25-hydroxycholecalciferol. The probable metabolically active form of vitamin D3: its identification and subcellular site of action

It has been assumed from the early work of Askew et al1and Windaus and co-workers2,3that vitamins D2and D3act directly in the organism without molecular change. Experiments by Kodicek4,5first demonstrated that vitamin D is in fact metabolized to biologically inactive "break down products" and lent further support to this assumption. In their hands, no evidence could be obtained for a biologically active metabolite of the vitamin. Clearly, their experiments were hindered by the low specific activity of vitamin D2labeled with radioactive carbon (14C) and by inadequate chromatographic methods so as to preclude the detection of a major biologically active metabolite. With the development of new and powerful chromatographic techniques6,7and the synthesis of a radioactive vitamin D preparation of high enough specific activity8-10to allow experiments with physiologic doses of the vitamin, the question of vitamin