Ferrous Ion Transport across Chloroplast Inner Envelope Membranes

Abstract

The initial rate of Fe²⁺ movement across the inner envelope membrane of pea (Pisum 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. Fe²⁺ transport was stimulated by an inwardly directed electrochemical proton gradient across right-side-out vesicles, an effect that was diminished by the addition of valinomycin in the presence of K⁺. 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.