Naphthoquinone-Dependent Generation of Superoxide Radicals by Quinone Reductase Isolated from the Plasma Membrane of Soybean

Author:

Schopfer Peter1,Heyno Eiri1,Drepper Friedel1,Krieger-Liszkay Anja1

Affiliation:

1. Universität Freiburg, Institut für Biologie II, D–79104 Freiburg, Germany (P.S., F.D.); and Commissariat à l'Energie Atomique, Institut de Biologie et Technologies de Saclay, CNRS Unité de Recherche Associée 2096, Service de Bioénergétique Biologie Structurale et Mécanisme, F–91191 Gif-sur-Yvette cedex, France (E.H., A.K.-L.)

Abstract

Abstract Using a tetrazolium-based assay, a NAD(P)H oxidoreductase was purified from plasma membranes prepared from soybean (Glycine max) hypocotyls. The enzyme, a tetramer of 85 kD, produces O2  ·− by a reaction that depended on menadione or several other 1,4-naphthoquinones, in apparent agreement with a classification as a one-electron-transferring flavoenzyme producing semiquinone radicals. However, the enzyme displayed catalytic and molecular properties of obligatory two-electron-transferring quinone reductases of the DT-diaphorase type, including insensitivity to inhibition by diphenyleneiodonium. This apparent discrepancy was clarified by investigating the pH-dependent reactivity of menadionehydroquinone toward O2 and identifying the protein by mass spectrometry and immunological techniques. The enzyme turned out to be a classical NAD(P)H:quinone-acceptor oxidoreductase (EC 1.6.5.2, formerly 1.6.99.2) that reduces menadione to menadionehydroquinone and subsequently undergoes autoxidation at pH ≥ 6.5. Autoxidation involves the production of the semiquinone as an intermediate, creating the conditions for one-electron reduction of O2. The possible function of this enzyme in the generation of O2  ·− and H2O2 at the plasma membrane of plants in vivo is discussed.

Publisher

Oxford University Press (OUP)

Subject

Plant Science,Genetics,Physiology

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