GENERATION OF HYDROGEN-PEROXIDE BY PLANT PEROXIDASES MEDIATED THIOL OXIDATION

Thiol oxidation by plant peroxidases is an oxygen consuming process in dependent of hydrogen peroxide addition. The in vitro system consists of two interdependent reactions, i.e. hydrogen peroxide mediated oxida tion of thiols via the enzymatic intermediates compound I and II yield ing thiyl radicals, and further the non-enzymatic reaction of thiyl ra dicals to the corresponding disulphides or reaction with molecular oxy gen yielding superoxide radicals and hydrogen peroxide. The thiyl radi cal generation per time (regulated by enzyme, thiol, and phenol concen tration) determines the amount of hydrogen peroxide generated and as a consequence the amount of phenol which is oxidized by this plant enzy me. In the presence of phenols and with increasing pH oxygen consumpti on, thiol oxidation, and as a consequence the amount of hydrogen perox ide generated, increases. Considering phenol oxidation the system appa rently 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 biosynt hetic route of hydrogen peroxide in plants.