[Vitamin D: its chemistry, metabolism and biological functions]

Vitamin D is metabolized to 25-hydroxyvitamin D(25(OH)D) in the liver and subsequently to 1 alpha,25-dihydroxyvitamin D(1 alpha,25(OH)2D) or 24R,25-dihydroxyvitamin D(24R,25(OH)2D) in the kidney. 1 alpha,25(OH)2D, the active form of vitamin D, stimulates intestinal calcium absorption and bone resorption, resulting in the elevation of plasma calcium. Recent studies have revealed that 1 alpha,25(OH)2D exerts a wide variety of biological actions such as cellular differentiation and proliferation in addition to its role in calcium homeostasis. Most cellular actions of 1 alpha,25(OH)2D are mediated by alterations in the transcription of vitamin D-dependent genes. In this review article, the mode of action and the biological function of 1 alpha,25(OH)2D are summarized.