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

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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 Fe^<2+> and ascorbate for its activity as well as ACC and O_2 as substrates. The in vitro enzyme activity was enhanced by CO_2. The apparent K_m value for ACC was 175 μM under atmospheric conditions. The enzyme activity was inhibited by sulthydryl inhibitors and divalent cations such as CO_2+, Cu^<2+>, and Zn^<2+> 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.