Abnormal renal mitochondrial 25-hydroxyvitamin D3-1-hydroxylase activity in the vitamin D and calcium deficient X-linked Hyp mouse.

Despite severe hypophosphatemia, plasma levels of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) are not significantly elevated in the Hyp mouse, a murine homologue of X-linked hypophosphatemia in man. To examine the effect of the Hyp mutation on vitamin D hormone biosynthesis, the metabolism of 25-hydroxyvitamin D3 (25-OH-D3) by isolated renal mitochondria was studied. The ability of a vitamin D deficient, low calcium diet to stimulate renal mitochondrial 25-hydroxyvitamin D3-1-hydroxylase activity (1-OHase) in normal mice (n = 22) and Hyp littermates (n = 20) was examined. Both genotypes responded to the diet with an increase in 1-OHase activity and a decrease in 25-hydroxyvitamin D3-24-hydroxylase activity (24-OHase). The increase in 1-OHase activity, however, was significantly lower in Hyp mice (8-fold) than in normal littermates (13-fold, P less than 0.001). In spite of depressed 1-OHase in the mutant strain, enzyme activity was significantly correlated with serum calcium concentration in both normal and Hyp mice. The present results provide direct evidence for an abnormal 1-OHase response in renal mitochondria of Hyp mouse.