Redox enzymes in the plant plasma membrane and their possible roles

Purified plasma membrane (PM) vesicles from higher plants contain redox pro teins with low-molecular-mass prosthetic groups such as flavins (both FMN a nd FAD), hemes, metals (Cu, Fe and Mn), thiol groups and possibly naphthoqu inone (vitamin K-1), all of which are likely to participate in redox proces ses. A few enzymes have already been identified: Monodehydroascorbate reduc tase (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 oxido reductases have been purified from the cytocolic surface of the PM, but the ir 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, althou gh both functions are speculative. Ferric-chelate-reducing enzymes (EC 1.6. 99.13) are localized and partially characterized on the inner surface of th e PM but they may participate only in the reduction of ferric-chelates in t he cytosol. Very recently a ferric-chelate-reducing enzyme containing bindi ng 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 i ron 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-insens itive NADH oxidase (possibly a protein disulphide reductase) has been chara cterized but its identity is still awaiting independent confirmation. Final ly, 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 cytochro me. In co-operation with vitamin K-1 and an NAD(P)H-quinone oxidoreductase, it may participate in trans-PM electron transport.