NADPH and ferredoxin : NADP(+) oxidoreductase-dependent reduction of quinones and their reoxidation

Molecular oxygen uptake was initiated by adding NADPH (1 mM) to the buffere d medium containing 0.6 mu M spinach ferredoxin:NADP(+) oxidoreductase and 20 mu M quinone (plastoquinone-2, decyl-plastoquinone, decyl-ubiquinone, or duroquinone). At pH 7.7 the rate of oxygen uptake was 2- to 12-fold higher during an initial phase (V-1) than in a subsequent phase (V-2). Except for duroquinone, the initial rate of oxygen consumption was ca. 2.7-fold highe r in alkaline than in acidic medium. Ferredoxin was not essential, although it stimulated the reaction investigated. Oxygen uptake was not detectable with plastoquinone-9 or alpha-tocoquinone. The possible mechanisms of the N ADPH and ferredoxin:NADP oxidoreductase dependent reduction of some quinone s and their reoxidation are discussed. (C) 1998 Elsevier Science Ltd. All r ights reserved.