Ferrous ion transport across chloroplast inner envelope membranes

The initial rate of Fe2+ movement across the inner envelope membrane of pea (Piston sativum) chloroplasts was directly measured by stopped-flow spectrofluorometry using membrane vesicles loaded with the Fe2+-sensitive fluorophore, Phen Green SK The rate of Fe2+ transport was rapid, coming to equilibrium within 3s. The maximal rate and concentration dependence of Fe2+ transport in predominantly right-side-out vesicles were nearly equivalent to those measured in largely inside-out vesicles. Fe transport was stimulated by an inwardly directed electrochemical proton radient across right-side-out vesicles, an effect that was diminished by the addition of valinomycin in the presence of V. Fe2+ transport was inhibited by Zn2+, in a competitive manner, as well as by Cu2+ and Mn2+. These results indicate that inward-directed Fe2+ transport across the chloroplast inner envelope occurs by a potential-stimulated uniport mechanism.