AA or Basic pH Causes in vitro and Nonenzymatic Cleavage of ACC to Ethylene

An in vitro assay was used to determine the effect of AA and pH on the enzymatic and nonenzymatic production of ethylene (C2H4) from ACC. We were interested in the effect of AA on C2H4 production from ACC because aldehydes, primarily AA, can accumulate in tissue as the result of ripening, storage under modified atmospheres, packaging, and stress. Using crude extracts of ACC oxidase from tomato (Lycopersicon esculentum Mill. `Castlemart') and apple (Malus ×domestica Borkh. `Golden delicious'), C2H4 production from ACC was shown to increase in response to an increase in pH above 7.2 and inclusion of 0.2 to 2 mm AA. Nonenzymatic C2H4 production from ACC also increased linearly with increasing AA concentrations in all the buffers tested. Removal of ascorbate or O2 suppressed AA-induced nonenzymatic C2H4 production. Nonenzymatic AA-induced production of C2H4 from ACC appeared to be an ascorbate dependent oxidation, which was augmented by O2 and was sensitive to minor pH fluctuation. The nonenzymatic AA-stimulated conversion of AEC to 1-butene lacked stereospecificity. Formaldehyde and propionaldehyde also stimulated C2H4 production from ACC. These data indicate that ACC oxidase assays or C2H4 measurements assessing physiological status can be seriously affected by the presence of aldehydes, such as AA. Chemical names used: AA, acetaldehyde; ACC, 1-aminocyclopropane-1-carboxylic acid; AEC, 1-amino-2-ethylcyclopropanecarboxylic acid; ADH, alcohol dehydrogenase; EtOH, ethanol.