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

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.