Iron-zinc interaction during uptake in human intestinal Caco-2 cell line: kinetic analyses and possible mechanism

Iron and zinc interact at the enterocyte during absorption, but the mechanism(s) remain elusive. The aim was, therefore, to understand the mechanism of interaction using kinetic analyses of iron and zinc uptake, individually and in combination under normal and altered cellular mineral concentrations in human intestinal Caco-2 cell line. Striking differences in kinetic parameters were observed between iron and zinc uptake. Iron uptake followed a two-component model, while zinc uptake followed a three-component model. Iron uptake had a Km of 3.6 microM and Vmax of 452 pmol/mg protein/min, while zinc uptake had a Km of 42 microM and Vmax of 3.09 pmol/mg protein/min. Zinc dose-dependently inhibited iron uptake through mixed-inhibition but iron marginally increased zinc uptake. Cellular zinc repletion doubled iron uptake and eliminated inhibition, but zinc depletion decreased iron uptake. Iron pre-treatment had no effect on zinc uptake. Based on these results, a two-transporter model of iron uptake, comprising the apical iron uptake transporter divalent metal ion transporter-1 (DMT-1) and an unknown putative transporter was derived. This model for DMT-1 was verified by immunoblotting. These results implied that cellular zinc status profoundly influenced iron uptake and its interactions with zinc during uptake. DMT-1 might not simultaneously transport iron and zinc, providing a mechanistic basis for observed interactions.