Suppression of rat hepatic vitamin D-25-hydroxylase by cholecalciferol, but not by 25-hydroxy- or 1,25-dihydroxymetabolites.

Hepatic vitamin D-25-hydroxylase activity is greater in vitamin D-depleted than replete animals. We investigated whether vitamin D itself or a metabolite of vitamin D was responsible for modulating the activity of vitamin D-25-hydroxylase. Accordingly, we repleted vitamin D-depleted rats with subcutaneous injections of 2600, 520, and 130 pmoles of cholecalciferol (D3), 25-hydroxycholecalciferol (25(OH)D3), and 1,25-dihydroxycholecalciferol (1,25(OH)2D3), respectively, for up to 3 weeks. Repletion resulted in accelerated weight gain and in increased activity of gut mucosal alkaline phosphatase. Using an improved assay to measure vitamin D-25-hydroxylase activity in liver homogenates, we found 78% reduction (P less than 0.001) in the D3-repleted group, maximal by 1 week, in contrast to no change in those groups treated with D3 metabolites. D3, 25(OH)D3, and D3-esters remaining in livers at the time of assay were estimated in a parallel experiment using [3H]D3-repleted rats. Residual D3 accounted for only a 9% dilution of substrate in the assay. 25(OH)D3 was present in the liver at concentrations two orders of magnitude lower than the amount required to inhibit vitamin D-25-hydroxylase activity in vitro. D3 esters had no inhibitory effect in vitro at 250-fold excess of that found in the repleted rat liver. Vitamin D appears to modulate its D-25-hydroxylase activity in biological systems by a mechanism other than feedback inhibition by 25(OH)D3, 1,25(OH)2D3, or D3-esters.