Biology of Disease

called Dcytb bound to the brush border of enterocytes reduces ferric iron to

the ferrous form. Ferrous iron is then transported into the cell by a H+ coupled

mechanism as illustrated in Figure 11.21. The transporter, called the divalent

metal transporter 1 (DMT1), is also able to effect the absorption of a number

of other divalent metal ions, such as those of cadmium, cobalt copper, lead,

nickel and zinc. Heme iron is absorbed into the enterocyte by a heme receptor

and, once internalized, its ferrous iron is released into the intracellular pool

by heme oxygenase activity (Figure 11.21). Ferrous iron is exported from the

enterocyte across the basal membrane by a membrane protein called ferro-

portin1 or Ireg1. It is then oxidized by hephaestin, a transmembrane copper

dependent ferroxidase, which is necessary for effective iron transport. The

ferric iron is bound by transferrin in the plasma and can be stored in erythro-

cytes in ferritin molecules (Chapter 13).

Calcium is absorbed in the upper part of the small intestine in an ionic form.

This absorption requires the active metabolite of vitamin D, 1,25-dihydroxy-

vitamin D 3 , and is inhibited by substances that form insoluble calcium salts,

such as phosphate and oxalate. The uptake of Na+ has been mentioned already

in relation to the active uptake of several nutrients and many anions, hydro-

gen carbonate, chloride and iodide, can passively follow it into enterocytes.

Phosphate is actively accumulated by enterocytes.

Amino acids, monosaccharides, urate and uracil, B vitamins, vitamin C and

minerals all leave the enterocytes through their basolateral membranes,

enter the hepatic portal vein and are delivered to the liver. Following their

absorption, many minerals are bound by intracellular proteins before being

expelled through the basolateral membrane into the bloodstream where they

X]VeiZg&&/ DISORDERS OF THE GASTROINTESTINAL TRACT, PANCREAS, LIVER AND GALL BLADDER

'.) W^dad\nd[Y^hZVhZ

Heme receptor

Capillary

GIT lumen

Fe3+

Fe3+

Fe2+

Fe2+

Fe2+

Fe2+

Fe2+

Dcytb Hephaestin

DMT1 Ferroportin

Heme

Heme

Intracellular

iron pool

Heme

oxygenase

Tight junction

Figure 11.21Uptake of iron. Transport

movements are denoted by colored lines,

chemical transformations in black. See text for

general details.