Tracer studies with avocado tissues indicate that methionine is converted to ethylene at stages of the climacteric rise and the climacteric peak, but not at the preclimacteric stage. The results suggest that the control of ethylene biosynthesis is at a step after methionine is synthesized. The endogenous content of methionine was found to be so low that methionine must be actively turned over for ethylene biosynthesis during the stages when the rate of ethylene production is high. Oxygen was found to be essential for this conversion, indicating that at least one of the steps in conversion of methionine to ethylene is oxygen-dependent. The ability of methionine and its keto analogue (a-keto-,y-methylthiobutyric acid) to serve as ethylene precursors by apple tissues was compared. Chemical and kinetic evidence support the view that methionine is a closer precursor of ethylene than its keto analogue.It is well known that in fruits the dramatic increase in ethylene production is closely associated with the increase in respiration and with ripening (3,4,10,14). Oxygen is essential for this endogenous production of ethylene (4, 10). Evidence from tracer studies in a number of plant tissues has shown that methionine serves as a precursor of ethylene in vivo (1,2,5,9,11, 18). The enzymic conversion of methionine analogues to ethylene catalyzed by peroxidase has been elucidated recently (7,12,13,15,17, 18); KMB,2 the methionine keto analogue, or methional (,B-methylthiopropionaldehyde) but not methionine, is the active substrate. A chemical mechanism accounting for such enzymatic reactions has been described (15,17, 18). On the basis of this information, Yang (16) has proposed the following scheme for the biosynthesis of ethylene in plants:methionine -* KMB -> (methional) -> ethylene. Later, we have shown that in apple tissues methional is not converted to ethylene (2). Although KMB was readily converted to ethylene, the efficiency of its conversion was found to be less than that from methionine (2). These data do not support the biosynthetic scheme proposed above. On the other hand, Mapson et al. (12) studied the relative conversion of methionine and KMB into ethylene by cauliflower florets and found that KMB is a more efficient precursor of ethylene than methionine. Therefore, they have supported the view that KMB is an intermediate in the formation of ethylene from methionine.
