摘要或動機

PPO 是一種含銅的多酚氧化? (E. C. 1. 14. 18. 1)，主要是將酚類 (phenol) 氧化成二酚類(diphenol)，更近一步的變成quinones，後者是一種不溶於水的褐色聚合物，在植物中造成褐化最主要原因，然而在人類皮膚則產生黑色素 (melanin)。我們提出有一種揮發性的抑制劑存在動植物體內，此種抑制劑會抑制生物體內PPO 的活性。植物在收成後，揮發性的抑制劑逐漸消失，導致內生性的PPO 活性逐漸上升，因此植物便會產生深褐色的斑點。在本研究中，我們以玉蘭花作為例子，以生化酵素動力實驗、部分純化黑色素抑制劑來解釋揮發性抑制劑在植物體內的存在及其作用，更進一步對水果快速褐化提出一種新的理論。研究發現PPO 的褐化反應就像是人體內酪氨酸? (tyrosinase) 的催化反應，酪氨酸?可以使人體產生黑色素而累積在皮膚上形成黑斑，利用〝人工皮膚〞模擬揮發性黑色素抑制物的作用，證明，防止PPO 抑制劑之揮發可能在未來美白保養品工業裡扮演一極具潛力的角色。Polyphenol oxidase (PPO) or tyrosinase (E.C. 1.14.18.1) is an important and ubiquitous enzyme responsible for browning in plants and melanization in animals. PPO is a copper-containing enzyme that catalyzes the chain-oxidation from monophenol or polyphenols to o-diphenols and subsequent o-quinones. The resulting quinones are large wate-insoluble polymers with dark brown color. We proposed that volatile inhibitors are associated with the plant PPO and block the PPO activity in vivo. While post-harvesting the volatile inhibitors evaporate, the endogenous PPO is then activated and therefore instantly produces dark quinone pigment. In the present study using magnolia flowers as an example, we show the presence of a potent volatile inhibitor(s) for PPO in plant. The novel finding clarifies the mechanism involved in the browning phenomenon of post-harvesting for most fruits. Since the PPO is also know present in human as tyrosinase responsible for the formation of “darkening spots” on skin, the finding of evaporation of potent PPO inhibitor may be potentially used as a strategy in developing a novel cosmetic product.