Page 204 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN017G-116 August 2, 2001 18:14

512 Vitamins and Coenzymes

The curative material, which is abundant in green leafy
vegetables, was named folic acid. However, this name is
usually reserved for the synthetic compound used in di-
etary supplementation. The natural forms are largely the
coenzymes (Fig. 6), which are collectively called folates.
The last of the accepted human vitamins to be discovered
was vitamin B 12 . A cobalt-containing organic compound
needed in very small amounts, it cures and prevents per-
nicious anemia, which was often a fatal disease of peo-
ple over 60 years of age. Its complex structure (Fig. 7)
was determined by X-ray diffraction after numerous ef-
forts at chemical characterization had failed. However,
FIGURE 5 The vitamin B 6 (pyridoxine) family: pyridoxol, pyri-
cyanocobalamin,thecompoundisolatedandtheformused
doxal, pyridoxamine, and the coenzyme forms pyridoxal and pyri-
doxamine phosphates. in nutritional supplementation, is an artifact of the isola-
tion and synthesis. The natural vitamin may have OH in
place of CN but consists largely of the coenzyme forms.
that had been prepared in 1867 by oxidation of nicotine, Have all of the vitamins been discovered? Rodents
but had not been recognized as a nutrient until 1935 when have been reared on almost completely synthetic diets.
it was shown to cure “black tongue” of dogs and shortly However, good health in human beings may require addi-
thereafter human pellagra. Biotin (Fig. 4) was first identi- tional materials. For example, some essential compounds
fied as a growth factor for yeast. Pantothenic acid (Fig. 4), might be made by intestinal bacteria. Some essential coen-
which is present in plentiful amounts in many foods, was zymes such as lipoic acid, ubiquinone (coenzyme Q),
recognized as a curative agent for a dermatitis of chicks. and pyrroloquinoline quinone (PQQ) may be vitamin-like.
Vitamin B 6 (Fig. 5) prevented a facial dermatitis in rats Their presence in foods may be beneficial. Some individ-
(“rat pellagra”). Folic acid, first described by Lucy Wills in uals may need as dietary components these compounds,
Bombay, was identified as a vitamin that cured a macro- which are normally made by the body. Another exam-
cytic anemia that was often associated with pregnancy. ple is the acyl-carrier molecule carnitine (Fig. 4), which

FIGURE 6 The coenzyme tetrahydrofolic acid (tetrahydropteroylglutamic acid). The vitamin folic acid has two addi-
tional double bonds (dashed) in the second ring. Most of this coenzyme exists within cells as more complex forms
containing additional glutamic acid units attached to the side chain at the upper right.