Strontium, calcidiol 1-hydroxylase

Calcidiol la-hydroxylase is not restricted to the kidney, but is also found in placenta, bone cells (in culture), mammary glands, and keratinocytes. The placental enzyme makes a significant contribution to fetal calcitriol, but it is not clear whether the calcidiol 1-hydroxylase activity of other tissues is physiologically significant or not. Acutely nephrectomized animals given a single dose of calcidiol do not form any detectable calcitriol, but there is some formation of calcitriol in anephric patients, which increases on the administration of cholecalciferol or calcidiol. However, thus extrarenal synthesis is not adequate to meet requirements, so that osteomalacia develops in renal failure (Section 3.4.1). The enzyme is inhibited, or possibly repressed, by strontium ions this is the basis of strontium-induced vitamin D-resistant rickets, which responds to the administration of calcitriol or la-hydroxycalciol, but not calciferol or calcidiol (Omdahl and DeLuca, 1971). 

Strontium intoxication can cause vitamin D-resistant rickets because strontium is a potent inhibitor of calcidiol 1-hydroxylase (Omdahl and DeLuca, 1971). 

Plasma Concentrations of Calcium and Phosphate Although the mean response to changes in plasma cedcium is a chemge in the secretion of peuathyroid hormone, the activity of calcidiol 1-hydroxyleise in kidney slices is decreased directly by high concentrations of cedcium in the incubation medium. Cedcium heis no direct effect on the activity of calcidiol 24-hydroxylase under these conditions. Strontium and cadmium edso inhibit ctdcidiol 1-hydroxylase. 

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