Catalyzes the conversion of 25-hydroxyvitamin D3 (25(OH)D3) to 1-alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)(2)D3), and of 24,25-dihydroxyvitamin D3 (24,25(OH)(2)D3) to 1-alpha,24,25-trihydroxyvitamin D3 (1alpha,24,25(OH)(3)D3). Is also active with 25-hydroxy-24-oxo-vitamin D3. Plays an important role in normal bone growth, calcium metabolism, and tissue differentiation. calcidiol + 2 H(+) + O2 + 2 reduced [adrenodoxin] = calcitriol + H2O + 2 oxidized [adrenodoxin] 2 H(+) + O2 + 2 reduced [adrenodoxin] + secalciferol = calcitetrol + H2O + 2 oxidized [adrenodoxin] Hormone biosynthesis; cholecalciferol biosynthesis. Kidney. Belongs to the cytochrome P450 family.