Page 373 - Color Atlas of Biochemistry

P. 373

364 Nutrition

Lipid-soluble vitamins the chromoprotein rhodopsin (see p. 358).
Retinoic acid, like the steroid hormones, in-
Vitamins are essential organic compounds fluences the transcription of genes in the cell
that the animal organism is not capable of nucleus. It acts as a differentiation factor in
forming itself, although it requires them in growth and development processes. Vitamin
small amounts for metabolism. Most vitamins A deficiency can result in night blindness, vis-
are precursors of coenzymes;in somecases, ual impairment, and growth disturbances.
they are also precursors of hormones or act as Vitamin D (calciol, cholecalciferol) is the
antioxidants. Vitamin requirements vary from precursor of the hormone calcitriol (1α,25-di-
species to species and are influenced by age, hydroxycholecalciferol; see p. 320). Together
sex, and physiological conditions such as with two other hormones (parathyrin and
pregnancy, breast-feeding, physical exercise, calcitonin), calcitriol regulates the calcium
and nutrition. metabolism (see p. 342). Calciol can be syn-
thesized in the skin from 7-dehydrocholes-
terol, an endogenous steroid, by a photo-
A. Vitamin supply
chemical reaction. Vitamin D deficiencies
A healthy diet usually covers average daily only occur when the skin receives insuf cient
vitamin requirements. By contrast, malnutri- exposure to ultraviolet light and vitamin D is
tion, malnourishment (e. g., an unbalanced lacking in the diet. Deficiency is observed in
diet in older people, malnourishment in alco- the form of rickets in children and osteomala-
holics,ready meals),orresorptiondisturban- cia in adults. In both cases, bone mineraliza-
ces lead to an inadequate supply of vitamins tion is disturbed.
from which hypovitaminosis, or in extreme Vitamin E (tocopherol) and related com-
cases avitaminosis, can result. Medical treat- pounds only occur in plants (e. g., wheat
ments that kill the intestinal flora—e. g., anti- germ). They contain what is known as a chro-
biotics—can also lead to vitamin deficiencies man ring. In the lipid phase, vitamin E is
(K, B 12 , H) due to the absence of bacterial mainly located in biological membranes,
vitamin synthesis. where as an antioxidant it protects unsatu-
Since only a few vitamins can be stored (A, rated lipids against ROS (see p. 284) and other
D, E, B 12 ), a lack of vitamins quickly leads to radicals.
deficiency diseases. These often affect the Vitamin K (phylloquinone) and similar sub-
skin, blood cells, and nervous system. The stances with modified side chains are in-
causes of vitamin deficiencies can be treated volved in carboxylating glutamate residues
by improving nutrition and by administering of coagulation factors in the liver (see
vitamins in tablet form. An overdose of vita- p. 290). The form that acts as a cofactor for
mins only leads to hypervitaminoses,with carboxylase is derived from the vitamin by
toxic symptoms, in the case of vitamins A enzymatic reduction. Vitamin K antagonists
and D. Normally, excess vitamins are rapidly (e. g., coumarin derivatives) inhibit this reduc-
excreted with the urine. tion and consequently carboxylation as well.
This fact is used to inhibit blood coagulation
in prophylactic treatment against thrombosis.
B. Lipid-soluble vitamins
Vitamin K deficiency occurs only rarely, as the
Vitamins are classified as either lipid-soluble vitamin is formed by bacteria of the intestinal
or water-soluble. The lipid-soluble vitamins flora.
includevitamins A,D,E,and K, allofwhich
belong to the isoprenoids (see p. 52).
Vitamin A (retinol) is the parent substance
of the retinoids, which include retinal and
retinoic acid. Theretinoids also can besynthe-
sized by cleavage from the provitamin β-car-
otene. Retinoids are found in meat-containing
diets, whereas β-carotene occurs in fruits and
vegetables (particularly carrots). Retinal is in-
volved in visual processes as the pigment of

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