Receptor, calcidiol

Both calcidiol and calcitriol are substrates for24-hydroxylation, catalyzed by a cytochrome P4so-dependent enzyme in kidneys, intestinal mucosa, cartilage, and other tissues that contain calcitriol receptors. This enzyme is induced by calcitriol the activities of calcidiol 1-hydroxylase and 24-hydroxylase in the kidney are subject to regulation in opposite directions, so that decreased requirement for, and synthesis of, calcitriol results in increased formation of 24-hydroxycalcidiol. Kidney epithelial cells in culture show increased formation of 24-hydroxycalcidiol, and decreased formation of calcitriol, after the addition of calcitriol or high concentrations of calcium to the culture medium. 

Calcitriol acts like a steroid hormone, binding to, and activating, nuclear receptors that modulate gene expression. More than 50 genes are known to be regulated by calcitriol (see Table 3.3), but vitamin D response elements have only been identified in a relatively small number, including calcidiol 1-hydroxylase and 24-hydroxylase calbindin, a calcium binding protein in the... 

Ritter, C.S. and Brown, A.J. 2011. Direct snppression of PTH gene expression by the vitamin D prohoraiones doxercalciferol and calcidiol requires the vitamin D receptor. J. Mol. 

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