Characterization and Induction of the Activity of 1-Aminocyclopropane-1-Carboxylate Oxidase in the Wounded Mesocarp Tissue of Cucurbita maxima

Abstract

1-Aminocyclopropane-1-carboxylate (ACC) oxidase (ethylene-forming enzyme) was isolated from wounded mesocarp tissue of Cucurbita maxima (winter squash) fruit, and its enzymatic properties were investigated. The enzyme required Fe2+ and ascorbate for its activity as well as ACC and O2 as substrates. The in vitro enzyme activity was enhanced by CO2. The apparent Km value for ACC was 175 μM under atmospheric conditions. The enzyme activity was inhibited by sulfhydryl inhibitors and divalent cations such as Co2+, Cu2+, and Zn2+. ACC oxidase activity was induced at a rapid rate by wounding in parallel with an increase in the rate of ethylene production. The exposure of excised discs of mesocarp to 2,5-norbornadiene(NBD), an inhibitor of ethylene action, strongly suppressed induction of the enzyme, and the application of ethylene significantly accelerated the induction of the activity of ACC oxidase in the wounded mesocarp tissue. These results suggests that endogenous ethylene produced in response to wounding may function in promoting the induction of ACC oxidase.