The chemistry and conformational and biological analysis of vitamin D3, its metabolites and analogues.

The chemical properties, stereochemical relationships and solution conformation, as assessed in part by proton NMR spectroscopy, for vitamin D3, its major metabolites [including 1alpha,25-(OH)2D3, its hormonally active form] and a number of A-ring and side chain analogues are evaluated and discussed in relation to their biological activity. In particular the relative ability of many of these seco-steroids to compete both with 25-OHD3 for its chick serum binding protein and 1alpha,25-(OH)2-D3 for its chick intestinal cytosol-chromatin receptor system was quantitated, in vitro. Further, the relative effectiveness of all these metabolites and analogues to mediate in vivo intestinal calcium absorption and bone calcium mobilization was determined. Collectively these chemical and biological studies constitute a "systems analysis" of the various steroid structural parameters both required and tolerated by the multi-stepped endocrine system associated with the biological actions of vitamin D.