Plasma 24,25‐dihydroxyvitamin D3 concentrations in x‐linked hypophosphatemic mice: Studies using mass fragmentographic and radioreceptor assays

Previous studies have suggested that both plasma 24,25‐dihydroxyvitamin D [24,25‐(OH)2D] concentrations and renal 25‐hydroxyvitamin D‐24‐hydroxylase activity are increased in mice with X‐linked hypophosphatemia (Hyp mice). However, because the plasma levels of 24,25‐(OH)2D seemed surprisingly high, we repeated these assays using two different techniques. Mass fragmentographic and radioreceptor assays were employed to compare the plasma concentrations of 25‐hydroxyvitamin D (25‐OHD) and 24,25‐(OH)2D in normal mice with those in Hyp mice. These assays yielded 24,25‐(OH)2D concentrations much lower than previously reported in mice (both normal and Hyp). The concentrations of 25‐OHD3, and 24,25‐(OH)2D3, determined by mass fragmentography, were lower in Hyp mice than in controls [25‐OHD3, 9.7 ± 0.4 versus 14.6 ± 0.6 ng/ml, p < 0.01; 24,25‐(OH)2D3, 7.1 ± 0.3 versus 10.4 ± 0.4 ng/ml, p < 0.01]. Plasma 25‐OHD concentration was the main determinant of plasma 24,25‐(OH)2D, and the ratio of 25‐OHD3 to 24,25‐(OH)2D3 obtained from mass fragmentographic measurements did not differ between the two groups (1.40 ± 0.05 versus 1.36 ± 0.03 ng/ml, NS in normal and Hyp groups, respectively). Separate measurement of plasma 25‐OHD, 24,25‐(OH)2D, and 25‐OHD3‐26,23‐lactone by radioreceptor assay showed no difference between either plasma 24,25‐(OH)2D, or the ratio of 25‐OHD concentration to 24,25‐(OH)2D concentration among Hyp and control animals. In neither study was plasma phosphate concentration related to the 25‐OHD3: 24,25‐(OH)2D3 ratio. We conclude that previous estimations of plasma 24,25‐(OH)2D3 concentrations in mice were erroneously high and, further, that in fact this metabolite is not present in increased concentration in Hyp mice