Monophenolase activity of polyphenol oxidase from Haas avocado

Avocado polyphenol oxidase (PPO) has been isolated and partially purified by using Triton X-114, reported here for the first time. The enzyme showed both monophenolase and diphenolase activities. The monophenolase activity of avocado PPO was characterized using 4-hydroxyanisole (4HA) as substrate with 3-methyl-2-benzothiazolinone hydrazone as coupled nucleophile. This continuous spectrophotometric method was reliable, with high sensitivity and precision. 4HA was a very good substrate for avocado PPO, showing a higher k(cat) than that for L-dopa at the optimum pH. Monophenolase activity of PPO showed a lag period (tau) prior to the attainment of the steady state rate (V-SS). The optimum pH was 5, the same for the monophenolase and diphenolase activities when the p-hydroxyphenylpropionic acid/3,4-dihydroxyphenylpropionic acid pair was assayed, as well as when 4HA was assayed. A reaction mechanism for explaining the kinetic behavior of the monophenolase activity of avocado PPO has been proposed and characterized.