Generation of Hydrogen Peroxide by Plant Peroxidases Mediated Thiol Oxidation

OBINGER C , BURNER U. & EBERMANN R. 1997. Generation of hydrogen peroxide by plant peroxidases mediated thiol oxidation. Phyton (Horn, Austria) 37 (3): (219) (226). Thiol oxidation by plant peroxidases is an oxygen consuming process independent of hydrogen peroxide addition. The in vitro system consists of two interdependent reactions, i.e. hydrogen peroxide mediated oxidation of thiols via the enzymatic intermediates compound I and II yielding thiyl radicals, and further the non-enzymatic reaction of thiyl radicals to the corresponding disulphides or reaction with molecular oxygen yielding Superoxide radicals and hydrogen peroxide. The thiyl radical generation per time (regulated by enzyme, thiol, and phenol concentration) determines the amount of hydrogen peroxide generated and as a consequence the amount of phenol which is oxidized by this plant enzyme. In the presence of phenols and with increasing pH oxygen consumption, thiol oxidation, and as a consequence the amount of hydrogen peroxide generated, increases. Considering phenol oxidation the system apparently becomes biphasic with a slow phenol oxidation phase A followed by a rapid phenol oxidation phase B. During phase A oxygen consumption occurs and thiol is oxidized completely. We present reaction schemes and discuss the biological relevance with regard to a possible biosynthetic route of hydrogen peroxide in plants.