About 2 g of samples were homogenized with 5 mL of sodium borate buffer (0.05 mmol L1, pH 8.8, containing 0.5 g PVP, 5 mmol L1 2-mercaptoethanol and 2 mmol L1 EDTA) in an external ice bath. The homogenates were centrifuged at 10000g for 15 min at 4 °C.