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Messenger ribonucleic acid (mRNA) levels and activities of renal 25-hydroxyvitamin D3-24-hydroxylase (24-OHase) were determined in 3 groups of rats: vitamin D-deficient, normal, and 1 alpha-hydroxyvitamin D3 (1 alpha OHD3)-administered rats. To measure renal 24-OHase mRNA, a DNA probe complementary to the reported sequence of the recently cloned P450 component was employed. The DNA probe hybridized with 24-OHase mRNA. Northern blot analysis indicated that the size of the transcript was approximately 3.4 kb, which was similar in size to that of the previous report. Oral administration of 1 alpha OHD3 (2 micrograms/kg bw/day x 7 days) markedly increased the plasma concentration of 1,25-dihydroxyvitamin D (1,25(OH)2D), renal 24-OHase activity and the renal 24-OHase mRNA level. However, the levels of 24-OHase mRNA were undetectable in vitamin D-deficient and normal rat kidneys, while a significant amount of 24-OHase activity was observed in the normal rat kidney. In vitamin D-deficient rats, renal 25-hydroxyvitamin D3-1 alpha-hydroxylase activity was markedly elevated and 24-OHase activity was completely abolished. When an oligonucleotide probe complementary to coding sequence of heme binding region was used for dot blot analysis, the mRNA was clearly detected in normal rat kidney. Furthermore, activity of the enzyme was attenuated by human parathyroid hormone (1-34) but the mRNA abundance did not change with 2 nmol (every 4 h, 5 times) of hormone treatment in 1 alpha OHD3-dosed thyroparathyroidectomized rats. The present study demonstrates that induction of 24-OHase activity by 1,25(OH)2D3 occurs at least in large part through increase of the gene expression in the kidney. Furthermore, these findings suggest that 2 forms (inducible and constitutive) of 24-OHase exist in rat kidney, and that the previously cloned P450 component is the inducible form of 24-OHase.

journal of nutritional science and vitaminology

Increase of Renal 25-Hydroxyvitamin D3-24-Hydroxylase Activity and Its Messenger Ribonucleic Acid Level in 1.ALPHA.-Hydroxyvitamin D3-Administered Rats: Possibility of the Presence of Two Forms of 24-Hydroxylase.