The ability of free radicals to convert l-aminocyclopropane-l-carboxylic acid (ACC) to ethylene under strictly chemical conditions has been investigated using the aerobic xanthine/xanthine oxidase reaction and the Fenton reaction. Ethylene is formed when 1 mM ACC is added to either of these reactions. Ethylene production by the xanthine/xanthine oxidase system can be stimulated by H2O2and inhibited by both catalase and superoxide dismutase, suggesting that the hydroxyl radical (OH·) formed by the Haber-Weiss reaction is reacting with ACC to form ethylene. Ethylene production from ACC by the Fenton reagent, which also produces OH·, shows a strong dependence upon H2O2. Involvement of the OH·radical was confirmed by spin-trap studies using 5,5-dimethyl-l-pyrroline-l-oxide (DMPO). Only the hydroxyl adduct of DMPO was detectable in both the xanthine/xanthine oxidase reaction and the Fenton reaction. When ACC was added to the Fenton reaction, an additional adduct of DMPO was detectable, which, on the basis of its hyperfine splitting constants, can be tentatively identified as the DMPO adduct of a carbon-centered free radical. The data are consistent with the view that formation of ethylene from ACC entails attack by OH·and the resultant formation of a carbon-centered radical, possibly of ACC. The chemical conversion of ACC to ethylene is less efficient than that characteristic of senescing tissues, in which the reaction is enzymatically media
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