Precursors of Ethylene

Two pathways for biosynthesis of ethylene in higher plants have been postulated (10). One is associated with the breakdown of peroxidized linolenic acid and the other involves the degradation of methionine. Although the formation of ethyilene from peroxidized linolenate has been demonstrated in model systems catalyzed by Cu24, oxygen and ascorbic acid (10) and by an apple extract in the presence of oxygen and ascorbic acid (4), there is no direct evidence that it occurs in plant tissues. The conversion of methionine to etlhylene in model systems (8,19) and in plant tissues (2,9) has been demonstrated. In the FMN-light mediated model system (19), it has been established that methionine is converted to ethylene via methional (,a-methylthiopropionaldehyde) as an intermediate. Enzymic conversion of methionine analogs to ethylene catalvzed by peroxidase has been elucidated recently (5, 6, 11, 12, 15, 17); a-keto-y-methylthiobutyric acid and methional, but not methionine, are the active substrates. A chemical mechanism accounting for suich enzymic formation of ethylene has been described (15-18). On tlle basis of this information, Yalng (16) has proposed the following scheme for the biosynthesis of ethylene in plants: methionine -> a-keto-y-methylthiobutyric acid -+ methional -) ethylene. In order to test the proposed pathways, radioactive linolenic acid and the appropriate radioactive methionine analogs were fed to apple tissue. None of these suggested precursors of ethylene was converted to ethylene as effectively as was methionine.