Regulation of Ethylene Biosynthesis --- Mechanism for SAM-Induced Inactivation of ACC synthase

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It was investigated the mechanism for the inactivation of 1-aminocyclopropane -1-carboxylate (ACC) synthase by the substrate, S-adenonosylmethionine (SAM), using an enzyme preparation partially purified from tomato ( Lycopersicon esculentum Mill.) fruits, and the following results were obtained.(1) SAM molecule has two sulfonium diastereomers, (+)- and (-)-SAM. When their activities as an inactivator of ACC synthase were compared, it was demonstrated that (+)-SAM inactivated the enzyme three times faster than (-)-SAM.(2) The contents of both (+)- and (-)-SAM were examined in some plant tissues, which produced ethylene at high rates. They contained (-)-SAM, but not (+)-SAM. Thus the natural (-)-SAM was thought to be responsible for the auto- inactivation process of the enzyme in vivo.(3) The enzyme was incubated with ^<14>C-SAM labeled at different positions, and the radioactivity incorporation into the enzyme protein was analyzed. It was revealed that 2-aminobutyric acid portion of SAM is linked covalently into ACC synthase.(4) L-Vinylglycine was shown to act as an irreversible inactivator of ACC synthase in the presence of pyridoxal phosphate. This finding coupled with that described in (3) suggested that the inactivation of ACC synthase by SAM proceeds through the formation of a vinylglycine-ACC synthase complex as an intermediate. A scheme for the mechanism-based inactivation of ACC synthase was postulated.(5) The specific labeling of ACC synthase by ^<14>C-SAM would facilitate the isolation of radio-labeled active-site peptide of ACC synthase and the subsequent determination of its amino-acid sequence.