Displacement Potency of Vitamm D2Analogs in Competitive Protein-Binding Assays for 25– Hydroxyvitamin D3, 24,25–Dihydroxyvitamin D3, and 1,25–Dihydroxyvitamin D3

Abstract

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 underestimate the contributions of D₂ metabolites. This is particularly important in the case of plasma extracts from patients given large doses of vitamin D₂. (J ClinEndocrinol Metab50: 773, 1980)