Specific binding of 24R,25-dihydroxyvitamin D3 to chick intestinal mucosa: 24R,25-dihydroxyvitamin D3 is an allosteric effector of 1,25-dihydroxyvitamin D3 binding

Add time:07/21/2019    Source:sciencedirect.com

We have previously described a significant decrease in the positive cooperativity level and affinity of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] binding to its chick intestinal chromatin receptor induced in vitro by a physiological 10-fold molar excess of (24R)-25-dihydroxyvitamin D 3 [24R,25(OH)2D3] [F. Wilhelm and A. W. Norman (1985)Biochem. Biophys. Res. Commun.126, 496–501]. In this report, we have initiated a comparative study of the binding of 24R,25(OH)2[3H]D3 and 1,25(OH)2[3H]D3 to the the intestinal chromatin fraction obtained from vitamin D-replete birds. 24R,25(OH)2[3H]D3 specific binding to this chromatin fraction was characterized by a dissociation constant (Kd) of 34.0 ± 6.4 nm, a positive cooperativity level (nH) of 1.40 ± 0.13, and a capacity (Bmax) of 47 ± 8 fmol/mg protein. The very low relative competitive index (RCI) of 24R,25(OH)2D3 (0.11 ± 0.03%) for the 1,25(OH)2D3 binding site/receptor, as well as the inability of 1,25(OH)2D3 to displace 24R,25(OH)2D3 from its binding site at a physiological molar ratio of 1:10, strongly suggest the independence of 24R,25(OH)2D3 and 1,25(OH)2D3 binding sites. Stereospecificity of the 24R,25(OH)2D3 binding sites was attested by the displacement of only 45 ± 6% of 24R,25(OH)2D3 specific binding by equimolar concentrations of 24S,25(OH)2D3. Collectively these results suggest the existence of a binding domain/ receptor for 24,25(OH)2D3 in the chick intestine which is independent of the 1,25(OH)2D3 receptor.

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