Page 208 - 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

516 Vitamins and Coenzymes

TABLE I Approximate Nutritional Requirements (mg/day) for
the Vitamins and Some Characteristic Deficiency Diseases or
Symptoms
Approximate Related
daily need Deficiency coenzyme
Vitamin (mg) diseases or function
Thiamin 0.8 or more a Beriberi Thiamin
diphosphate
Pantothenic 10–15 Coenzyme A
acid
Riboflavin 1.5 FMN, FAD
Nicotinamide 2.5 b Pellagra NAD, NADP
(or nicotinic
acid)
Biotin 0.15–0.3 Bound as prosthetic
group
Pyridoxine 1.5–2 Pyridoxal or
(vitamin B 6 ) pyridoxamine
phosphate
Folic acid 0.2–0.4 c Tetrahydrofolate
Vitamin C 50–200 Scurvy Antioxidant,
electron carrier

Vitamin B 12 0.002 Pernicious 5 -Deoxycobalamin,

FIGURE 11 The vitamin biotin and the vitamin-like compound (cobalamin) anemia 5 -methylcobalamin
lipoic acid and their covalent attachments to selected lysine side Vitamin A 0.7 Retinol, bound
chains in proteins (polypeptides). Both of these compounds func- (retinol) as prosthetic
tion as catalytic prosthetic groups, biotin for CO 2 and lipoic acid group
for hydrogen. The fragment biocytin was isolated from autolysates Vitamin D 0.02 Rickets Hormonal role
of rapidly growing yeast. in calcium
metabolism
and high in maize, a grain whose protein is deficient Vitamin E 8–10 Antioxidant
in tryptophan. Tryptophan can be converted to nicoti- Vitamin K 0.05–0.08 Bleeding Blood clotting
namidewithanefficiencyofabout1/60.Hence,mostdiets a Amount should be at least 0.5 mg per 1000 kcal (Cal) of food energy.
provide the necessary minimum. However, persons with b Some may be obtained from metabolism of the amino acid trypto-
pellagra often died after suffering from characteris- phan, about 1/60 of which can be converted into this vitamin.
tic symptoms of dermatitis, diarrhea, and dementia. c The larger amount is recommended for women of child-bearing age.
Deficiency of vitamin D was widespread, especially in
northern regions, prior to the use of supplementation of species are unable to synthesize this important antioxi-
milk. Deficiencies of the B vitamins, pantothenic acid, ri- dant compound. The need for ascorbic acid is high, but
boflavin, biotin, and vitamin B 6 , are not often met in the the optimum amount needed for good nutrition is uncer-
human population. Except for the sensitivity of riboflavin tain. Furthermore, there has been some concern that exces-
to light, these compounds are quite stable. Nevertheless, sive intake of vitamin C, especially in combination with
some infants are born with unusually high requirements iron ions, may generate damaging free radicals. However,
for specific vitamins. Some cases of sudden infant death ascorbic acid seems to have predominantly an antioxida-
have been attributed to biotin deficiency and convulsions tive effect in animals.
in infants to a deficiency of vitamin B 6 in a nutritional Vitamin B 12 is required in minute amounts, one mi-
formula. Vitamin B 6 is a family of three forms, an alco- crogram per day supplying the needs for the human body.
hol pyridoxol, an amine pyridoxamine, and an aldehyde However, absorption of this small amount of vitamin from
pyridoxal (Fig. 5). Of these, pyridoxol, a very stable com- the gut and transport to its sites of action requires special
pound, predominates in plants. More of the vitamin is transport proteins. One of these, the “intrinsic factor,” is
present as the less stable pyridoxal and pyridoxamine in synthesized by cells of the intestinal mucosa and is uti-
foods of animal origin. lized for absorption of vitamin B 12 . Synthesis of the in-
Vitamin C is made not only by plants but also by most trinsic factor is defective in some individuals, and is of-
animals who use the sugar glucose as the starting mate- ten inadequate in persons older than about 60 years. If
rial. However, human beings, guinea pigs, and a few other untreated, this deficiency leads to pernicious anemia, a