Redox enzymes in the plant plasma membrane and their possible roles

Purified plasma membrane (PM) vesicles from higher plants contain redox proteins with low‐molecular‐mass prosthetic groups such as flavins (both FMN and FAD), hemes, metals (Cu, Fe and Mn), thiol groups and possibly naphthoquinone (vitamin K₁), all of which are likely to participate in redox processes. A few enzymes have already been identified: Monodehydroascorbate reductase (EC 1.6.5.4) is firmly bound to the cytosolic surface of the PM where it might be involved in keeping both cytosolic and, together with a b‐type cytochrome, apoplastic ascorbate reduced. A malate dehydrogenase (EC 1.1.1.37) is localized on the inner side of the PM. Several NAD(P)H‐quinone oxidoreductases have been purified from the cytocolic surface of the PM, but their function is still unknown. Different forms of nitrate reductase (EC 1.6.6.1–3) are found attached to, as well as anchored in, the PM where they may act as a nitrate sensor and/or contribute to blue‐light perception, although both functions are speculative. Ferric‐chelate‐reducing enzymes (EC 1.6.99.13) are localized and partially characterized on the inner surface of the PM but they may participate only in the reduction of ferric‐chelates in the cytosol. Very recently a ferric‐chelate‐reducing enzyme containing binding sites for FAD, NADPH and hemes has been identified and suggested to be a trans‐PM protein. This enzyme is involved in the reduction of apoplastic iron prior to uptake of Fe²⁺ and is induced by iron deficiency. The presence of an NADPH oxidase, similar to the so‐called respiratory burst oxidase in mammals, is still an open question. An auxin‐stimulated and cyanide‐insensitive NADH oxidase (possibly a protein disulphide reductase) has been characterized but its identity is still awaiting independent confirmation. Finally, the only trans‐PM redox protein which has been partially purified from plant PM so far is a high‐potential and ascorbate‐reducible b‐type cytochrome. In co‐operation with vitamin K₁ and an NAD(P)H‐quinone oxidoreductase, it may participate in trans‐PM electron transport.