Metabolism and mechanism of action of vitamin D.

Progress in the definition of mechanism of action of Vitamin D indicates that the vitamin functions as a prohormone giving rise to at least one hormone that plays a central role in the metabolism of calcium and phosphorus. Vitamin D3 is converted to 25-hydroxyvitamin D3 (25-OH-D3) in the liver. This compound, which can be regarded as the major circulating metabolite of Vitamin D does not act directly at physiologic concentrations but is altered in the kidney to I alpha, 25-dihydroxy-vitamin D3. It is this form of Vitamin D which is then transferred to the intestine, bone, and kidney where it or a further metabolite stimulates intestinal calcium transport, intestinal phosphate transport, the mobilization of calcium from bones, and renal absorption of calcium. Evidence has been presented for the existence of a receptor protein in intestinal mucosa that binds specifically I alpha, 25-dihydroxyvitamin in D3 and that may function in the nucleus to initiate transcription of genomes that code for proteins involved in the transport phenomenon. The discovery of this new endocrine system has brought with it many implications for the understanding and treatment of metabolic bone disease. (MFB)