INFLUENCE OF SUBSTITUTION AT C-24 ON THE CALCIUM-BINDING PROTEIN-STIMULATING ACTIVITY OF VITAMIN-D METABOLITES IN CHICK EMBRYONIC DUODENUM

The production of calcium-binding protein, in vitro, by embryonic chick duodenum has been used to assess the potency of vitamin D compounds. The introduction of an hydroxyl on 1-, 25-, or 24R-position enhanced biological activity while the introduction of both 1α- and 25-hydroxyls produced maximal activity. However 24R-hydroxylation of 1,25-dihydroxyvitamin D3 diminished activity. The vitamin D2 side chain on 25-hydroxyvitamin D or 1,25-dihydroxyvitamin D did not greatly diminish activity in contrast to the fact that the vitamin D2 compounds are 10% as active as the vitamin D3 compounds in vivo in the chick. These results support the idea that the target organs of the chick do not discriminate against the vitamin D2 side chain and that the discrimination in this species is at the level of metabolism. © 1979.