Physiological importance of vitamin D metabolism.

1,25(OH)2D3 and 24,25(OH)2D3 are biosynthesized from 25(OH)D3 in kidney. A cytosol receptor for 1,25(OH)2D3, but not 24,25(OH)2D3, is detected in kidney as well as bone and gut. The function of 1,25(OH)2D3 in kidney is probably to enhance calcium-binding protein and mineral reabsorption and to induce vitamin D-24-OHase enzyme. Receptors for 1,25(OH)2D3 are present also in endocrine sites such as parathyroid, pancreas, pituitary, and placenta, possibly to mediate central control of mineral homeostasis. Finally, 1,25(OH)2D3 elicits normal bone mineralization and fracture healing, contrary to other reports that 24,25(OH)2D3 is uniquely required for such physiologic vitamin D actions.