1Alpha,25-dihydroxyvitamin D3 receptor: competitive binding of vitamin D analogs

The intestinal nuclear receptor for lα,25-dihydroxyvitamin D₃ has been utilized to determine the ability of vitamin D-active sterols to compete with this hormone at the molecular level. 25-Hydroxyvitamin D₃ and lα-hydroxyvitamin D₃ must be present in 150 and 450 times the concentration respectively of lα,25-dihydroxyvitamin D₃, $\text{in}$$\text{vitro}$, to displace the physiologic hormone. These data indicate that: i) superphysiologic levels of 25-hydroxyvitamin D₃ may simulate lα,25-dihydroxyvitamin D₃ and act directly on isolated target organs and ii) the biologic potency observed for low doses of lα-hydroxyvitamin D₃, $\text{in}$$\text{vivo}$, is probably the result of 25-hidroxylation of the lα-derivative to form lα,25-dihydroxyvitamin D₃.