715
CHAPTER EIGHTEEN
Nutrition and Metabolism
Various tissues store cyanocobalamin, particularly
those of the liver. An average adult has a reserve sufF cient
to supply cells for three to F ve years. This vitamin
is essential for the functions of all cells. It is part of
coenzymes required for the synthesis of nucleic acids and
the metabolism of carbohydrates and fats. Vitamin B
12
is
important to erythrocyte production. Cyanocobalamin
also helps form myelin in the central nervous system.
Only foods of animal origin contain cyanocobalamin.
Good sources include liver, meats, milk, cheese, and
eggs. Excessive intake does not appear to be toxic.
In most countries, dietary lack of this vitamin is rare,
although strict vegetarians may develop a deF ciency.
7.
Folacin,
or
folic acid.
±olacin is a yellow crystalline
compound that exists in several forms. It is easily
A daily adult intake of 4–7 mg of pantothenic acid
is adequate. Most diets provide sufF
cient amounts, so
deF ciencies are rare. Good sources of pantothenic acid
include meats, whole-grain cereals, legumes, milk,
fruits, and vegetables.
5.
Vitamin B
6
.
Vitamin B
6
is a group of three compounds,
pyridoxine, pyridoxal,
and
pyridoxamine,
which are
chemically similar
(F g. 18.14)
. These compounds have
similar actions and are fairly stable in the presence
of heat and acids. Oxidation or exposure to bases
or ultraviolet light destroys them. The vitamin B
6
compounds function as coenzymes essential in several
metabolic pathways, including those that synthesize
proteins, amino acids, antibodies, and nucleic acids, as
well as the reaction of tryptophan to produce niacin.
Vitamin B
6
functions in the metabolism of nitrogen-
containing substances, so the requirement for this vitamin
varies with the protein content of the diet rather than
with caloric intake. The recommended daily allowance
of vitamin B
6
is 2.0 mg, but because it is so widespread
in foods, deF ciency conditions are rare. Good sources
of vitamin B
6
include liver, meats, bananas, avocados,
beans, peanuts, whole-grain cereals, and egg yolk.
Excess vitamin B
6
produces burning pains, numbness,
clumsiness, diminished refl exes, and paralysis.
6.
Cyanocobalamin,
or
vitamin B
12
.
Cyanocobalamin has
a complex molecular structure, including a single atom
of the element
cobalt
(F g. 18.15)
. In its pure form, this
vitamin is red. It is stable to the effects of heat but is
inactivated by light or strong acids or strong bases.
Secretion of
intrinsic factor
from the parietal cells of
the gastric glands regulates cyanocobalamin absorption.
Intrinsic factor combines with cyanocobalamin and
facilitates its transfer through the epithelial lining of the
small intestine and into the blood. Calcium ions must be
present for the process to take place.
When the gastric glands of some individuals fail to secrete adequate
amounts of intrinsic factor, cyanocobalamin is poorly absorbed. This
leads to
pernicious anemia,
in which abnormally large red blood cells
called macrocytes are produced when bone marrow cells do not
divide properly because of defective DNA synthesis.
Pyridoxine
CH
2
OH
CH
3
HO
CH
2
OH
C
CH
C
C
C
N
Pyridoxal
Vitamin B
6
CH
2
OH
CH
3
HO
CHO
C
CH
C
C
C
N
Pyridoxamine
CH
2
OH
CH
3
HO
CH
2
NH
2
C
CH
C
C
C
N
Vitamin B
12
(cyanocobalamin)
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CN
CH
2
CO
CH
2
CH
2
N
N
N
N
CONH
2
CH
2
CH
2
CO
CH
2
CH
2
CH
2
CH
3
CH
3
NH
2
CO
CH
2
CH
3
CH
2
CO
CH
2
NH
2
CO
CONH
2
NH
2
NH
2
NH
CH
2
CH
CH
2
HO
Co
+
H
O
OO
H
C
H
C
H
C
HC
N
N
CH
3
CH
3
O
O
O
P
FIGURE 18.14
±Vitamin±B
6
includes three similar chemical compounds.
FIGURE 18.15
±Vitamin±B
12
, which has the most complex molecular
structure of the vitamins, contains cobalt (Co).
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