Food Lipids: Chemistry, Nutrition, and Biotechnology

Toxic symptoms of hypervitaminosis A include drowsiness, headache, vomiting,
and muscle pain. Vitamin A can be teratogenic at high doses [38]. Vitamin A
deficiency results in night blindness and ultimately total blindness, abnormal bone
growth, increased cerebrospinal pressure, reproductive defects, abnormal cornification,
loss of mucus secretion cells in the intestine, and decreased growth. The importance
of beef liver, an excellent source of vitamin A, in cure of night blindness
was known to the ancient Egyptians about 1500 BC [39].
2. Vitamin D
Although as many as five vitamin D compounds have been described (Fig. 29), only
two of these are biologically active: ergocalciferol (vitamin D 2) and cholecalciferol
(vitamin D 3). Vitamin D 3 can be synthesized in humans from 7-dehydrocholesterol,
which occurs naturally in the skin via light irradiation (Fig. 30).
The actual hormonal forms of the D vitamins are the hydroxylated derivatives.
Vitamin D is converted to 25-OH-D in the kidney and further hydroxylated to 1,25diOH-D
in the liver. The dihydroxy form is the most biologically active form in
humans.
3. Vitamin E
Vitamin E compounds include the tocopherols and tocotrienols. Tocotrienols have a
conjugated triene double bond system in the phytyl side chain, while tocopherols do
not. The basic nomenclature is shown in Figure 31. The bioactivity of the various
vitamin E compounds is shown in Table 9. Methyl substitution affects the bioactivity
of vitamin E, as well as its in vitro antioxidant activity.
4. Vitamin K
Several forms of vitamin K have been described (Fig. 32). Vitamin K 1 (phylloquinone)
is found in green leaves and vitamin K 2 (menaquinone) is synthesized by
intestinal bacteria. Vitamin K is involved in blood clotting as an essential cofactor
in the synthesis of �-carboxyglutamate necessary for active prothrombin. Vitamin K
deficiency is rare because of intestinal microflora synthesis. Warfarin and dicoumerol
prevent vitamin K regeneration and may result in fatal hemorrhaging.
J. Hydrocarbons
The hydrocarbons include normal, branched, saturated, and unsaturated compounds
of varying chain lengths. The nomenclature for hydrocarbons has already been discussed.
The hydrocarbons of most interest to lipid chemists are the isoprenoids and
their oxygenated derivatives.
The basic isoprene unit (2-methyl-1,3-butadiene) is the building block for a
large number of interesting compounds, including carotenoids (Fig. 33), oxygenated
carotenoids (Fig. 34), sterols, and unsaturated and saturated isoprenoids (isopranes).
Recently, it has been discovered that 15-carbon and 20-carbon isoprenoids are covalently
attached to some proteins and may be involved in control of cell growth
[40]. Members of this class of protein-isoprenoid molecules are called prenylated
proteins.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.