The crude enzymatic extract was prepared from leaf tissue (0.5 g)
macerated using liquid N2 and homogenized with 5.0 mL of 100 mM
potassium phosphate buffer (pH 7.5) containing 1.0 mM EDTA,
3.0 mM dithiothreitol (DTT) and 2% polyvinylpolypyrrolidone (PVPP)
(m/v) (Azevedo et al., 1998). The homogenate was centrifuged at
14,000 g for 30 min at 4 °C, the supernatant was collected, and 0.5 mL
aliquots were stored in a freezer at −80 °C until enzyme and total
protein quantification. The total protein concentration of each sample
was assessed in a spectrophotometer at 595 nm as described by
Bradford (1976) using bovine serum albumin (BSA) as standard.