Chisari Marco, Barbagallo Riccardo N, Spagna Giovanni
Dipartimento di OrtoFloroArboricoltura e Tecnologie Agroalimentari, DOFATA, Via Santa Sofia 98, 95100, Catania, Italy.
J Agric Food Chem. 2008 Jan 9;56(1):132-8. doi: 10.1021/jf0721491. Epub 2007 Dec 11.
Polyphenol oxidase (PPO) and peroxidase (POD) were extracted from two different varieties of melon ( Cucumis melo L. cantalupensis cv. Charentais and C. melo L. inodorus cv. Amarillo) and characterized using reliable spectrophotometric methods. In both cases the enzymes followed Michaelis-Menten kinetics, showing different values of kinetics parameters between the two cultivars: K m = 7.18 +/- 0.70 mM ('Charentais') and 6.66 +/- 0.20 mM ('Amarillo') mM; V max = 7.93 +/- 0.35 units/min ('Charentais') and 13.82 +/- 0.37 units/min ('Amarillo'), relative to PPO; K m = 24.0 +/- 2.10 mM ('Charentais') and 5.05 +/- 0.19 mM ('Amarillo') mM; V max = 344.83 +/- 10.32 units/min ('Charentais') and 80.64 +/- 2.01 units/min ('Amarillo'), relative to POD. Optimum pH for PPO was 7.0 for 'Charentais' and 7.5 for 'Amarillo, whereas it was 4.5 for both cultivars relative to POD. Melon PPO had maximum activity at 60 degrees C in both 'Charentais' and 'Amarillo' cultivars, whereas POD maximum activity was found at 45 degrees C in 'Charentais' and at 25 degrees C in 'Amarillo'. POD from both cultivars showed higher thermolability compared with PPO, losing >90% of relative activity after only 5 min of incubation at 70 degrees C. POD's activation energy was much higher than that of PPO (Delta E (#) = 86.3 and 160.6 kJ mol (-1) for 'Charentais' and 'Amarillo', respectively). PPO and POD activities from both cultivars showed a decreasing pattern as sugar concentration in the assay medium increased, except in POD extract from 'Charentais', which maintained its activity in the presence of high d-glucose concentration (up to 5 M). Changes in L*, a*, b*, chroma, and hue angle values were chosen to describe the browning development in the samples during storage at 5 degrees C. A slight decrease in L* value and a more marked reduction of a* value were noted in both cultivars above all at the end of storage period. POD activity during storage at 5 degrees C was highly correlated with changes of parameters a*, b*, chroma, and hue angle ( r (2) from 0.82 to 0.97) for cultivar 'Charentais'. According to these results, only POD activity seemed to be involved in browning of minimally processed melon.
从两种不同品种的甜瓜（甜瓜变种卡塔卢佩西斯品种“Charentais”和甜瓜变种无香味品种“Amarillo”）中提取了多酚氧化酶（PPO）和过氧化物酶（POD），并使用可靠的分光光度法对其进行了表征。在这两种情况下，酶均遵循米氏动力学，两个品种之间的动力学参数值不同：相对于PPO，米氏常数（Km）分别为7.18±0.70 mM（“Charentais”）和6.66±0.20 mM（“Amarillo”）；最大反应速度（Vmax）分别为7.93±0.35单位/分钟（“Charentais”）和13.82±0.37单位/分钟（“Amarillo”）；相对于POD，Km分别为24.0±2.10 mM（“Charentais”）和5.05±0.19 mM（“Amarillo”）；Vmax分别为344.83±10.32单位/分钟（“Charentais”）和80.64±2.01单位/分钟（“Amarillo”）。“Charentais”品种PPO的最适pH为7.0，“Amarillo”品种为7.5，而两个品种POD的最适pH均为4.5。“Charentais”和“Amarillo”两个品种的甜瓜PPO在60℃时活性最高，而“Charentais”品种的POD在45℃时活性最高，“Amarillo”品种在25℃时活性最高。与PPO相比，两个品种的POD热稳定性更高，在70℃孵育仅5分钟后，相对活性损失>90%。POD的活化能远高于PPO（“Charentais”和“Amarillo”的活化能分别为86.3和160.6 kJ mol⁻¹）。随着测定培养基中糖浓度的增加，两个品种的PPO和POD活性均呈下降趋势，但“Charentais”品种的POD提取物除外，其在高浓度d-葡萄糖（高达5 M）存在下仍保持活性。选择L*、a*、b*、色度和色相角值的变化来描述样品在5℃储存期间的褐变发展。在储存期结束时，两个品种的L值均略有下降，a值下降更为明显。“Charentais”品种在5℃储存期间的POD活性与参数a*、b*、色度和色相角的变化高度相关（r²为0.82至0.97）。根据这些结果，似乎只有POD活性与最少加工甜瓜的褐变有关。
