The Biological Activity and Metabolism of 24,25-Dihydroxyvitamin D3

Abstract

Abstract 24,25-Dihydroxyvitamin D3 (24,25-(OH)2D3) gives a marked increase in intestinal calcium transport and serum calcium concentration of vitamin D-deficient rats fed a diet containing adequate calcium and phosphorus. These responses occur after a considerable time lag following administration of the metabolite. The 24,25-(OH)2D3 on a long term basis and at low doses has striking activity in the stimulation of intestinal calcium transport but not bone calcium mobilization. Radioactive 24,25-(OH)2D3 is metabolized to a more polar compound which increases in concentration in serum, intestinal mucosa, and bone over a 48-hour period. Nephrectomy or the feeding of a high calcium diet abolishes the stimulation of intestinal calcium transport by 24,25-(OH)2D3 and the in vivo genesis of the more polar compound from tritiated 24,25-(OH)2D3. It is concluded that 24,25-(OH)2D3, the usual dihydroxy metabolite of vitamin D3 in normal rats, has potential physiological importance through its conversion to a more polar metabolite. This metabolite possibly acts to facilitate intestinal calcium absorption without stimulating bone calcium mobilization.