Mechanism of zinc uptake by microvilli isolated from human term placenta

The mechanism of zinc (Zn) uptake by microvillous membrane vesicles prepared from human term placenta has been studied. The uptake was complex, with two processes being identified. In the first process, uptake was rapid, reaching equilibrium within 2 min, and was temperature dependent, with a Q10 of 1.5. Equilibrium Zn levels were sensitive to osmotic pressure, with Zn binding at infinite osmolarity being 69% iso-osmotic value. The uptake was saturable, with a Vmax of 58 +/- 2 nmol/mg protein/min and an apparent Kt of 128 +/- 13 microM. Uptake was inhibited by increasing extravesicular K+ concentration, dropping from 0.91 +/- 0.03 nmol/mg/min at 0 extravesicular K+ to 0.47 +/- 0.03 at an extravesicular K+ concentration of 150 mM ([Zn] = 1.0 microM). In the presence of both valinomycin, an electrogenic ionophore, and nigericin, an electroneutral exchanger, an outwardly directed K+ gradient stimulated Zn uptake. Similarly, preloading vesicles with Zn and imposing an inward gradient resulted in a temperature dependent efflux of Zn. The data suggest that there is a K+ dependent Zn transporter in vesicle membranes, and we suggest that the evidence is biased in favour of a Zn/K+ exchanger rather than Zn being dependent on the membrane potential.