PloS one

BACKGROUND: The 1alpha,25-dihydroxy-3-epi-vitamin-D3 (1alpha,25(OH)2-3-epi-D3), a natural metabolite of the seco-steroid vitamin D3, exerts its biological activity through binding to its cognate vitamin D nuclear receptor (VDR), a ligand dependent transcription regulator. In vivo action of 1alpha,25(OH)2-3-epi-D3 is tissue-specific and exhibits lowest calcemic effect compared to that induced by 1alpha,25(OH)2D3. To further unveil the structural mechanism and structure-activity relationships of 1alpha,25(OH)2-3-epi-D3 and its receptor complex, we characterized some of its in vitro biological properties and solved its crystal structure complexed with human VDR ligand-binding domain (LBD). METHODOLOGY/PRINCIPAL FINDINGS: In the present study, we report the more effective synthesis with fewer steps that provides higher yield of the 3-epimer of the 1alpha,25(OH)2D3. We solved the crystal structure of its complex with the human VDR-LBD and found that this natural metabolite displays specific adaptation of the ligand-binding pocket, as the 3-epimer maintains the number of hydrogen bonds by an alternative water-mediated interaction to compensate the abolished interaction with Ser278. In addition, the biological activity of the 1alpha,25(OH)2-3-epi-D3 in primary human keratinocytes and biochemical properties are comparable to 1alpha,25(OH)2D3. CONCLUSIONS/SIGNIFICANCE: The physiological role of this pathway as the specific biological action of the 3-epimer remains unclear. However, its high metabolic stability together with its significant biologic activity makes this natural metabolite an interesting ligand for clinical applications. Our new findings contribute to a better understanding at molecular level how natural metabolites of 1alpha,25(OH)2D3 lead to significant activity in biological systems and we conclude that the C3-epimerization pathway produces an active metabolite with similar biochemical and biological properties to those of the 1alpha,25(OH)2D3