A Mechanism for the Production of Ethylene from Methional

Ethylene was produced when methional (β-methylthio-propionaldehyde) was incubated under aerobic conditions with milk xanthine oxidase plus xanthine. The accumulation of ethylene under these conditions exhibited a brief lag phase which was overcome by small amounts of hydrogen peroxide. The production of ethylene was inhibited by either superoxide dismutase or by catalase, indicating that both superoxide anion radicals and hydrogen peroxide were required. We have proposed that the hydroxyl radical is the chemical species actually responsible for the production of ethylene from methional and that hydroxyl radicals are generated in the aerobic xanthine oxidase system. This proposal generated several predictions which were experimentally affirmed. 1. Compounds which act as scavengers of hydroxyl radicals inhibited the production of ethylene. 2. Xanthine oxidase acting on limiting amounts of xanthine accumulated less than the theoretical amount of hydrogen peroxide and superoxide dismutase increased the amount of peroxide accumulated. 3. Xanthine oxidase plus ample xanthine reduced cytochrome c to a plateau representing a balance between the reduction of ferricytochrome c by O2- and the oxidation of ferrocytochrome c by ·OH. Scavengers of the hydroxyl radical allowed more nearly complete reduction of cytochrome c. Catalase did likewise. 4. Xanthine oxidase exhibited a xanthine-dependent ferrocytochrome c peroxidase activity which was inhibited by catalase, superoxide dismutase, or by scavengers of the hydroxyl radical. These results have led to the conclusion that xanthine oxidase generates O2- and H2O2 as the primary products of the reduction of oxygen and generates ·OH as a secondary consequence of the interaction of O2- with H2O2 and that it is the hydroxyl radical rather than the superoxide anion radical which reacts with methional to generate ethylene.