中文摘要：

以蘑菇多酚氧化酶（PPO）为原料，研究不同浓度的柠檬酸处理对PPO相对酶活性、动力学参数（米氏常数K～最大反应速率Vmax）、褐变测量参数的影响。结果表明：随着柠檬酸浓度的增加，PPO的相对酶活性逐渐降低，当柠檬酸浓度达到70mmol／L时，PPO的相对酶活性仅为原酶活性的1％；随着底物邻苯二酚浓度的增加，柠檬酸处理的PPO反应速率逐渐提高，当邻苯二酚浓度为15mmol／L时，反应速率达到最大值。随着柠檬酸浓度的增大，PPO的Vmax基本不变，而Vmax逐渐降低，经Lineweaver．Burk作图得出0、20、40mmol／L柠檬酸处理PPO的Vmax分别为578．4、437．8、111．6U／min。表明柠檬酸是PPO与底物邻苯二酚反应的非竞争抑制剂。对褐变测量参数影响表明，随着处理的柠檬酸浓度的增大，L＋（亮度）、a＊（红值）变化较小，而西。（黄值）急剧下降，40、70mmol／L柠檬酸处理后扫。分别降为原酶活性的57．5％和13．7％；Hue、Chroma、褐变指数（BI）也明显降低；这与PPO的相对酶活性和Vmax变化一致，表明相对酶活性、Vmax与b＊、Hue、Chroma、BI有直接的相关性。

英文摘要：

The effect of different concentrations of citric acid treatment on the relative activity, kinetic parameters （gm and Vmax） and browning parameters （Hue, Chroma and BI） of polyphenol oxidase （PPO） from mushroom. The results showed that the relative activity of PPO decreased gradually with increasing citric acid concentration and was only 1% of its original activity at 70 mmol/L. In the presence of citric acid, the reaction rate of PPO showed a gradual increase with increasing concentration of catechol, the substrate and reached its maximum level at a substrate concentration of 15 mmol/L. However, the Km remained basically unchanged despite an increase in citric acid concentration, while the Vmax gradually decreased. Lineweaver-Burk plots demon- strated that the Vmax values of PPO in the presence of citric acid at concentrations of 0, 20 mmol/L and 40 mmol/L were 578.4, 437.8 U/min and 111.6 U/min, respectively, suggesting that citric acid is a non-competitive inhibitor for the reaction between PPO and catechol. Increasing citric acid concentration resulted in small changes in L＊ and a＊ and a sharp decrease in b＊. The b＊ values from browning catalyzed by PPO in the presence of 40 mmol/L and 70 mmol/L ciWic acid were reduced to 57.5% to 13.7% of its origin value observed when citric acid was absent. In addition, Hue, Chroma and BI （browning index） presented a dramatic decline as well. These consistent changes implied that relative activity and Vmax had direct correlations with b＊, Hue, Chroma and BI.