Biosynthesis of ethylene. 4-Methylmercapto-2-oxobutyric acid, an intermediate in the formation from methionine

Abstract

The enzyme responsible for the conversion of methionine into a precursor of ethylene in cauliflower florets is a transaminase. The formation of 4-methyl-mercapto-2-oxobutyric acid by this enzyme has been shown. The oxo acid stimulates the synthesis of ethylene when added to floret tissue, and tracer experiments have shown that (14)C is incorporated into ethylene from the labelled oxo acid. The evidence is consistent with the view that the oxo acid is an intermediate in the formation of ethylene from methionine.

Ethylene Biosynthesis in Fruit Tissues

“…This phenomenon can be due to a high Michaelis constant of the enzymes in question, or perhaps it can also be due to the nonenzymatic decomposition of KMB to ethylene. The possibility that KMB could be degraded nonenzymatically is indicated by their observation that heat-inactivated cauliflower tissue resumes ethylene production from KMB when ascorbic acid is added back (12). Alternatively, it is equally possible that the pathway of ethylene biosynthesis from methionine is different in apple fruits than in cauliflower floret tissues.…”

“…The rate of endogenous ethylene production from the plugs which received methionine and KMB was 5.8 and 6.4 m,umole/hr, respectively. (12) have recently studied the relative incorporation of methionine and KMB into ethylene by cauliflower florets and have found that KMB is a more efficient precursor than methionine. In order to get more insight into this question, we have administered radioactive methionine or KMB to apple plugs and studied their interconversion as related to the efficiency of ethylene formation.…”

“…On the contrary, when KMB-U-"C was fed to the tissues, it was very actively converted to a variety of products, including several which were not identified. (12), who have studied the conversion of methionine and KMB to ethylene in cauliflower floret tissues. Their conclusion that KMB is a closer precursor of ethylene than methionine was largely based on the following observations: KMB greatly stimulated ethylene production by cauliflower floret tissues, ETHYLENE B14 KMB was incorporated into ethylene more efficiently than methionine, and unlabeled methionine was ineffective in reducing the conversion of radioactive KMB into ethylene, while KMB was effective in reducing the conversion of radioactive methionine to ethylene.…”

“…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).…”

“…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.…”