714
UNIT FIVE
Rich sources of niacin (and tryptophan) include
liver, lean meats, peanut butter, and legumes. Milk is a
poor source of niacin but a good source of tryptophan.
Excess niacin can cause acute toxicity with effects
such as fl
ushing, wheezing, vasodilation, headache,
diarrhea, and vomiting. Chronic toxicity effects include
liver problems.
Historically, niacin deF
ciencies have been
associated with diets largely consisting of corn and
corn products, which are very low in niacin and lack
adequate tryptophan. Such a deF
ciency causes a disease
called
pellagra
that produces dermatitis, infl
ammation
of the digestive tract, diarrhea, and mental disorders.
Pellagra is rare today, but it was a serious problem
in the rural South of the United States in the early 1900s.
Pellagra is less common in cultures that extensively
treat corn with lime (CaCO
3
) to release niacin bound
to protein. It sometimes affects people with chronic
alcoholism who drink alcohol rather than eat.
4.
Pantothenic acid,
or
vitamin B
5
.
Pantothenic acid is
a yellowish oil destroyed by heat, acids, and bases.
It functions as part of a complex molecule called
coenzyme A,
which, in turn, reacts with intermediate
products of carbohydrate and fat metabolism to yield
acetyl coenzyme A,
which enters the citric acid cycle.
Pantothenic acid is therefore essential to cellular energy
release.
gastrointestinal disturbances, mental confusion,
muscular weakness and paralysis, and heart
enlargement. In severe cases, the heart may fail.
In developed nations, beriberi affects mostly people with chronic
alcoholism who have substituted alcohol for foods. Moreover,
because thiamine is required for the metabolic oxidation of alcohol,
people with alcoholism are particularly likely to develop a thiamine
deF
ciency.
2.
Ribofl
avin,
or
vitamin B
2
.
Ribofl avin is a yellowish
brown crystalline substance that is relatively stable to
the effects of heat, acids, and oxidation but is destroyed
by exposure to bases and ultraviolet light. This vitamin
is part of several enzymes and coenzymes known as
avoproteins
. One such coenzyme, ±AD, is an electron
carrier in the citric acid cycle and electron transport chain
of aerobic respiration. ±lavoproteins are essential for
the oxidation of glucose and fatty acids and for cellular
growth. An active transport system controls the amount
of ribofl avin entering the intestinal mucosa. Ribofl avin
is carried in the blood combined with proteins called
albumins
. Excess ribofl avin in the blood is excreted in
the urine, turning it yellow-orange, and any that remains
unabsorbed in the intestine is lost in the feces.
The amount of ribofl
avin the body requires varies
with caloric intake. About 0.6 mg of ribofl
avin per 1,000
calories is sufF
cient to meet daily cellular requirements.
Ribofl
avin is widely distributed in foods, and rich
sources include meats and dairy products. Leafy green
vegetables, whole-grain cereals, and enriched cereals
provide lesser amounts. Vitamin B
2
deF
ciency produces
dermatitis and blurred vision.
3.
Niacin
or
vitamin B
3
.
Niacin, also known as
nicotinic
acid,
is in plant tissues and is stable in the presence of
heat, acids, and bases. After ingestion, it is converted
to a physiologically active form called
niacinamide
(F
g. 18.12)
. Niacinamide is the form of niacin in foods
of animal origin.
Niacin functions as part of two coenzymes
(coenzyme I, also called NAD
[F g. 18.13]
, and coenzyme
II, called NADP) essential in glucose oxidation. These
coenzymes are electron carriers in glycolysis, the citric
acid cycle, and the electron transport chain, as well as in
the synthesis of proteins and fats. They are also required
for the synthesis of the sugars (ribose and deoxyribose)
that are part of nucleic acids.
Niacin is readily absorbed from foods, and human
cells synthesize it from the essential amino acid
tryptophan
. Consequently, the daily requirement for
niacin varies with tryptophan intake. Nutritionists
recommend a daily niacin (or niacin equivalent) intake
of 6.6 mg per 1,000 calories.
Niacin (nicotinic acid)
C
O
OH
HC
HC
C
CH
N
C
H
Niacinamide
C
O
NH
2
HC
HC
C
H
N
C
H
C
FIGURE 18.12
Enzymes catalyze reactions that convert niacin from
foods into physiologically active niacinamide.
NAD or nicotinamide adenine dinucleotide
Niacinamide
P
O
OCH
2
H
2
CO
P
NH
2
C
H
H
H
O
H
O
O
OH
OH
H
OH
OH
NH
2
C
C
C
C
N
+
H
H
H
H
N
N
O
O
N
HH
H
OH
OH
N
FIGURE 18.13
Niacinamide is incorporated into molecules of NAD.
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