Cholecalciferol (vitamin D3) is rapidly metabolized, via a 25-hydroxy intermediate, to an as yet unidentified polar compound. This compound, designated Metabolite 4B, has previously been shown to be the most potent and rapidly-acting form of cholecalciferol yet found, and probably represents the biologically active form of this steroid in the intestine. Studies on the metabolism of a mixed dose of 1, 2-H-and 4-14C-labeled cholecalciferol in the rachitic chick reveal a loss of tritium in the formation of Metabolite 4B. The tritium loss involves the removal of a single hydrogen from the carbon-1 position. The implications of this tritium loss on the possible structure of Metabolite 4B are discussed. © 1971.
Myrtle, J. F., & Norman, A. W. (1971). Studies on calciferol metabolism II. Tritium loss from tritiated vitamin D3and the possible structure of the proposed nuclear regulator of intestinal calcium transport. Steroids, 17(6), 619–630. https://doi.org/10.1016/0039-128X(71)90078-X