Calcium interactions with divalent metal–ion transporter–1 (DMT1)

Abstract

Iron deficiency is the most prevalent micronutrient deficiency worldwide. Whereas dietary calcium is known to reduce the bioavailability of iron, the molecular basis of this interaction is not understood. DMT1 is the principal or only mechanism by which nonheme iron is taken up at the intestinal brush border and, since the scallop DMT1 homolog has been reported to operate as a calcium transporter [FEBS Lett 579, 2727 (2005)], we tested the hypothesis that human DMT1 also transports calcium. We expressed human DMT1 in RNA‐injected Xenopus oocytes and examined DMT1 activity using radiotracer assays and the voltage clamp. At pH 5.5, DMT1 did not mediate significant uptake of 2 μM or 100 μM 45Ca2+ (P > 0.25). Instead, we found that 20 mM Ca2+ inhibited the uptake of 2 μM 55Fe2+ (P < 0.001) and blocked the Fe2+‐evoked currents at all membrane potentials from −150 to +50 mV. Ca2+ decreased the Vmax for 55Fe2+ uptake without effect on K0.5 suggesting that Ca2+ is a noncompetitive inhibitor of DMT1 (Ki ≈ 10 mM). The mechanism of calcium inhibition did not involve intracellular Ca2+ signaling since the effect persisted even after microinjection of EGTA. The alkaline earth metal ions Ba2+, Sr2+ and Mg2+ also inhibited DMT1‐mediated iron‐transport activity. We conclude that Ca2+ is a low‐affinity noncompetitive inhibitor—but not a transported substrate—of DMT1, explaining in part the effect of dietary calcium on iron bioavailability.