Basic Studies on the Mechanism of Action of Vitamin D

Abstract

Studies on the cellular and subcellular localization of vitamin D and its metabolites have led to the suggestion that the vitamin is first metabolized to an as yet uncharacterized more polar compound that then associates stereospecifically with a receptor in the chromatin or DNA-containing portion of the mucosal cells. This in turn activates the biochemical expression of genetic information, which ultimately results in the physiological responses characteristic of vitamin D. The following evidence has been obtained in support of this proposal: a) The administration of low doses of actinomycin D to rachitic chicks prevents subsequently administered vitamin D₃ from promoting intestinal calcium absorption. b) When physiological doses of radioactive vitamin D₃ are adiministered to rachitic chicks, most of the radioactivity recovered in the mucosa is associated with the crude nuclear fraction and its chromatin fraction. c) Extraction and chromatographic analysis of this radioactivity from the chromatin indicates that 86% of it exists as a polar metabolite of vitamin D₃. d) This polar metabolite has biological activity equivalent to that of the parent vitamin. e) This polar metabolite is found only in the chromatin fraction from intestinal mucosa, and not from liver, presumably not a target organ for vitamin D. Pretreatment of the rachitic chick with non-radioactive vitamin D or its analogs specifically inhibits the chromatin binding of the metabolite resulting from subsequently administered radioactive vitamin D₃. f) The time course of localization of this metabolite in the intestinal mucosa and its chromatin fraction is in accord with the time delay in the physiological response to vitamin D administration. g) Administration of the vitamin to rachitic chicks has been shown to stimulate the synthesis of mucosal RNA and protein.