In vitro regulation of kidney 25-hydroxyvitamin D3-hydroxylase enzyme activities by vitamin D3 metabolites. Molecular specificity and mechanism of action.

An acute chick kidney tubule model was used to evaluate the molecular specificity of steroids which act to inhibit kidney 25-hydroxyvitamin D3-1-hydroxylase and induce 25-hydroxyvitamin D3-24-hydroxylase enzyme activities. Such hydroxylase-regulatory activity was confined to the vitamin D family of secosteroids. Vitamin D3 per se was not active. Rather, expression of hydroxylase-regulatory activity required acquisition of a C-25 hydroxyl grouping. Of the metabolites tested, 25-hydroxyvitamin D3 demonstrated the greatest hydroxylase-regulatory activity. Metabolites of 25-hydroxyvitamin D3 which contained hydroxyl groups at carbon atoms 1, or 24, or both, were also active in regulating the kidney hydroxylase enzymes. The hydroxylase-regulatory action of 1,25-dihydroxyvitamin D3 was blocked by inhibitors of RNA or protein synthesis, implicating a requirement for de novo protein synthesis. It was suggested that the regulatory process involves both hydroxylase-enzyme synthesis and turnover as well as enzyme-level modulation of endogenous enzyme activities.