Vitamin D: A Cholecalciferol Metabolite Highly Active in Promoting Intestinal Calcium Transport

A major polar metabolite of cholecalciferol (vitamin D$_{3}$) obtained from chick intestines is over four times as effective as cholecalciferol and over two times as effective as 25-hydroxycholecalciferol in stimulating intestinal calcium transport 24 hours after administration. Following a considerable lag, cholecalciferol and its 25-hydroxy derivative produce a maximum stimulation of the transport response at 24 to 48 hours. The polar intestinal metabolite greatly shortens this lag, stimulating maximum calcium transport by 9 hours. At 9 hours this metabolite is at least 13 times as active as the parent cholecalciferol and as such is a likely candidate for the biologically active form of cholecalciferol in the intestine.