Food Lipids: Chemistry, Nutrition, and Biotechnology

E. The Chemistry of Vitamin D and Related Sterols
The discovery of vitamin D dates back to the 1930s, following studies of rickets, a
well-known disease resulting from deficiency of vitamin D [50]. Vitamin D has
basically two functions in mammals: to stimulate the intestinal absorption of calcium
and to metabolize bone calcium. A deficiency of vitamin D results in rickets in young
growing animals and osteomalacia in adult animals. In both cases, the collagen fibrils
are soft and pliable and are unable to carry out the structural role of the skeleton.
As a result, bones become bent and twisted under the stress of the body’s weight
and muscle function.
Vitamin D is obtained from dietary uptake or via biosynthesis in the skin by
means of the ultraviolet irradiation of 7-dehydrocholesterol. The UV irradiation of
7-dehydrocholesterol first produces provitamin D3, which results from a rupture in
the 9–10 bond followed by a 5,7-sigmatropic shift (Scheme 9). Provitamin then
undergoes the thermally dependent isomerization to vitamin D3 in liver, and further
is metabolized to 1,25-dihydroxyvitamin D3, which is 10 times more active than
vitamin D3, whereas 25-hydroxyvitamin D3 (Scheme 10) is approximately twice as
active as vitamin D3. The most important nutritional forms of vitamin D are shown in Figure 3. Of
these structures, the two most important are vitamin D2 and vitamin D3. These two
forms of vitamin D are prepared from their respective 5,7-diene sterols. Vitamins
D4,D5, and D6 have also been prepared chemically, but they have much lower
biological activity than vitamins D2 and D3. Also, many analogs of vitamin D metabolites
have been synthesized. Some of these compounds exhibit similar vitamin
D hormone responses and have found use in the treatment of vitamin D deficiency
diseases (Fig. 4). Recently, vitamin D metabolites were found to be potent inducers
of cancer cells, which make this steroid hormone and its analogs (biosynthetic inhibitors)
potential candidates for the treatment of cancers and other diseases [51].
Scheme 9 Photochemical synthesis of vitamin D 3.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.