On the specificity of vitamin D compounds binding to chick pig intestinal 1,25-dihydroxyvitamin D3 receptor

The binding activity of four vitamin D metabolites and/or analogs for the intestinal 1,25-dihydroxyvitamin D3 receptor was evaluated after incubation at 25 degrees C for 1 h or at 0-4 degrees C for 18 h. The incubation conditions, which had no effect on the binding of 1,25-dihydroxyvitamin D3, had a dramatic effect on the binding of 25-hydroxyvitamin D3 and 1 alpha-hydroxyvitamin D3 and a small but reproducible effect on 24,25-dihydroxyvitamin D3 binding to receptor. Affinities 10- to 20-fold higher were obtained for 25-hydroxyvitamin D3 and 1 alpha-hydroxyvitamin D3, and affinities 3-fold higher were obtained for 24,25-dihydroxyvitamin D3 at the 0-4 degrees C/18-h incubation. A comparison of intestinal receptor from chick and pig with nine vitamin D compounds showed no major differences between the two species. The relative affinity of the vitamin D analogs to compete with tritiated 1,25-dihydroxyvitamin D3 for the receptor in pig nuclear extract, expressed as ratios of the molar concentration required for 50% binding of the tritiated 1,25-dihydroxyvitamin D3 compared to nonradioactive 1,25-dihydroxyvitamin D3, are as follows: 1,25-dihydroxyvitamin D3 (1) = 1,25-dihydroxyvitamin D2 = 24-homo-1,25-dihydroxyvitamin D3 greater than 1,24,25-trihydroxyvitamin D3 (4) greater than 25-hydroxyvitamin D3 (21) = 10-oxo-19-nor-25-hydroxyvitamin D3 = 1 alpha-hydroxyvitamin D3 (37) greater than 24,25-dihydroxyvitamin D2 (257) much much greater than vitamin D3 (greater than 10(6)).