2.7. Extraction and assay of ACC synthase (ACS) activity
The homogenate (1 g pulp tissue) was prepared in extraction buffer as described as Kato et al. (2000). One gram frozen pulp tissue were homogenized with a mortar and pestle in 3 mL of extraction buffer (0.1 M EPPS-KOH buffer, pH 8.5, 10 mM 2-mercaptoethanol, and 10 mM pyridoxal phosphate) at 4 _C. The homogenate was centrifuged at 25,000 _ g for 20 min at 4 _C. The supernatant was desalted by passing through PD-10 column (Amersham Pharmacia Biotech) with 10 mM EPPS-KOH buffer, pH 8.5, containing 10 mM 2-mercaptoethanol and 10 mM pyridoxal phosphate. The eluent was assayed for ACC synthase activity in a reaction mixture consisting of 50 mM EPPS-KOH buffer, pH 8.5, 50 mM SAM, and the enzyme preparation with a total volume of 1 mL. The reaction mixture was incubated for 30 min at 30 _C and then the reaction was stopped by adding 0.2 mL of 10 mM HgCl2. ACC formed in the reaction was assayed by the method of Lizada and Yang (Concepcion et al., 1979). ACS activity was expressed as the ethylene production in the reaction (milligram per kilogram per second).