Profile of ligand specificity of the vitamin D binding protein for 1α,25-dihydroxyvitamin d3 and its analogs

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Abstract
The profile of structural preference for the ligand binding domain of the human vitamin D binding protein (DBP) was determined by steroid competition assay of 71 analogs of 1α,25‐dihydroxyvitamin D3 [1α,25(OH)2D3]. The following categories of structural modification were evaluated [values represent fold change; R = reduction, I = increase in binding to the DBP from the reference 1α,25(OH)2D3]: (1) deletion in the A ring of the 1α‐hydroxyl (20‐1600I); (2) conversion of the triene system to the previtamin form (6‐40R); (3) addition of substituents to carbon 11 of the C ring (4‐14R); (4) inversion of the C/D ring junction (8‐20R); (5) unsaturation of the D ring (16‐ene; 4‐140R); (6) replacement of hydrogen with deuterium atoms (no effect); alteration of the side chain by (7) adding or deleting carbon atoms (5‐12R); (8) addition of fluorines (0.2‐10R); (9) presence of unsaturation (22‐ene, 0‐5R; 23‐ene, 3R‐10I; 23‐yne, 5‐20R); (10) addition of hydroxyls (2‐100R); and (11) addition of an aromatic ring (0‐20I). Thus the DBP ligand binding domain could tolerate only modest changes to the structure of 1α,25(OH)2D3 without a reduction in binding of the analog. The increases in binding seen in the aromatic side chain and with a triple bond at carbon‐23 may be indicative of a preferred conformation of the flexible 1α,25(OH)2D3 side chain. In addition, a comparison was made of the DBP ligand binding domain with that of the human HL‐60 cell 1α,25(OH)2D3 nuclear receptor. Both ligand binding domains could equivalently accommodate to the presence of (1) a side‐chain cyclopropyl group, (2) 22‐ene or 23‐yne, (3) lengthening the side chain by two carbons, (4) presence of four to six fluorine atoms, (5) substitution of an oxygen for carbon 22, and (6) presence of a 22‐[m‐(dimethylhydroxymethyl)phenyl] aromatic group in the side chain. The DPB could tolerate better than the HL‐60 cell receptor the presence of a 22‐(p‐hydroxyphenyl) aromatic group in the side chain and the absence of the 1α‐hydroxyl. In contrast, the HL‐60 cell receptor could tolerate better than the DBP the following structural modifications: presence of a 16‐ene, or 16‐ene plus 23‐yne unsaturation, and presence of an 11β‐hydroxyl.
Funding Information
  • USPHS (DK-09012 (Norman), DK-16595 (Okamura), CA-43,277 (Norman and Okamura))

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