蜜梨果实多酚氧化酶酶学特性的研究

多酚氧化酶（PPO）是酶促褐变的关键酶，与果蔬加工制品的色泽、抗氧化能力密切相关。以蜜梨果实为原料，邻苯二酚为底物，采用分光光度法研究蜜梨多酚氧化酶的酶学特性。结果表明：pH和温度对蜜梨PPO活性有明显的影响，其最适pH为4．5，最适温度为34℃。在加工过程中，可通过调节pH和温度来降低蜜梨PPO活性，减少褐变的发生：蜜梨PPO催化底物邻苯二酚的酶促反应动力学与米氏方程高度符合，R^2-0．9972，其动力学方程为专1/V=0．17371/[S]＋0．4775，最大反应速率Vmax=2．09U·min^-1，米氏常数Km=0．36mol·L^-1；蜜梨PPO具有一定的热稳定性，随着温度的提高。完全抑制PPO活性所需要的时间逐渐减少。采用短时高温（90℃，1min）的热处理，不仅可有效降低蜜梨加工过程中的酶促褐变．而且可减少蜜梨汁营养成分的损失．较好地保持其固有色泽。

Study on Enzymatic Characterization of Pyphenol Oxidase of Pear Fruit

Abstract：Polyphenol oxidase （PPO） was the key enzyme of enzymatic browning, and was closely related to the color and antioxidant capacity of processed fruit and vegetable products. The enzymatic characterization of PPO from pears were studied in this paper with spectrophotometry method and based on substrate of catechol. The results showed that, both of pH and temperature had a significant effect on the activity of PPO of pear, the PPO had an optimum pH at 4.5 and optimum temperature at 34 ℃. In the process, the PPO activity of pear could be decreased by the regulation of pH and temperature to reduce the browning. The kinetics of the enzyme-catalyzed reaction of PPO was established and was in accord with Michaelis-Menten ecluation, and R2 was 0.997 2. The Km,Vmax and kinetic equation were respectively 0.36 mol·L^-1, 2.09 U· min^-1 and 1/V=0.17371/[S]＋0.4775, using catechol as substrate. The PPO possessed certain thermal stability, furthermore, the higher the heating temperature was, the shorter time inhibiting PPO activity needed. It could effectively inhibit the enzymatic browning reactions under 90 ℃ for 1 rain. This approach avoided the loss of nutrition and was closer to natural color in fresh pear juice.