REDUCTION OF FE(III) IS REQUIRED FOR UPTAKE OF NONHEME IRON BY CACO-2CELLS

Differentiated cultures of Caco-2 human colonic cells were used to exa mine the importance of reduction of nonheme ferric iron, Fe(III), for transport across the brush border surface. Cultures accumulated simila r to 100 pmol Fe/(h . mg protein) when 10 mu mol Fe(III) as the nitril otriacetic acid complex (1Fe:2NTA) was added to the apical compartment . Ascorbic acid enhanced cellular acquisition of iron in a dose-depend ent manner, with a concentration as low as 8 mu mol/L ascorbate increa sing iron uptake by 50%. Similarly, the rate of iron transport from th e apical to the basolateral compartment increased 5.6- and 30-fold whe n 100 and 1000 mu mol/L ascorbic acid, respectively, were present in t he apical chamber. Ascorbate-mediated stimulation of iron uptake was t emperature dependent and required the reduction of Fe(III) to Fe(II), because it was inhibited by ascorbate oxidase and chelators of Fe(II). Moreover, Caco-2 cells recycled dehydroascorbic acid to ascorbic acid . Ferricyanide and Fe(II) chelators also partially inhibited iron upta ke from a medium devoid of ascorbic acid. Intact Caco-2 cells exhibite d a ferrireductase activity on the apical surface that accounted for t he majority of iron accumulated by cells incubated in the absence of e xogenous reductant. These data suggest that reduction of Fe(III) withi n the lumen or at the cell surface is required for transfer of this es sential micronutrient across the intestinal brush border surface.