COMPETITION BY 24,25-DIHYDROXYCHOLECALCIFEROL IN THE COMPETITIVE PROTEIN BINDING RADIOASSAY OF 25-HYDROXYCALCIFEROL

Abstract

24,2.5-dihydroxycholecalcif erol (24,25-(OH)₂D₃) is equipotent to 25-hydroxycholecalciferol in the displacement of ₃H₂₅-OHD₃ from rat serum binding sites, and is extracted by the ethanol procedure recommended in non-chromatographic competitive protein binding radioassays for 25-OHD. Serum 24,25-(OH)₂D content was measured following 24,25-(OH)₂D₃ isolation from lipid extracts by liquid-gel partition chromatography. Since normal serum 24,25-(OH)₂D concentration is appreciable, non-chromatographic 25-OHD assays most probably overestimate serum 25-OHD levels as a result of their recognition of 24,25-(OH)₂D₃.