Calcium-binding protein of intestine: Induction by biologically significant cholecalciferol-like steroids in vitro

Abstract

Vitamin D (cholecalciferol-like) steroids added to the culture medium induce a specific calcium-binding protein (CaBP) ‡ in embryonic chick duodenum maintained in organ culture. Responses of the isolated duodenum faithfully mimic several other cholecalciferol-dependent responses observable in vivo attesting to their physiological significance. Therefore, this system provides a relevant bioassay, i.e. a physiological response (CaBP-induction) in a principal target organ, for the study of structure-activity relationships of biologically-significant cholecalciferol-like steroids. The several cholecalciferol-like steroids normally circulating in the intact animal, including cholecalciferol itself, 25-hydroxycholecalciferol, 24R,25-dihydroxycholecalciferol and 1α,25-dihydroxycholecalciferol, induced CaBP biosynthesis in vitro at or below physiological concentrations. This clearly demonstrates the relative rather than absolute specificity of the intestine for cholecalciferol-like steroids in the induction of CaBP. Maximal activity was observed with either 1α-hydroxycholecalciferol (not a native steroid), 1α,25-dihydroxycholecalciferol or 1α,24R,25-trihydroxycholecalciferol indicating a requirement for the 1α-hydroxyl function but suggesting the non-essentiality of the 25-hydroxyl function. The cis triene structure and an intact side-chain were also found to be essential to optimal activity. Preliminary studies with impure analogs suggest an absolute requirement for the 3β-hydroxyl group. The minimal structure inducing CaBP among naturally occurring steroids is cholecalciferol itself.