苹果中90%左右的多酚氧化酶以结合态形式存在。在正常状态下，细胞内的多酚氧化酶与质体、线粒体和细胞内膜结合，活性很低，但当细胞膜受到损伤后，潜在的多酚氧化酶被激活，从而导致果蔬制品的褐变量。随着研究的深入，发现膜结合态多酚氧化酶对苹果鲜榨汁酶促褐变的影响非常重要。但膜结合态多酚氧化酶的性质、结构及褐变机理尚未开展深入系统的研究。本项目对苹果中膜结合态多酚氧化酶进行分离纯化，进行结构表征和特性研究。对两个苹果品种膜结合态多酚氧化酶酶学性质的研究及结构的模拟、表征。分析其与底物结合的活性中心结构，从分子水平上探讨酶促褐变的机理，探索膜结合态多酚氧化酶在酶促褐变中的作用机制，以期为控制酶促褐变新方法的开发提供理论依据，为我国高品质鲜榨苹果汁产品的开发奠定基础。

PPO is located in the thylakoid lumen of chloroplasts in two forms: membrane-bound (mPPO) and soluble (sPPO). mPPO is either weakly or strongly bound to the thylakoid membrane under normal condition and contributes to a percentage of 90% in total PPO. Even though the role of mPPO in plant tissue is not well understood, it has been hypothesized that mPPO can be easily activated, thereby contributing to browning and rotting. Few studies have focused on the effect or structure of mPPO in apples. This research is focused on the separation and purification of mPPO from two kinds of apples (Fuji and Granny Smith) to deal with their constructions and properties. Purification, characterization, property and active center of mPPO and browning mechanism of mPPO from apple will be studied. Thus to figure out the mechanism of enzymatic browning and the role of mPPO in this progress. This study will gain a better understanding of the behavior of mPPO in apples, thus enable a proper procedure for the PPO inactivation to be developed. In conclusion, this study would have been a theory basis for new study in enzymatic browning control and contribute to the fruit juice development of apples.