The interaction of 1,25-dihydroxyvitamin D3 with its intestinal mucosa receptor: kinetic parameters and structural requirements.

Vitamin D3 and its metabolites comprise an endocrine system which plays a critical role in calcium homeostasis. The active form of vitamin D3 is 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. Chromatin localization of 1,25(OH)2D3 and sucrose density gradient centrifutation have demonstrated the presence of an intestinal mucosa cytosol receptor which specifically binds 1,25(OH)2D3. The kinetic parameters of 1,25(OH)2D3 binding to its receptor have been determined by hydroxylapatite and reconstituted chromatin cytosol assays. Utilization of these assays has also permitted a determination of the precise structural requirements of the vitamin D ligand for the intestinal receptor. Furthermore, it has been possible to propose two receptor-ligand models which are capable of accommodating the conformationaly modile A ring of the vitamin D seco-steroids.