogs in Competitive Protein-binding Assays for 25-hydroxyvitamin D3, 24,25-dihydroxyvitamin D3, and 1,25-dihydroxyvitamin D3

24(R),25-Dihydroxyergocalciferol [24,25-(OH)2-D2] is 1.7 times less potent than 24 (R), 25-(OH)2D3, 25-Hydroxyvitamin D2 (25OHD2), or 25OHD3 in the displacement of (3H)25OHD3 from rat serum binding proteins. 1,25-(OH)2D2 is 1.3 times less potent than 1,25-(OH)2D3 in the displacement of (3H)1,25-(OH)2D3 from a chick intestinal binding receptor. In light of binding affinity and chromatographic differences between vitamin D3 and its D2 analogs, it is our view that methods which purport to measure 1,25-(OH)2D and 24,25-(OH)2D probably understimate the contributions of D2 metabolites. This is particularly important in the case of plasma extracts from patients given large doses of vitamin D2.