Role of polyphenol oxidase in anthocyanin degradation of lychee pericarp

中国人. 荔枝果皮褐变与花色素苷含量、多酚氧化酶（PPO）活性的关系极为密切，本研究的目的是探讨PPO催化花色素苷降解而引起果皮褐变的机理。利用纯化的荔枝果皮PPO、花色素苷、花色素、花色素的降解产物及固定化PPO，探讨PPO在花色素苷降解中的作用。纯化的荔枝果皮PPO虽不能催化花色素苷降解，但在邻苯二酚存在下，花色素苷被PPO迅速降解，并形成褐色产物；PPO可直接催化氧化荔枝果皮花色素及其降解产物并形成褐色产物；利用固定化PPO建立了PPO-酚-花色素苷反应系统，表明花色素苷的降解不依赖于PPO，而仅依赖于PPO催化形成的醌类物质。在荔枝果皮褐变过程中，首先是PPO催化氧化酚类物质、花色素及其降解产物形成相应的醌类物质，醌类物质进一步氧化花色素苷，导致花色素苷含量降低、褪色或变色。

英语. It is well established that anthocyanin and polyphenol oxidase (PPO) are involved in pericarp browning of harvested lychee fruit. However, the mechanism by which PPO catalyzes the anthocyanin degradation and pericarp browning has not been well understood yet. To understand the role of PPO in anthocyanin degradation, the in vitro degradation of anthocyanins was analyzed in the reactions containing the purified anthocyanin, purified PPO or its immobilized style, exogenous phenol, anthocyanidin or its degraded products. Purified PPO did not catalyze the degradation of purified anthocyanin. However, in the presence of catechol, anthocyanin was degraded rapidly by PPO and then formed to be brown by-products. PPO directly catalyzed the oxidation of anthocyanidin or its degraded by-product. A sequential reaction system of PPO-phenol-anthocyanin was established by application of the immobilized PPO. The study indicated that the anthocyanin degradation was independent of PPO, but dependent of the quinones generated by PPO. It was suggested that, during the pericarp browning of lychee fruit, PPO first catalyzes the oxidation of phenols, anthocyanidin and/or its degraded products, then forms quinones, and finally quinones oxidize anthocyanin to brown by-products, leading to the decoloration, browning and reduced anthocyanin content.