Inactivation of 1-aminocyclopropane-lcarboxylic acid synthase of etiolated mung bean hypocotyl segments by its substrate

ACC synthase, isolated from mung bean hypocoty! segments treated with IAA and BA, was inactivated by its substrate, SAM, during its catalytic action. The reaction products, ACC and MTA, had no effect on ACC synthase activity. The half-life of the enzyme was 12 min with an initial concentration of 150 ̂ M SAM, but this was extended to 23.5 min when the SAM concentration was reduced to 40 /JM, near to the endogenous concentration of SAM in mung bean hypocotyl tissue. Addition of AVG, a competitive inhibitor of ACC synthase, to the reaction mixture containing 40 /IM SAM, prevented ACC synthase inactivation and increased the half-life about 2-fold. We suggest that ACC synthase inactivation is caused by SAM acting as an enzyme-activated irreversible inactivator (k^-type inactivator), besides being the substrate for the enzyme. This SAM-dependent inactivation of ACC synthase may explain the rapid inactivation of the enzyme in intact mung bean hypocotyl segments previously found by Yoshii and Imaseki (1982). Key words: S-Adenosyl-L-methionine — 1-Aminocyclopropane-l-carboxylic acid (ACC) synthase — Ethylene biosynthesis — Inactivation (ACC synthase) — kca