A comparison of the conversion of 1-amino-2-ethylcyclopropane-1-carboxylic acid stereoisomers to 1-butene by pea epicotyls and by a cell-free system

Abstract

The characteristics of the conversion of 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene by pea (Pisum sativum L.) epicotyls and by pea epicotyl enzyme are compared. Of the four stereoisomers of 1-amino-2-ethylcyclopropane-1-carboxylic acid (AEC), only (1R,2S)-AEC is preferentially converted to 1-butene in pea epicotyls. This conversion is inhibited by ACC, indicating that butene production from (1R,2S)-AEC and ethylene production from ACC are catalyzed by the same enzyme. Furthermore, pea epicotyls efficiently convert ACC to ethylene with a low K _m (66 μM) for ACC and do not convert 4-methylthio-2-oxo-butanoic acid (KMB) to ethylene, thus demonstrating high specificity for its substrate. In contrast, the reported pea epicotyl enzyme which catalyzes the conversion of ACC to ethylene had a high K _m (389 mM) for ACC and readily converted KMB to ethylene. We show, moreover, that the pea enzyme catalyzes the conversion of AEC isomers to butene without stereodiscrimination. Because of its lack of stereospecificity, its low affinity for ACC and its utilization of KMB as a substrate, we conclude that the reported pea enzyme system is not related to the in-vivo ethylene-forming enzyme.