title: Quantitation of endogenously occupied and unoccupied binding sites for 1,25-dihydroxyvitamin D3 in rat intestine.
abstract: The quantitative reversible dissociation of the 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]-receptor complex by the mercurial reagent mersalyl was used to develop an assay for endogenously occupied and unoccupied 1,25-(OH)2D3 binding sites. Incubation of intestinal cytosol preparations in buffer containing 50 mM Tris . HCl, 300 mM KCl, and 1.5 mM EDTA, pH 7.4, with 1 mM mersalyl for 60 min was effective in inhibiting 98% of 1,25-(OH)2D3 specific binding activity. Dissociation of bound 1,25-(OH)2[26,27-3H]D3 from the hormone-receptor complex approached completion by 180 min. In cytosol incubated with saturating levels of nonradioactive hormone, 96% of total binding activity was measurable with the hormone binding assay after displacement of bound nonradioactive ligand with 1 mM mersalyl. Endogenously occupied 1,25-(OH)2D3 binding sites contributed 0, 9, and 19% of total binding activity in rats with plasma 1,25-(OH)2D3 levels averaging 2, 121 +/- 36 and 516 +/- 92 pg/ml, respectively. Therefore, the major fraction of cytosolic 1,25-(OH)2D3 specific binding activity is unoccupied in rat intestine. The results suggest that only a small proportion of the measurable receptors are in the bound form to provide maximal 1,25-(OH)2D3-induced calcium transport.
mesh_terms: D000818:Animals; D001665:Binding Sites; D001693:Biological Transport, Active; D002117:Calcitriol / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D002499:Centrifugation, Density Gradient; D002645:Chickens; D005260:Female; D007413:Intestinal Mucosa / Q000378:metabolism*; D008297:Male; D051381:Rats; D018167:Receptors, Calcitriol; D011987:Receptors, Steroid / Q000378:metabolism*
chemical_list: D018167:Receptors, Calcitriol; D011987:Receptors, Steroid; D002117:Calcitriol; D002118:Calcium