A cytochrome P450 monooxygenase involved in activation of vitamin D precursors (PubMed:12867411, PubMed:24019477). Catalyzes hydroxylation at C-25 of both forms of vitamin D, vitamin D(2) and D(3) (calciol). Can metabolize vitamin D analogs/prodrugs 1alpha-hydroxyvitamin D(2) (doxercalciferol) and 1alpha-hydroxyvitamin D(3) (alfacalcidol) forming 25-hydroxy derivatives. Mechanistically, uses molecular oxygen inserting one oxygen atom into a substrate, and reducing the second into a water molecule, with two electrons provided by NADPH via cytochrome P450 reductase (CPR; NADPH-ferrihemoprotein reductase) (PubMed:12867411, PubMed:24019477) (By similarity). calciol + O2 + reduced [NADPH--hemoprotein reductase] = calcidiol + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] O2 + reduced [NADPH--hemoprotein reductase] + vitamin D2 = 25-hydroxyvitamin D2 + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] 1alpha-hydroxyvitamin D2 + O2 + reduced [NADPH--hemoprotein reductase] = 1alpha,25-dihydroxyvitamin D2 + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] alfacalcidol + O2 + reduced [NADPH--hemoprotein reductase] = calcitriol + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] Hormone biosynthesis; vitamin D biosynthesis. Homodimer. Highly expressed in the liver and testis. Mutant mice are born at the expected Mendelian rate and have normal development. They display substantial reduction of serum calcidiol levels. Belongs to the cytochrome P450 family.