25-Hydroxyvitamin D ₃ 1α-Hydroxylase and Vitamin D Synthesis

Abstract

Renal 25-hydroxyvitamin D3 1alpha-hydroxylase [1alpha(OH)ase] catalyzes metabolic activation of 25-hydroxyvitamin D3 into 1alpha, 25-dihydroxyvitamin D3 [1alpha,25(OH)2D3], an active form of vitamin D, and is inhibited by 1alpha,25(OH)2D3. 1alpha(OH)ase, which was cloned from the kidney of mice lacking the vitamin D receptor (VDR-/- mice), is a member of the P450 family of enzymes (P450VD1alpha). Expression of 1alpha(OH)ase was suppressed by 1alpha, 25(OH)2D3 in VDR+/+ and VDR+/- mice but not in VDR-/- mice. These results indicate that the negative feedback regulation of active vitamin D synthesis is mediated by 1alpha(OH)ase through liganded VDR.

De-orphanization of Cytochrome P450 2R1

“…These experiments proved challenging for three reasons. First, 1␣-hydroxyvitamin D 3 generated by the CYP27B1 enzyme has some agonist activity on the vitamin D receptor (18). Second, 25-hydroxyvitamin D 3 generated by the CYP2R1 or CYP27A1 enzymes also has significant agonist activity (Fig.…”

“…This hypothesis was tested further in a series of transfection experiments with different cytochrome P450 cDNAs, vitamin D receptor constructs, and reporter genes. Vitamin D 3 rather than 1␣-hydroxyvitamin D 3 was utilized as a precursor because 25-hydroxyvitamin D 3 also activates the vitamin D receptor, albeit with decreased potency compared with the 1␣,25-dihydroxy compound (18). Transfection of the mouse or human CYP2R1 cDNA increased luciferase expression ϳ7-10-fold in the GAL4-VDR/GAL4-luciferase reporter system (Fig.…”

“…In the current study, a cDNA library made from hepatic mRNA of mice deficient in the gene encoding the mitochondrial CYP27A1 enzyme was screened using a vitamin D receptorbased, ligand activation assay (18). A single cDNA specifying a microsomal P450 enzyme termed CYP2R1 with previously unknown substrate specificity was identified.…”

“…25-Hydroxylation is performed in the liver by two different enzymes, one located in the mitochondria (10), identified as the CYP27A1 sterol 27-hydroxylase (11)(12)(13)(14)(15), and a second in microsomes (16,17), which has not been identified in most species. A second mitochondrial P450 (CYP27B1), for which an encoding cDNA was isolated by expression cloning (18), catalyzes 1␣-hydroxylation of 25-hydroxyvitamin D in the kidney, and a third mitochondrial P450, vitamin D 24-hydroxylase (CYP24A1), inactivates the vitamin in this tissue (19 -21). Together, these enzymes regulate the systemic and local levels of vitamin D through complex feedback mechanisms mediated in part by the vitamin D receptor (22).…”

Differential Regulation of Vitamin D Receptor and Its Ligand in Human Monocyte-Derived Dendritic Cells

“…Thus, the impact of 1␣25(OH) 2 D 3 upon DC function is likely to be dependent not only upon cellular expression of VDR but also its local synthesis and or breakdown. 1␣25(OH) 2 D 3 is generated upon the 1-␣ hydroxylation of an inactive precursor metabolite, 25-hydroxyvitamin D 3 (25(OH)D 3 ), which exists at a 2-3 log higher concentration than the daughter molecule (10). The level of 1␣25(OH) 2 D 3 is, thus, critically dependent upon cellular expression of the enzyme, 25-hydroxyvitamin D 3 (25(OH)D 3 ) 1-␣-hydroxylase (1␣OHase) which catalyzes this reaction.…”