MEASUREMENT OF CIRCULATING 1,25-DIHYDROXYVITAMIN-D IN MAN - CHANGES IN SERUM CONCENTRATIONS DURING TREATMENT WITH 1-ALPHA-HYDROXYCHOLECALCIFEROL

A sensitive assay is described for measurement of circulating 1,25-dihydroxyvitamin D (1,25-(OH)2D) using rachitic chick intestinal cytosol binding protein. The metabolite was extracted from serum by diethyl ether and chromographed on a Sephadex LH-20 column followed by high-pressure liquid chromatography. The concentration of 1,25-(OH)2D was then measured by a competitive protein-binding assay. The binding capacity of the protein was unchanged for 4 months. The inter- and intra-assay variations were both 11%. The standard curve was useful at serum concentrations ranging from 14 to 395 pmol/l (6 to 165 pg/ml). The normal mean concentration was 79.6±36.8 pmol/l (33.1±15.3 pg/ml). The serum 1,25-(OH)2D was studied in elderly subjects following administration of 1α-hydroxycholecalciferol (1α-OHD3) at different doses. Large variations occurred indicating individual rates of metabolism of 1,25-(OH)2D.