Normal 24-hydroxylation of vitamin D metabolites in patients with vitamin D-dependency rickets type I. Structural implications for the vitamin D hydroxylases.

Abstract

The steady state serum concentration of 1,25-dihydroxyvitamin D [1,25-(OH)2D] is determined by the relative rates of its biosynthesis via the renal mitochondrial 1-hydroxylase and catabolism via renal and target cell 24-hydroxylases. It is not yet known whether the two catalytic activities are mediated by the product of a single gene or products of distinct genes. To address this question, we undertook to assess 24-hydroxylase function in patients with vitamin D-dependency rickets type I (VDDR-I), a Mendelian disorder of 1,25-(OH)2D synthesis attributable to a defect in renal 1-hydroxylase activity. To assess renal 24-hydroxylase activity, we measured the serum concentration of 24,25-dihydroxyvitamin D [24,25-(OH)2D] and its 25-hydroxyvitamin D (25OHD) precursor. We also measured target cell, 1,25-(OH)2D3-inducible 24-hydroxylase activity and calcitroic acid production in skin fibroblasts from VDDR-I patients and age- and sex-matched controls. Serum levels of 24,25-(OH)2D and 25OHD were similar in VDDR-I patients and controls [ratio of product to substrate, 0.062 +/- 0.013 (n = 5) vs. 0.067 +/- 0.005 (n = 10), mean +/- SEM, for patients and controls, respectively]. Circulating levels of 1,25-(OH)2D were also comparable in both groups [80.6 +/- 15.5 (n = 5) vs. 86.1 +/- 5.2 (n = 10) pmol/L, for patients and controls, respectively], presumably indicative of compliance with calcitriol therapy. Skin fibroblasts from VDDR-I patients exhibited 24-hydroxylase activity which was indistinguishable from that observed in control fibroblasts [108 +/- 14 (n = 5) vs. 96 +/- 25 fmol/10(6) cells.min (n = 6), for patients and controls, respectively]. Similarly, calcitroic acid production was comparable in fibroblast cultures derived from the two groups of subjects [31 +/- 6 vs. 33 +/- 3 fmol/10(6) cells.min (n = 3), for patients and controls, respectively]. Our data demonstrate that renal and target cell 24-hydroxylase activities are normal in patients with VDDR-I and suggest that the renal 1- and 24-hydroxylases likely represent, or contain, distinct polypeptides encoded by different genes.