The reaction mixture consisted of 0.5 mL of crude enzyme and
2.5 mL of 0.2 mol/L catechol dissolved in 0.05 mol/L sodium phosphate
buffer (pH 7.0). Enzyme activity was measured by the
increase in absorbance at 398 nm. The increase in absorbance at
398 nm was recorded every 1 min for 3 min using a spectrophotometer
(Evolution 201, Thermo Fisher Scientific Inc., Madison,
WI, USA). One unit of PPO activity was defined as an increase in
absorbance at 0.001/min. The specific PPO activity was expressed
as units of enzyme/mg protein.