Agriculture Reference
In-Depth Information
complexes that are formed may then be destroyed by complement proteins produced
mainly in the liver or by cytokines produced by white blood cells, such as T-helper
cells or macrophages. Prenatal or childhood protein-energy malnutrition may have
permanent adverse affects on immune function.
n
u t r i e n t
t
r a n s P o r t
Proteins that combine with substances requiring transport in the blood, within cells,
or across cell membranes include albumin, transthyretin (formerly prealbumin),
heme proteins (hemoglobin, myoglobin), transferrin, and ceruloplasmin. Albumin
transports several nutrients, including calcium, zinc, and vitamin B
6
. Transthyretin
complexes with retinol-binding protein to transport retinol (vitamin A). Hemoglobin
and myoglobin are iron-containing proteins that bind or transport oxygen. Transferrin
transports iron, and ceruloplasmin transports copper.
PRoteIn dIgestIon And AbsoRPtIon
Digestion of dietary protein begins in the stomach through the action of hydrochloric
acid and pepsin. End products of gastric digestion include primarily large polypep-
tides, some oligopeptides (polymers of just a few amino acids), and free amino acids.
These end products are emptied from the stomach into the duodenum, stimulating
release of alkaline pancreatic juice containing water, bicarbonate, various electro-
lytes, and proenzymes that undergo activation to trypsin, chymotrypsin, and car-
boxypeptidases. Peptidases are also produced by the brush border of enterocytes
(intestinal mucosal cells), extending from the duodenum into the distal small intes-
tine (ileum). The main end products of digestion in the small intestine are dipeptides,
tripeptides, and free amino acids.
Absorption of amino acids from the intestinal lumen into the enterocyte involves
multiple energy-dependent transport systems through the brush border, some of
which also require sodium cotransport. Dipeptide and tripeptide transport through
the brush border involves carrier systems different from those for amino acids, and
peptide absorption tends to be more rapid. The inward movement of hydrogen ions
provides an absorptive driving force, followed by outward pumping of hydrogen ions
into the lumen in exchange for inward movement of sodium ions. Evidence suggests
that about two thirds of the amino acids are absorbed as small peptides, whereas
the rest are absorbed in free form. Peptides inside the enterocytes are hydrolyzed by
cytoplasmic peptidases, generating free amino acids.
A significant proportion of absorbed amino acids is used by the intestinal cell for
synthesis of new digestive enzymes, hormones, and other nitrogen-containing com-
pounds and for energy. Transport through the enterocyte basolateral membrane into
the interstitial fluid (and ultimately into the capillaries and portal vein) appears to be
largely by diffusion and sodium-independent systems, although sodium-dependent
pathways may be important when luminal amino acid concentrations are low.
