Enzyme reactions with phenolic compounds: Effects of O-methyltransferase and high pH on the polyphenol oxidase substrates in apple

Abstract

An anaerobic methylating treatment with a catechol O-methyltransferase system at pH 8 permanently prevented oxidative darkening of apple juice and fruit sections by modifying their polyphenol oxidase (PPO) substrates. Control non-methylating treatments of juice resulted in normal PPO darkening of endogenous substrates. Similar control treatments applied to fruit sections resulted in less darkening than occurred in untreated sections, and the observed effect resembled that of O-methyltransferase treatment in that the oxidative substrates in the tissue appeared to be permanently modified. This effect on the substrates was found to be entirely a result of the mildly alkaline pH of the system. The possibility that the effect was due to alkaline extraction of PPO substrates from the apple surfaces was shown to be unlikely, suggesting that substrate modification may occur as a result of in situ O-methyltransferase, when it is brought to its active pH range.