Purification and Properties of 1-Aminocyclopropane-1-carboxylate Synthase of Mesocarp of Cucurbita maxima Duch. Frults

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1-Aminocyclopropane-1-carboxylate (ACC) synthase, which forms ACC from S-adenosylmethionine (SAM), was purified to homogeneity from sliced and aged mesocarp tissue of Cucurbita maxima Duch, cv Ebisu fruits, and its enzymatic properties were determined. The specific activity of the purified enzyme was 220 mU/mg protein at 30℃ at 50 μM SAM. Native ACC synthase has a relative molecular mass of 160 ± 10 kDa and consisted of two subunits of about 84 ± 3 kDa. S-adenosylhomocysteine (SAH), S-methylmethionine (SMM) and L-methionine did not serve as substrate. The enzyme reaction was competitively inhibited by aminoethoxyvinylglycine (AVG) (Ki, 2.5 μM), aminooxyacetic acid (Ki, 40 μM) and SAH (Ki, 30 μM). The reaction was also strongly inhibited by semicarbazide, and less effectively by homocysteine. The enzyme was rapidly inactivated by its substrate, SAM in the presence of pyridoxal phosphate (PLP), but in the absence of PLP, SAM-induced inactivation was much slower. Inactivation did not occur by SAH and SMM,SAM analogs without substrate activity. Pyridoxal phosphate was an essential cofactor to be added to a reaction mixture for maximum activity, but an enzyme preparation from which pyridoxal phosphate was removed by Sephadex G-25 gel filtration exhibited one-eighth activity which was inhibited by semicarbazide, this indicating that a small amount of pyridoxal phosphate is firmly bound to the enzyme.
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