2.5. PPO Activity

The relative activity of crude and purified enzyme was measured by following the method presented by Liu et al. [11] with slight modifications Crude enzyme was made by adding 0.1% polyvinylpolypyrrolidone (PVPP) and (0.1%) Triton X-100 to 2 mL quince juice. After a storage time of 1 h at 4 °C, the juice was centrifuged at 10,000 rpm for 20 min. This crude enzyme was used for PPO activity. An aliquot of 50 μL supernatant was mixed with 0.1 M catechol substrate solution (200 μL) and with 0.05 M phosphate buffer (pH 7.2). Absorbance of the solution was determined at a wavelength of 420 nm at 30 s intervals for 3 min (30 °C incubation) in a spectrophotometer (Multiskan FC-Thermo Scientific, Waltham, MA, USA). The PPO specific activity (Abs/min) was taken as the first linear part of the slope from the reaction curve. The relative activity (RA) was determined using the following equation: