A Bioassay Capable of Measuring 1 Picogram of 1,25-Dihydroxyvitamin D3*

Abstract

Fetal rat bones in organ culture constitute a sensitive system of assay of the vitamin D metabolite, 1.alpha.,25-dihydroxyvitamin D3. Significant bone resorption is obtained with as little as 2 pg 1,25-dihydroxyvitamin D3 after 48 h of culture and with 1 pg after 64 h of culture. In the current study, organ cultures of fetal rat bone are used as a bioassay for 1,25-dihydroxyvitamin D in normal human plasma, which was prepared for assay by extraction with dichloromethane, chromatography on Sepadex LH-20 and purification on silicic acid by high pressure liquid chromatography. The concentration of 1.alpha.,25-dihydroxyvitamin D3 in normal adult human plasma was 24.8 .+-. 2.0 pg/ml (n = 19) by this assay.