Main Characteristics of Polyphenol Oxidase from Asparagus Lettuce

Polyphenol oxidase (PPO) was isolated from asparagus lettuce (Lactuca sativa var. angustana Irish), a widely used vegetable in China. Biochemical characteristic of PPO were studied. PPO from asparagus lettuce showed the highest PPO activity in pH buffer 6.5 at 20 degrees C. PPO was of good stability up to 60 degrees C. But relative activities were 50% at 70 degrees C for 9 min and there was no activity at 80 degrees C for 3 min. Cysteine was the most effective inhibitor and relative order of PPO inhibition was cysteine > ascorbic acid > citric acid. Total phenols were also determined in the presence of these inhibitors at room temperature. The presence of cysteine or ascorbic acid decreased total phenols slightly, while citric acid resulted in a notable decrease. Lettuce phenolics were protected from oxidation by cysteine and ascorbic acid.