Vitamin D Family

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Rickets was first reported in the mid-seventeenth century. It could be lethal, but bone deformation was more common. In the United States, rickets continued to be a problem into the 1930s until vitamin D-fortified milk became common. Older adults show bowlegs characteristic of childhood rickets. There were many attempts at giving calcium and/or phosphorous supplements. Finally, it was realized that rickets was not found in sunny climates, and populations whose main source of protein was fish did not have rickets. In 1924, Professor Henry Steenbock, University of Wisconsin, demonstrated that irradiation of foods, including milk, produced food that was antirachitic (16,17).

When there is adequate sunlight, no dietary source of the vitamin is required. Indeed, an argument can be made that the calciferols are not normal components of the diet. In the United States. it is added to milk. other dairy products, and dairy substitutes. Fish is about the only natural food source. Cholecal-ciferol is produced in the body from endog-enously synthesized 7-dehydrocholecalciferol (Fig. 8.10). Consistent with a hormone model, excess amounts of cholecalciferol can result in excess calcium uptake from the intestinal tract, leading to calcification of soft tissues.

3.2.1 Chemistry. There are two forms of vitamin D, and both are considered biologically equivalent. Irradiation of the major plant sterol, ergosterol, produces ergocalciferol, also known as vitamin D2 (Fig. 8.11). Because they are photochemical reactions and in contrast to enzyme-catalyzed biochemical reactions, the formation of cholecalciferol is not clean. Exposure of human skin to sunlight of 295-300 nm converts 7-dehydrocholesterol to previtamin D,. The isomerization to cholecalciferol (vitamin D3) is heat catalyzed. Continuous exposure to ultraviolet radiation from the sun results in the reversible formation of lumisterol

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Figure 8.10. Photochemical formation of cholecalciferol (vitamin D3).

HO 3

Cholecalciferol (Vitamin D3)

HO 3

Tachysterol

Figure 8.10. Photochemical formation of cholecalciferol (vitamin D3).

sand tachysterol (18, 19). Once the B-ring of pie two steroids has been cleaved, the prod-cts should no longer be referred to as steroids. It is incorrect to call these compounds roidal vitamins.

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