Mechanism of the reduction of organic nitro compounds by chloroplasts.

Abstract

Abstract The mechanism of the reduction of nitro- and nitrosoaryl compounds by spinach chloroplasts has been investigated. The photoreduction of these compounds can be envisaged as a new type of a noncylic, ferredoxin-dependent, photophosphorylation reaction. The reduction in the dark requires, in addition to chloroplasts, NADPH and ferredoxin. Evidence is presented in support of the conclusion that the chloroplast enzyme ferredoxin-NADP + reductase (NADPH:ferredoxin oxidoreductase, EC 1.6.99.4) mediates the electron transfer from NADPH to ferredoxin, and that the reduction of the nitro compound by reduced ferredoxin proceeds non-enzymatically. Ferredoxin-NADP + reductase can be substituted for the chloroplast preparation, and neither chloroplasts nor chloroplast extract stimulate the reduction of 2,4-dinitrophenol by the system NADPH + ferredoxin-NADP + reductase + ferredoxin. The kinetics of the reduction of 2,4-dinitrophenol by this enzyme system have been studied in more detail. Further it is shown that the reduction of nitro- and nitrosoaryl compounds can also be accomplished by chemically-reduced ferredoxin. The stoichiometry of this reaction confirms the previous finding that ferrodoxin is a one-electron carrier. The possible role of iron flavoproteins in the reduction of organic nitro compounds is discussed.