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A Ferric Reductase Activity Is Found in Brush Border Membrane Vesicles Isolated from Caco-2 Cells¹

University of Vienna, Medical School, Department of Medical Physiology, A-1090 Vienna, Austria

Brush border membrane vesicles isolated from Caco-2 cells were used to examine whether there is an apical membrane-associated ferric reductase activity in small intestinal enterocytes. A ferric reductase activity which was dependent on NADH or NADPH as reductants was shown. Reduction of Fe(III) was quantified by the formation of a stable Fe(II)/ferrozine complex. The ferric reductase revealed saturation kinetics with a K_m of 4.12 ± 0.65 µmol/L and a V_max of 3.11 ± 0.043 nmol/(min · mg protein) for NADH. About 25% of the electrons for the NADH-dependent ferric iron reduction were transferred indirectly from the superoxide anion as verified by the superoxide dismutase inhibitable ferric iron reduction rate. However, the main part of Fe(III) reduction occurs directly by catalyzed electron transfer from NADH to ferric iron through (an) enzyme(s) located in the brush border membrane. The ferric reductase activity was inhibited by Pt(II) and especially p-chloromercuribenzoate. Ferricyanide, which is also reduced by the enzyme, is a competitive inhibitor of the Fe(III)/nitrilotriacetate (NTA) complex with a K_i of 43 µmol/L. These results suggest that brush border membranes of enterocytes possess a ferric reductase that reduces ferric to ferrous iron before the iron is transported through the microvillous membrane.

KEY WORDS: • microvillous membranes • Caco-2 cells • ferric reductase • superoxide

¹ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

² To whom correspondence should be addressed.

Manuscript received 21 November 1995. Revision accepted 23 May 1996.

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