Metabolically Active Form of Vitamin D Isolation, Identification, and Subcellular Location’

that a lag exists between the time of vitamin D administration and the onset of its physiological action (1, 2). Until recently no evidence had been developed to support one of several possible explanations. Then, it was shown in this laboratory that the effect of vitamin D on both the active trans-port of calcium across the intestine and the mobilization of bone mineral is blocked by actinomycin D (3, 4). A similar observation was reported for intestinal absorption of calcium in chicks (5). Through a series of critical experiments, it was shown (4) that actinomycin D was acting in the expected fashion, i.e., it blocked the transcription of DNA into mRNA, indicating that the ex-pression of vitamin D action involves the transcription of DNA and protein synthesis. This was confirmed by Stohs and others (6, 7), who dlemonstrated that vitamin D stimu-lated the incorporation of 3H orotic acid into nuclear RNA of intestine. Hence, the lag in vitamin D action must be partly due to the production of mRNA and the subse-quent synthesis of a protein component of the calcium transport system of intestine and probably also of bone (2). This by itself, however, was not sufficient to account for the entire lag in vitamin D action. This led to further studies aimed at elucidating what happens to the vitamin D molecule between the time of vitamin D administration an