The enzymatic malonylation of 1-aminocyclopropane-1-carboxylic acid in homogenates of mung-bean hypocotyls

Homogenates of hypocotyls of light-grown mung-bean (Vigna radiata (L.) Wilczek) seedlings catalyzed the formation of 1-(malonylamino)cyclopropane-1-carboxylic acid (MACC) from the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and malonyl-coenzyme A. Apparent K_m values for ACC and malonyl-CoA were found to be 0.17 mM and 0.25 mM, respectively. Free coenzyme A was an uncompetitive inhibitor with respect to malonyl-CoA (apparent K_i=0.3 mM). Only malonyl-CoA served as an effective acyl donor in the reaction. The d-enantiomers of unpolar amino acids inhibited the malonylation of ACC. Inhibition by d-phenylalanine was competitive with respect to ACC (apparent K_i=1.2 mM). d-Phenylalanine and d-alanine were malonylated by the preparation, and their malonylation was inhibited by ACC. When hypocotyl segments were administered ACC in the presence of certain unpolar d-amino acids, the malonylation of ACC was inhibited while the production of ethylene was enhanced. Thus, a close-relationship appears to exist between the malonylation of ACC and d-amino acids. The cis- as well as the trans-diastereoisomers of 2-methyl- or 2-ethyl-substituted ACC were potent inhibitors of the malonyltransferase. Treatment of hypocotyl segments with indole-3-acetic acid or CdCl₂ greatly increased their content of ACC and MACC, as well as their release of ethylene, but had little, or no, effect on their extractable ACC-malonylating activity.Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - MACC 1-(malonylamino)-cyclopropane-1-carboxylic acid Dedicated to Professor Dr. Hubert Ziegler on the occasion of his 60th birthday