Approximately 1 g fresh weight of frozen tomato tissue was ground in extraction buffer (50 mmol L-1 Hepes (pH7.5), 1 mmol L-1 EDTA, 10 mmol L-1 MgCl2, 2.5 mmol L-1 DTT, 10 mmol L-1 ascorbate) according to the method of Wang and Zhang (2000). Nought point five gram of insoluble polyvinylpolypyrrolidone (PVPP) and three times sample volume buffer were used for each extraction. Homogenates were centrifuged at 12 000×g for 20 min at 4°C and the pellets were discarded. Ammonia sulphate was gradually added to the supernatant to 80% saturation and the solution was again centrifuged at 12 000×g for 30 min at 4°C. The supernatant was discarded and the pellet was dissolved in 2-5 mL of extraction buffer, then dialyzed against a ten-fold volume of extraction buffer for 20 h. All steps were carried out at 0-4°C.