The Metabolism of 1,25-Dihydroxyvitamin D3*

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Abstract
VITAMIN D plays a major role in the maintenance of extracellular fluid calcium and phosphorus concentrations, the mineralization of bone, and the maintenance of skeletal integrity (1–3). The vitamin increases active calcium and phosphorus transport in the duodenum and proximal intestine; rachitic animals and humans have increased feeal losses of calcium that are reversed by the administration of vitamin D. Furthermore, there is evidence that the vitamin or its metabolites have direct effects upon muscle metabolism and on isolated bone cells, and upon the mobilization of calcium from isolated bone (1). Finally, the vitamin has pronounced effects upon the maintenance of adequate growth in experimental animals and in man (1, 3). The present review will focus upon some of the more recent findings concerning the regulation of 25-hydroxyvitamin D3 lα-hydroxylase activity and upon the peripheral metabolism of vitamin D3 homologs. History In 1919, Sir Edward Mellanby demonstrated that rickets could be cured by the administration of cod liver oil (4, 5). Steenbock and colleagues (6) demonstrated that the antirachitic activity of certain oils was increased after irradiation with ultraviolet light. McCollum et al. (7) showed that the growth-promoting fat-soluble vitamin A was different from the antirachitic fat-soluble vitamin D. Huldshinsky in Vienna demonstrated the healing of rickets by adequate exposure to sunlight (8). Therefore, vitamin D is not a vitamin in the strict sense but has become so because of changes in the exposure to sunlight that have accompanied evolution (8). The chemical work of Askew, Windaus, and their colleagues (9–11) resulted in the elucidation of the structure of vitamin D2 and vitamin D3. Orr et al. (12) made the important observation that individuals with rickets had high losses of calcium in the feces. Subsequently, work by Nicolaysen and co-workers (13–16) definitely established the role of vitamin D in the transport of calcium in the intestine.

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