Plant Physiol. Journal of Pharmacology and Experimental Therapeutics
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Published on April 11, 2008; 10.1104/pp.108.118745

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Received March 5, 2008
Accepted April 8, 2008

Naphthoquinone-dependent generation of superoxide radicals by quinone reductase isolated from the plasma membrane of soybean

Peter Schopfer , Eiri Heyno , Friedel Drepper , and Anja Krieger-Liszkay *

Universitat Freiburg, Institut fur Biologie II, Schanzlestrasse 1, D-79104 Freiburg, Germany; CEA, Institut de Biologie et Technologies de Saclay (iBiTecS), CNRS URA 2096, Service de Bioenergetique Biologie Structurale et Mecanisme, F-91191 Gif-sur-Yvette Cedex, France

* Corresponding author; email: anja.krieger-liszkay{at}cea.fr.

Using a tetrazolium-based assay, an NAD(P)H oxidoreductase was purified from plasma membranes prepared from soybean hypocotyls. The enzyme, a tetramer of 85 kDa, 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 towards 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 (NQR, EC 1.6.5.2, formerly 1.6.99.2) that reduces menadione to menadionehydroquinone that subsequently undergoes autoxidation at pH ≥ 6.5. Autoxidation involves the production of the semiquinone as an intermediate creating the condition 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.




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