The enzyme 1-aminocyclopropane-1-carboxylate (ACC) oxidase, which catalyses the final step in the biosynthesis of ethylene, showed a non-linear time-course in vitro and activity decayed with a half-life of around 14 min. This loss of activity was studied using tomato ACC oxidase purified from Escherichia coil transformed with the cDNA clone pTOM13. Inactivation was not due to end-product inhibition by dehydroascorbic acid or cyanide. Preincubatlon of enzyme in the combined presence of Fe2+ ascorbate and ACC, which together allowed catalytic turnover, resulted in almost total loss of ACC oxidase activity. Enzyme Inactivated by catalysis could not be reactivated by passage through Sephadex G-25 or by treating with combina tions of DTT and CO2 A non-linear time-course and inactivation in the presence of all substrates and cofactors was also shown for the enzyme assayed in vivo with melon fruit discs. Using the purified tomato enzyme a distinct ascorbate-dependent inactivation was also observed, which occurred In the absence of catalysis and was prevented, although not reversed, by catalase. This ascorbate-dependent inactivation may thus be due to H2O2 attack on ACC oxidase.

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