2.4.3. PPO activity
The PPO activity was determined according to the method
described by Gonzalez, de Ancos, and Cano (1999), with some
modifications.
2.4.3.1. Enzyme extraction. All steps of PPO extraction were carried
in ground ice, to avoid enzyme inactivation. Seven grams of peach
cubes were extracted with 20 ml of 0.1 mol/l phosphate buffer pH
7.3 containing 1 g/100 ml insoluble polyvinylpolypyrrolidone
(PVPP) and 0.5 mmol/l phenylmethylsulfonyl fluoride (PMSF), by
using a Homogenizer Power Gen 1000 (Fisher Scientific, Germany).
The homogenate was kept in agitation for 30 min, then centrifuged
at 10,000 g for 15 min at 4 C, and the supernatant used as the
PPO enzyme source in the following experiment.
2.4.3.2. Enzyme assay. PPO activity was determined by using a
spectrophotometric method based on the initial rate of increase in
absorbance at 420 nm. A volume of 2.55 ml of phosphate buffer
solution 0.01 mol/l pH 5.5 with 0.01 mol/l cathecol as a substrate
and 50 ml of the enzyme extract was pipetted into a test tube and
mixed thoroughly. Then, the mixturewas rapidly transferred to a 1-
cm path length cuvette. The absorbance at 420 nm was recorded
continuously at 30 C for 5 min by using an ultravioletevisible
spectrophotometer (Gilford, USA). The activity is expressed as units