Inactivation of 1-Aminocyclopropane-1-carboxylic Acid Synthase of Etiolated Mung Bean Hypocotyl Segments by its Substrate, S-Adenosyl-L-methionine

ACC synthase, isolated from mung bean hypocotyl segments treatedwith IAA and BA, was inactivated by its substrate, SAM, duringits catalytic action. The reaction products, ACC and MTA, hadno effect on ACC synthase activity. The half-life of the enzymewas 12 min with an initial concentration of 150µM SAM,but this was extended to 23.5 min when the SAM concentrationwas reduced to 40 µM, near to the endogenous concentrationof SAM in mung bean hypocotyl tissue. Addition of AVG, a competitiveinhibitor of ACC synthase, to the reaction mixture containing40 µM SAM, prevented ACC synthase inactivation and increasedthe half-life about 2-fold. We suggest that ACC synthase inactivationis caused by SAM acting as an enzyme-activated irreversibleinactivator (k_cat-type inactivator), besides being the substratefor the enzyme. This SAM-dependent inactivation of ACC synthasemay explain the rapid inactivation of the enzyme in intact mungbean hypocotyl segments previously found by Yoshii and Imaseki(1982). (Received October 15, 1985; Accepted December 6, 1985)