Enzymology of ferric chelate reduction at the root plasma membrane

Our research has focused on the characterization and purification of plasma membrane reductases of root cells that are involved in ferric chelate reduction in the rhizosphere of “Fe‐efficient”; tomato (Lycopersicon esculentum Mill). Fe chelate reductase activity in plasma membranes from Fe‐deficient plants exhibited Michaells‐Menten kinetics with regard to the substrate Fe3+ citrate. The kinetic data, as well as sensitivity of Fe chelate reduction to the protease trypsin, confirmed the essential enzymatic nature of the tomato root plasma membrane Fe chelate reductase. Differential enzyme rates obtained upon altering the sequence of addition of reactants in vitro suggested that the root plasma membrane Fe chelate reductase may require an ordered substrate binding mechanism for optimal activity. Partial inhibition of enzyme activity by catalase added to reaction mixtures suggested some participation by hydrogen peroxide to Fe chelate reduction in vitro. Electrophoretic separation of proteins from three different isoforms of tomato root plasma membrane Fe chelate reductase revealed similar polypeptide polypeptide profiles shared by the three isoforms. The similarity of polypeptide composition is the ftrst step to understanding the relationship between these isoforms and the amplification of their activity under Fe‐deficiency conditions.