In Vivo and in Vitro 1-Aminocyclopropane-1-carboxylic Acid Oxidase Activity in Pear Fruit: Role of Ascorbate and Inactivation during Catalysis

Abstract

1-Aminocyclopropane-1-carboxylic acid (ACC) oxidase catalyzes the final step in the biosynthesis of ethylene. The enzyme extracted from ripe pear fruits (Pyrus communis L. cv. Blanquilla) uses ascorbate as an essential cosubstrate, nonreplaceable by other reductants, and is inhibited by 2-oxoglutarate and catalase. ACC oxidase has a half-life of 60 min. Inactivation of the enzyme is partly associated with low protein concentration during incubation. The inclusion of ascorbate results in additional loss of its activity, maximum loss occurring with ascorbate, Fe2+, and ACC. Inactivation is not affected by the inclusion of CO2, and it is not due to inhibition by ethylene. In vivo, activity requires ACC, Fe2+, and CO2 for maximal activity. Ascorbate appears to act as an inhibitor in vivo, despite its requirement for the in vitro assay. Keywords: 1-Aminocyclopropane-1-carboxylic acid oxidase; ascorbate; ethylene; enzyme inactivation; pear (Pyrus communis L.)