Adams D O, Yang S F
Department of Vegetable Crops, University of California, Davis, California, 95616.
Plant Physiol. 1977 Dec;60(6):892-6. doi: 10.1104/pp.60.6.892.
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-(14)C] or [(35)S]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 [(35)S] or [Me(14)C]methionine into MTR was similarly inhibited. Furthermore, the formation of MTA and MTR from [(35)S]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 [(35)S]MTA was fed to climacteric or preclimacteric apple tissue, radioactivity was efficiently incorporated into MTR and methionine. However, when [(35)S]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 [(35)S, Me-(3)H]MTA indicates that the CH(3)S 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.
如果如先前提出的那样，S-腺苷甲硫氨酸（SAM）是乙烯的直接前体，那么预计5'-S-甲基-5'-硫代腺苷（MTA）会是片段核苷。当将[甲基-（14）C]或[（35）S]甲硫氨酸饲喂给跃变型苹果（苹果属森林苹果）组织时，放射性的5-S-甲基-5-硫代核糖（MTR）被鉴定为主要产物，而MTA是次要产物。当甲硫氨酸向乙烯的转化被（l）-2-氨基-4-（2'-氨基乙氧基）-反式-3-丁烯酸抑制时，[（35）S]或[甲基-（14）C]甲硫氨酸向MTR的转化也同样受到抑制。此外，仅在产生乙烯并将甲硫氨酸积极转化为乙烯的跃变型组织中观察到由[（35）S]甲硫氨酸形成MTA和MTR，而在未产生乙烯或未将甲硫氨酸转化为乙烯的跃变前组织中未观察到。这些观察结果表明，甲硫氨酸向MTA和MTR的转化与乙烯生物合成密切相关，并提供了间接证据表明SAM可能是甲硫氨酸转化为乙烯过程中的中间体。当将[（35）S]MTA饲喂给跃变型或跃变前型苹果组织时，放射性有效地掺入到MTR和甲硫氨酸中。然而，当施用[（35）S]MTR时，放射性有效地掺入到甲硫氨酸中但未掺入到MTA中。这表明MTA的硫通过MTR掺入到甲硫氨酸中。一项用[（35）S，甲基-（3）H]MTA进行的双标记实验表明，MTA的CH（3）S基团作为一个单元转移以形成甲硫氨酸。本文提出了一个由甲硫氨酸生成乙烯的方案，第一步是甲硫氨酸被ATP激活生成SAM。SAM裂解生成乙烯、MTA和其他产物。然后MTA水解为MTR，MTR将其甲硫基提供给一个四碳受体以重新生成甲硫氨酸。
