Reduction of ferric chelates by leaf plasma membrane preparations from Fe-deficient and Fe-sufficient sugar beet

Abstract

The ferric chelate reductase activities of leaf plasma membranes isolated from the leaves of Fe-deficient and Fe-sufficient sugar beet have been characterized. Substrates used were the complexes of ferric iron with ethylene diamine tetraacetic acid, citric acid and malic acid. Iron deficiency was associated with 1.5- to 2-fold increases in leaf plasma membrane ferric chelate reductase activity when rates were calculated on a protein basis. The natural complexes of ferric iron with citrate and especially with malate were good substrates for the ferric chelate reductase enzyme present in leaf plasma membrane preparations. The apparent affinities were higher for the ferric malate complex. The optimal pH for the activity of the ferric chelate reductase in sugar beet leaf plasma membranes was in the range 6.5-7.0. The ferric chelate reductase activity decreased by approximately 30% when the assay pH was decreased to 5.8 or increased to 7.5. Therefore, our data provide evidence against the hypothesis that changes in apoplastic pH could decrease markedly the activity of the ferric chelate reductase enzyme in plasma membrane preparations from the leaves of Fe-deficient plants.