A new 1-aminocyclopropane-1-carboxylic acid-conjugating activity in tomato fruit

A new conjugate, 1-(γ-L-glutamylamino)cyclopropane-1-carboxylic acid (GACC), of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) is identified. The only previously identified conjugate of ACC is 1-(malonylamino)cyclopropane-1-carboxylic acid (MACC). GACC, not MACC, was the major conjugate formed by crude protein extracts of tomato (Lycopersicon esculentum Mill cv Ailsa Craig) fruit pericarp and seeds incubated with [¹⁴C]ACC. GACC was resolved from [¹⁴C]ACC and [¹⁴C]MACC by reversed-phase C₁₈ thin-layer chromatography and subsequently detected and quantified using a radioisotope-imaging system. Proteins precipitated from crude extracts failed to catalyze formation of GACC unless the supernatant was added back. Reduced glutathione, but not other reducing agents, replaced the crude supernatant. When [³⁵S-cysteine]glutathione and [³H-2-glycine]glutathione were used as substrates, neither radiolabeled glycine nor cysteine from the glutathione tripeptide was incorporated into GACC. Oxidized glutathione, S-substituted glutathione, and di- and tripeptides having an N-terminal γ-L-glutamic acid, but lacking cysteine and glycine, also served as substrates for GACC formation. Peptides lacking the N-terminal γ-L-glutamic acid did not serve as substrates. Acid hydrolysis of GACC yielded ACC, suggesting that GACC is an amide-linked conjugate of ACC. Taken together, these results indicate that GACC is 1-(γ-glutamylamino)cyclopropane-1-carboxylic acid and that its formation is catalyzed by a γ-glutamyltranspeptidase. Gas chromatography-mass spectrometry analysis of the N-acetyl dimethyl ester of GACC confirmed this structure.