Agriculture Reference
In-Depth Information
oxidation-reduction reactions within intermediary metabolic cycles that are central
to energy production. They function in these reactions as oxidizing agents through
their ability to accept a pair of hydrogen atoms. Absorption of riboflavin requires
that it be freed from any bound forms (protein bound or phosphorylated) before
transport into the intestinal mucosa via a saturable, energy-dependent carrier. If
food riboflavin concentrations are high, some may be absorbed by diffusion. In the
enterocyte, riboflavin is phosphorylated to FMN and then dephosphorylated at the
basolateral membrane before entering the blood. Free riboflavin or riboflavin bound
to plasma proteins (albumin and globulins) is carried via the portal vein to the liver,
where it is phosphorylated again to FMN or FAD. Riboflavin is found in many foods,
but animal products (such as milk, cheese, eggs, and meat) and legumes are particu-
larly good sources. Green vegetables supply some, and grains and fruits are gener-
ally low, although cereals and breads may be enriched with riboflavin supplements.
Recommended dietary allowances range from 0.5 mg per day for 1- to 3-year-old
children to 1.6 mg per day for lactating women.
n
i a C i in
Niacin
is a generic term for nicotinic acid and nicotinamide, both of which have
vitamin activity as a consequence of their interconvertability. As nicotinamide
adenine dinucleotide (NAD) or nicotinamide adenine dinucleotide phosphate
(NADP), niacin serves coenzyme functions as a hydrogen donor or electron acceptor
in oxidation-reduction reactions. NAD is reduced to NADH during catabolism of
carbohydrates (glycolysis), β-oxidation of fatty acids, oxidation of ethanol, oxida-
tive decarboxylation of pyruvate, and oxidation of acetyl coenzyme A (CoA) via the
Krebs cycle. NAD also catalyzes the conversion of vitamin B
6
, as pyridoxal, to its
excretory product pyridoxic acid. NAD can serve as a donor of adenosine diphos-
phate ribose (ADP-ribose) in nonredox reactions involving proteins that function in
repair, replication, and differentiation of nuclear DNA. NADP is reduced to NADPH
during synthesis of fatty acids, cholesterol, steroid hormones, and DNA precursors
(deoxyribonucleotides); oxidation of glutamate; and regeneration of glutathione,
vitamin C, and thioredoxin. NADPH is required in several reactions involving folate
metabolism.
Absorption of nicotinamide or nicotinic acid from the stomach has been demon-
strated, but most absorption takes place in the small intestine. When dietary concen-
trations are low, niacin is absorbed by sodium-dependent, carrier-mediated diffusion.
At high concentrations, most is absorbed by passive diffusion. If NAD or NADP
is present in the diet, hydrolysis in the intestinal lumen or enterocyte is required to
release free nicotinamide for further transport. Nicotinamide is the predominant nia-
cin form in plasma, but nicotinic acid is also present—up to a third bound to plasma
proteins. These free forms move across cell membranes by simple diffusion, but once
they are converted to NAD or NADP within cells, they are trapped until hydrolyzed.
Foods of animal origin, such as fish and organ and muscle meats, are good
sources of niacin. Legumes and enriched breads and cereals also provide appre-
ciable amounts. Nicotinamide is the usual supplemental form. Niacin is complexed
with carbohydrates (niacytin) or small peptides (niacinogens) in some foods and is
