Agriculture Reference
In-Depth Information
C
h L of r i D e
Chloride (Cl
−
) constitutes about 0.15% of the body mass of a 70-kg adult human,
with about 88% in extracellular fluid and 12% inside the cells. As a negative ion,
it neutralizes the positive charge of sodium and is important in maintaining elec-
trolyte balance. Other roles include production of hydrochloric acid by the parietal
cells of the stomach, chloride release by white blood cells during phagocytosis to
aid destruction of microorganisms, and as an exchange ion for bicarbonate (HCO
3
−
)
during the movement of the latter out of cells as CO
2
is generated during metabolism
(the so-called chloride shift). This involves a protein transporter that moves Cl
−
and
HCO
3
−
in opposite directions across the cell membrane so HCO
3
−
can enter plasma,
be transferred to red blood cells, and be carried to the lungs for exhalation of CO
2
.
Most of ingested chloride comes from salt (NaCl). Salt is about 60% chloride, and
processed foods and salty snacks supply liberal amounts. Eggs, meat, and seafood
are good natural sources. Estimated adequate chloride intakes range from 1500 mg
per day for 1- to 3-year-old children to 2300 mg per day for teenagers and men and
women to 50 years of age. Estimated adequate intakes then decline to a low of 1800
mg per day at ages greater than 70.
i
r o n
The adult human body contains about 2 to 4 g of iron, with more than 65% in hemo-
globin, about 10% in myoglobin, about 1-5% in enzymes, and the rest in transport
form (transferrin) or storage forms (ferritin and hemosiderin). It exists in two oxida-
tion states in the body, ferrous (Fe
2+
) and ferric (Fe
3+
). The presence of iron in heme
is vital for transport of oxygen in hemoglobin to tissues and for transitional storage of
oxygen in myoglobin in muscle. Oxygen is held in a loose coordinate bond with iron
in these heme proteins, allowing for ready oxygen transfer to tissues for support of
cellular metabolism. There also are heme-containing enzymes, such as catalase, the
cytochromes, and myeloperoxidase, and nonheme, iron-containing enzymes, such
as the oxygenases and oxidoreductases. These enzymes are variously involved in
electron transfer; amino acid metabolism; synthesis of carnitine, collagen, DNA, and
vitamin A; and destruction of cytotoxic compounds.
Absorption of iron found in heme, as part of hemoglobin or myoglobin in animal
products, requires that heme be hydrolyzed from globin by proteases in the stomach
and small intestine. The released heme is soluble and enters the enterocyte intact
via an uncharacterized transporter. Within the mucosal cell, heme is hydrolyzed,
and ferrous iron is released and used within the cell or transported to the basolateral
membrane. Bound nonheme iron in food is freed for absorption by hydrochloric acid
and pepsin in gastric secretions and by intestinal proteases. Most of the iron released
is present in the ferric form and remains soluble as long as the milieu is acidic.
However, intestinal and pancreatic juices tend to be alkaline, and some of the ferric
iron may undergo reduction to ferrous iron. Ferrous iron may enter the enterocyte
in association with transporter proteins that, to a lesser extent, can also transport
copper, zinc, manganese, nickel, and lead. Absorption of ferric iron is facilitated by
association with chelators, small organic compounds that form ligands and maintain
