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Metabolism and Enzymology

S-Adenosylmethionine-Dependent Inactivation and Radiolabeling of 1-Aminocyclopropane-1-Carboxylate Synthase Isolated from Tomato Fruits ¹

Mann Laboratory, Department of Vegetable Crops, University of California, Davis, California 95616

1-Aminocyclopropane-1-carboxylic acid (ACC) synthase was partially purified from the homogenate of wounded tomato (Lycoperiscon esculentum Mill.) pericarp tissue by (NH₄)₂SO₄ fractionation followed by conventional column chromatography with diethylaminoethyl-Sepharose, Sephadex G-150, Affi-Gel blue and hydroxylapatite. The partially purified ACC synthase preparation attained a specific activity of about 12,000 nmoles per hour per milligram protein. Employing this enzyme preparation, we confirmed that the ACC synthase was inactivated by its substrate, S-adenosyl-L-methionine (SAM), during its catalytic action. When the partially purified enzyme preparation was incubated with [3,4-¹⁴C]SAM and the resulting proteins were analyzed by sodium dodecyl sulfate-gel electrophoresis, only one radioactive protein band was observed. This protein was thought to be ACC synthase based on its molecular mass of 50 kD and on the fact that it was specifically bound to a monoclonal antibody against ACC synthase (AB Bleecker et al. 1986 Proc Natl Acad Sci USA 83, 7755-7759). These results suggest that the substrate SAM acts as an enzyme-activated inactivator of ACC synthase by covalently linking a fragment of SAM molecule to the active site of ACC synthase, resulting in the inactivation of the enzyme.

¹ This work was supported by grant DMB-8414971 from the National Science Foundation.

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