The purpose of this project is a detailed investigation at the molecular level of the mechanism of action of vitamin D and its most biologically active metabolite, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). The major thesis of the study is that in terms of its chemical structure and postulated mode of action the seco-steroid vitamin D is similar to that of other classical steroid hormones. In this regard the kidney is postulated to be the endocrine gland which produces in a physiologically regulated fashion small amounts of 1,25(OH)2D3. After systemic transport, the steroid interacts in its target tissues, the intestinal mucosa and bone to generate the usual biological responses attributed to vitamin D. The specific studies for the forth coming year include: (a) a detailed kinetic evaluation of the rates of association and dissociation of the ligand 1,25(OH)2D3 with its intestinal receptor; (b) a comparative evaluation of the rates of synthesis and degredation of the vitamin D-dependent CaBP in the intestine and Kidney; and (c) development of a radioimmunoassay for CaBP.