Evidence for Involvement of the Superoxide Radical in the Conversion of 1-Aminocyclopropane-1-Carboxylic Acid to Ethylene by Pea Microsomal Membranes

Abstract

Electron spin resonance (ESR) spectroscopy has provided evidence for involvement of the superoxide anion (O₂⁻) radical in the conversion of l-aminocyclopropane-l carboxylic acid (ACC) to ethylene by microsomal membranes from etiolated pea seedlings. Formation of ethylene from ACC by the membrane system is oxygen-dependent, heat denaturable, inhibited by the radical scavenger n-propyl gallate and sensitive to superoxide dismutase (SOD) and catalase. Addition of 1,2-dihydroxybenzene-3,5-disulfonic acid (Tiron) to the reaction mixture results in formation of the Tiron semiquinone (Tiron radical) ESR signal derived from O₂⁻, and also inhibits ethylene production. The radical signal is oxygen-dependent and inhibited by SOD and catalase, but is formed both in the presence and absence of ACC. Heat denaturation of the microsomal enzyme system completely blocks formation of the radical signal. The data collectively suggest that O₂⁻ generated by a membrane-bound enzyme facilitates the conversion of ACC to ethylene.