716
UNIT FIVE
Vitamin C,
or
ascorbic acid.
Ascorbic acid is a crystal-
line compound that has six carbon atoms. Chemically, it is
similar to the monosaccharides
(f
g. 18.16)
. Vitamin C is one
of the least stable vitamins in that oxidation, heat, light, or
bases destroy it. However, vitamin C is fairly stable in acids.
Ascorbic acid is necessary for the production of the con-
nective tissue protein
collagen,
for conversion of folacin to
folinic acid, and in the metabolism of certain amino acids. It
also promotes iron absorption and synthesis of certain hor-
mones from cholesterol.
Overall, vitamin C is not stored in any great amount, but
the adrenal cortex, pituitary gland, and intestinal glands con-
tain high concentrations of it. Excess vitamin C is excreted in
the urine or oxidized.
Individual requirements for ascorbic acid may vary. Ten
mg per day is sufF cient to prevent deF ciency symptoms, and
80 mg per day saturate the tissues within a few weeks. Many
nutritionists recommend a daily adult intake of 60 mg, which
is enough to replenish normal losses and satisfy cellular
requirements.
Ascorbic acid is fairly widespread in plant foods, with
high concentrations in citrus fruits and tomatoes. Leafy
green vegetables are also good sources.
Prolonged deF ciency of ascorbic acid leads to
scurvy,
which is more likely to affect infants and children. Scurvy
impairs bone development and causes swollen, painful
joints. The gums may swell and bleed easily, resistance to
infection is lowered, and wounds heal slowly
(f
g. 18.17)
.
If a woman takes large doses of ascorbic acid during preg-
nancy, the newborn may develop symptoms of scurvy when
the daily dose of the vitamin drops after birth because it is
no longer delivered through the placenta.
Table 18.9
summarizes the water-soluble vitamins and
their characteristics.
PRACTICE
49
What factors destroy vitamin C?
50
What are functions of vitamin C?
51
Which foods are good sources of vitamin C?
52
What are symptoms of vitamin C deF
ciency?
oxidized in an acid environment and is destroyed by
heat in alkaline solutions; consequently, this vitamin
may be lost in stored or cooked foods.
±olacin is readily absorbed from the digestive tract
and is stored in the liver, where it is converted to a
physiologically active substance called
folinic acid.
±olinic acid functions as a coenzyme necessary for the
metabolism of certain amino acids and for the synthesis
of DNA. It also acts with cyanocobalamin in producing
normal red blood cells.
Good sources of folacin include liver, leafy green
vegetables, whole-grain cereals, and legumes. Due to
excess excretion in the urine, toxicity is rare. ±olacin
deF ciency leads to
megaloblastic anemia,
in which
the number of normal red blood cells is reduced and
large, nucleated red cells appear. ±olacin deF
ciency
has been linked to neural tube defects, in which the
tube that becomes the central nervous system in a
fetus fails to entirely close. Neural tube defects include
spina biF
da and anencephaly. Taking synthetic folic
acid supplements just before and during pregnancy can
greatly reduce the risk of a neural tube defect.
Naturally occurring folate is a mixture of compounds that collectively
have the same activity as synthetic folic acid, also called pteroylmono-
glutamic acid. However, synthetic folic acid is much more stable and
enters the bloodstream much more readily. This di±
erence has led to
confusion. ²or example, 200 micrograms of synthetic folic acid is pre-
scribed to treat anemia, but the same e±
ect requires 400 micrograms
of folate from foods. Synthetic folic acid used to enrich grain foods
has a greater e±
ect on health than folate.
8.
Biotin.
Biotin is a simple compound, that is stable to the
effects of heat, acids, and light but may be destroyed
by oxidation or bases. (See F
g. 18.18 for the molecular
structure of biotin.) It is a coenzyme in metabolic
pathways for amino acids and fatty acids. It also plays a
role in the synthesis of the purine nitrogenous bases of
nucleic acids.
Metabolically active organs such as the brain, liver,
and kidneys store some biotin. Bacteria that inhabit the
intestinal tract synthesize biotin. The vitamin is widely
distributed in foods, and dietary deF ciencies are rare.
Good sources include liver, egg yolk, nuts, legumes, and
mushrooms. Excess biotin does not produce toxic effects.
PRACTICE
45
Which biochemicals comprise the vitamin B complex?
46
Which foods are good sources of vitamin B complex?
47
Which of the B-complex vitamins can be synthesized from
tryptophan?
48
What is the general function of each member of the vitamin B
complex?
OH
HC
OH O
HC
H
HO
C
OH
HC
HC
OH
HC
H
C
O
O
C
HO
HO
C
HC
HC
Vitamin C
(ascorbic acid)
Glucose
(a monosaccharide)
OH
HC
OH
H
FIGURE 18.16
Vitamin C is chemically similar to some 6-carbon
monosaccharides.
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