Agriculture Reference
In-Depth Information
diffusion, primarily in the jejunum, followed by release into the portal blood plasma.
When taken up by the liver, most of the free forms are converted to PLP and stored
or bound to albumin and delivered to peripheral tissues by the systemic circulation.
Good food sources of vitamin B
6
include meats, vegetables, bananas, whole-grain
breads and cereals, and nuts. Pyridoxine hydrochloride is commonly used in vitamin
supplements. Recommended dietary allowances range from 0.5 mg per day for 1- to
3-year-old children to 2.0 mg per day for pregnant women.
b
i of t i n
Biotin is a white, water-soluble vitamin that functions as a coenzyme for four impor-
tant carboxylases. Pyruvate carboxylase replenishes oxaloacetate for the Krebs cycle
and is necessary for carbohydrate synthesis from noncarbohydrate precursors (glu-
coneogenesis). Acetyl CoA carboxylase furnishes acetate for synthesis of fatty acids.
Propionyl CoA carboxylase is involved in metabolism of some amino acids and fatty
acids with an odd-numbered chain. β-Methylcrotonyl CoA carboxylase is involved
in catabolism of leucine and certain isoprenoid compounds. Biotin also may be
involved in gene expression.
Biotin in foods is often bound to protein or to lysine (biocytin). These bound
forms must be hydrolyzed by proteases in the jejunum and ileum for absorption to
occur. Some biocytin may be absorbed, but it is not functional and is excreted in the
urine. Biotin transport into the enterocyte at low concentrations is sodium dependent
and carrier mediated. Transport across the basolateral membrane is carrier mediated
but does not require sodium. Biotin synthesized by colonic bacteria can be absorbed
in the proximal and midtransverse regions. In plasma, biotin is mostly in the free
form, although some may be bound to albumin, α- and β-globulins, and biotini-
dase. Biotin is taken up by tissues via active, sodium-dependent carriers. Good food
sources include liver, legumes, egg yolk, nuts, and some cereals. Raw egg white con-
tains a glycoprotein, called avidin, that binds biotin. However, avidin is heat labile,
and cooking frees biotin for absorption. The amount of biotin synthesized by bacte-
ria in the gut of cattle, sheep, and horses is considerable, but the amounts available
from colonic bacterial synthesis in humans are unlikely to meet needs. Estimated
adequate intakes range from 8 µg per day for 1- to 3-year-old children to 35 µg per
day for lactating women.
f
o L a C i n
Folacin
(or
folate
) is a generic term for compounds with the activity of folic acid (pte-
roylmonoglutamate). The basic structure of folic acid consists of a pteridine nucleus
conjugated with para-aminobenzoic acid, forming pteroic acid, which in turn is bound
to glutamic acid. In food, folacin may contain up to nine glutamate residues. Before
absorption, the polyglutamate forms must be hydrolyzed to the monoglutamate form.
This hydrolysis is performed by glutamate carboxypeptidases in pancreatic juice,
bile, and the intestinal cell brush border. This last enzyme is zinc dependent, and zinc
deficiency will inhibit folic acid absorption. Transport into the enterocyte is energy
and sodium dependent. Within the enterocyte, folic acid is reduced to dihydrofolate
