Monomeric and Dimeric Forms and the Mechanism-Based Inactivation of 1-Aminocyclopropane-1-Carboxylate Synthase

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The molecular mass of 1-aminocyclopropane-1-carboxylate (ACC) synthase from a variety of sources was examined by both high-performance gel-filtration chromatography and polyacrylamide gel electrophoresis in the presence of sodium dodecylsulfate. Enzymes used were prepared from wounded or non-wounded pericarp of ripe tomato fruits and wounded mesocarp of winter squash fruits, as well as from cells of E. coli that had been transformed with cDNAs for the wound-induced or ripening-induced ACC synthases of tomato and the wound-induced or auxin-induced enzymes from winter squash. The enzymes from tomato fruit tissues were isolated in a monomeric form, whereas the enzymes synthesized in E. coli from cDNAs for tomato ACC synthase were isolated in a dimeric form. ACC synthases of winter squash obtained either from fruit tissues or from transformed E. coli cells were isolated in dimeric forms. ACC synthase in the monomeric form was less sensitive to the inactivation that is associated with the catalytic reaction (the mechanism-based inactivation) than the enzyme in the dimeric form. A plausible mechanism relating the difference in molecular form to sensitivity to the mechanism-based inactivation of tomato ACC synthase is discussed.