It is largely through historical accident in the interval of 1920-1940 that vitamin D(3) became classified as a vitamin rather than as a steroid hormone. The formal definition of a vitamin is that it is a trace dietary constituent required to produce the normal function of a physiological process or processes. The emphasis here is on trace and the fact that the vitamin must be supplied regularly in the diet; this implies that the body is unable to metabolically synthesize the vitamin in question. However, the ultraviolet exposure of 7-dehydrocholesterol present in the skin results in the photochemical production of vitamin D(3). Thus, vitamin D(3) becomes a true vitamin only when the animal or human does not have regular access to sunlight or ultraviolet light. Under normal physiological circumstances, all mammals, including humans, can generate, via ultraviolet exposure of 7-dehydrocholesterol present in the skin, adequate quantities of vitamin D(3) to meet their nutritionally defined requirements. There is a vibrant historical record beginning in 1650 and culminating in 1963 concerned with the determination of the chemical structures of vitamin D(3) and vitamin D(2). A surprising aspect concerning vitamin D(3) is that it is itself biologically inert. There are no known essential biological actions or contributions that rely specifically on the molecule vitamin D(3). While chemists had certainly appreciated the strong structural similarity between the vitamins D and other steroids, this correlation was never widely acknowledged in the biological, clinical, or nutritional sciences until 1965-1970. The biological role of vitamin D(3) is to serve as a substrate for the liver 25-hydroxylase which produces 25-hydroxyvitamin D(3) [25(OH)D(3)]. 25(OH)D(3) in turn serves as the substrate for the kidney proximal tubule 25(OH)D(3)-1α-hydroxylase enzyme which produces the steroid hormone 1α,25(OH)(2)-vitamin D(3) [1α,25(OH)(2)D(3)].

The history of the discovery of vitamin D and its daughter steroid hormone.

Edward Mellanby used the experimental method to investigate medical problems In 1918, working at King's College for Women, London, he provided conclusive evidence that rickets is a dietary deficie

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Sir Edward Mellanby (1884-1955) GBE KCB FRCP FRS: nutrition scientist and medical research mandarin.

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Sir Edward Mellanby, G.B.E., K.C.B., M.D., F.R.C.P., F.R.S.

Sir Edward MelIanby had accepted a cordial invitation from the Editorial Committee of the Annual Review of Biochemistry to contribute this Prefatory Chapter. Unhappily, before he had completed his seventy-first year, he died suddenly on Sunday, January 30, 1955, about mid-day, after he had spent the morning at his research. His draft of the introduction to his contribution to this volume is printed below in the section“ to British Biochemistry.” For nearly six years after retiring from the post of Secretary of the Medical Research Council, he continued to engage in research work and in many scientific affairs. He was, for example, active in the Royal Society, of which he had been a Fellow for 30 years, and he was one of its Vice-Presidents. Sir Henry Dale has recently completed an Obituary Notice of Mellanby for the Royal Society (1955); this tribute is of such grandeur as perhaps only a man of Sir Henry's eminence could contribute, and I shall quote from this to give the weight of wisdom, experience, and...

B. S. Platt

Papers

Sir Edward Mellanby, G.B.E., K.C.B., M.D., F.R.C.P., F.R.S. (1884-1955): The Man, Research Worker, and Statesman