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Methionine Metabolism in Apple Tissue

Implication of S-Adenosylmethionine as an Intermediate in the Conversion of Methionine to Ethylene ¹

^a Department of Vegetable Crops, University of California, Davis, California, 95616

If S-adenosylmethionine (SAM) is the direct precursor of ethylene as previously proposed, it is expected that 5'-S-methyl-5'-thioadenosine (MTA) would be the fragment nucleoside. When [Me-¹⁴C] or [³⁵S]methionine was fed to climacteric apple (Malus sylvestris Mill) tissue, radioactive 5-S-methyl-5-thioribose (MTR) was identified as the predominant product and MTA as a minor one. When the conversion of methionine into ethylene was inhibited by _l-2-amino-4-(2'-aminoethoxy)-trans-3-butenoic acid, the conversion of [³⁵S] or [Me¹⁴C]methionine into MTR was similarly inhibited. Furthermore, the formation of MTA and MTR from [³⁵S]methionine was observed only in climacteric tissue which produced ethylene and actively converted methionine to ethylene but not in preclimacteric tissue which did not produce ethylene or convert methionine to ethylene. These observations suggest that the conversion of methionine into MTA and MTR is closely related to ethylene biosynthesis and provide indirect evidence that SAM may be an intermediate in the conversion of methionine to ethylene.

When [³⁵S]MTA was fed to climacteric or preclimacteric apple tissue, radioactivity was efficiently incorporated into MTR and methionine. However, when [³⁵S]MTR was administered, radioactivity was efficiently incorporated into methionine but not MTA. This suggests that the sulfur of MTA is incorporated into methionine via MTR. A dual label experiment with [³⁵S, Me-³H]MTA indicates that the CH₃S group of MTA was transferred as a unit to form methionine.

A scheme is presented for the production of ethylene from methionine, the first step being the activation of methionine by ATP to give SAM. SAM is fragmented to give ethylene, MTA, and other products. MTA is then hydrolyzed to MTR which donates its methylthio group to a four-carbon acceptor to reform methionine.

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