Vitamin D is metabolized to 25-hydroxyvitamin D (25-OH-D) in the liver and subsequently to 1α, 25-dihydrovitamin D [1, 25-(OH)2D] or 24 R, 25- dihydroxyvitamin D [24, 25-(OH)2D] in the kidney when plasma calcium concentrations are lower or higher than normal (10 mg/ 100 mL). 1, 25-(OH)2D, which is known as an active form of vitamin D, is transported to the target organs, i.e., intestine, bone and others, to show the physiological activity. The most important role of 1, 25-(OH)2D is to keep the calcium homeostasis by accerelating the intestinal calcium absorption and bone remodeling. Recently, it has been clarified that the metabolite has the activity to differentiate myeloid leukemia cells to macrophages. The results suggested that 1, 25-(OH)2D and its modified compounds are available for the differentiation therapy for curing and preventing some kinds of cancer, i.e., myeloid leukemia, skin cancer, colon cancer and others. The metabolite has been also confirmed to be effective for curing the psoriasis.
