Evidence for Cadmium Uptake through Nramp2: Metal Speciation Studies with Caco-2 Cells

Abstract

The specific uptake of 0.3 μM ¹⁰⁹Cd by the TC7 clone of the human enterocytic-like Caco-2 cells increased 4-fold as the pH_out was lowered from 7.5 to 5.5; the stimulatory effect of acidic media being more pronounced when the level of the free ion ¹⁰⁹Cd²⁺, relative to total ¹⁰⁹Cd, was increased. The initial uptake rate was 12-fold higher under conditions, optimizing ¹⁰⁹Cd²⁺ accumulation over that of ¹⁰⁹CdCl²⁻ⁿ_n (NO⁻₃/pH_out 5.5); a saturable system of transport has been characterized (K_m = 1.1 ± 0.1 μM, V_max = 87 ± 3 pmol/3 min/mg protein). An excess of Fe²⁺ failed to affect ¹⁰⁹Cd uptake when the pH_out was 7.4, whereas a strong inhibition was observed under NO⁻₃/pH_out 5.5 conditions. In contrast, the maximal inhibitory effect of Zn²⁺ was observed under Cl⁻/pH_out 7.4 conditions. This results strongly suggest that Fe²⁺ may compete with Cd²⁺ for Nramp2, whereas Zn and CdCl²⁻ⁿ_n compete for another system of transport that has yet to be identified.