Biochemical characterization of 1-aminocyclopropane-1-carboxylate oxidase in mung bean hypocotyls

The final step in ethylene biosynthesis is catalyzed by the enzyme 1-aminocyclopropane-1-carboxylate (ACC) oxidase. ACC oxidase was extracted from mung bean hypocotyls and its biochemical characteristics were determined. In vitro ACC oxidase activity required ascorbate and Fe²⁺, and was enhanced by sodium bicarbonate. Maximum specific activity (approximately 20 nl ethylene h^-1 mg protein^-1) was obtained in an assay medium containing 100 mM MOPS (pH 7.5), 25 μM FeSO₄, 6 mM sodium ascorbate, 1 mM ACC, 5 mM sodium bicarbonate and 10% glycerol. The apparent K_m for ACC was 80 ± 3 μM. Pretreating mung bean hypocotyls with ethylene increased in vitro ACC oxidase activity twofold. ACC oxidase activity was strongly inhibited by metal ions such as Co²⁺, Cu²⁺, Zn²⁺, and Mn²⁺, and by salicylic acid. Inactivation of ACC oxidase by salicylic acid could be overcome by increasing the Fe²⁺ concentration of the assay medium. The possible mode of inhibition of ACC oxidase activity by salicylic acid is discussed.