Inhibition of palmito (Acanthophoenix rubra) polyphenol oxidase by carboxylic acids

The inhibition of palmito (Acanthophoenix rubra) polyphenol oxidase (PPO) is reported. Recently, two forms of palmito PPO were partially purified by hydrophobic chromatography. Inhibitory effects of various carboxylic acids on these two forms have been studied. Both forms showed identical behaviour towards the inhibitors studied. Cinnamic acid was found to have the greatest inhibitory effect (Ki = 0·06 mM). When the inhibitory effects of acids from the benzoic acid family and from the cinnamic acid family were compared, it was found that acids which possess a double bond between the benzene ring and the carboxylic function showed the highest inhibitory effect. This inhibitory effect was decreased by substitutions on the benzene ring. The influence of pH on the inhibitory effect of carboxylic acids on palmito PPO has also been investigated. Ki decreased with a decrease in pH. This effect is due to the fact that it is only the undissociated form (AH) of carboxylic acids that is responsible for inhibition of PPO. Inhibition constants (Ki for the AH form have been recalculated and were found to remain constant in the pH range studied (for benzoic acid Ki = 0·14 mM, for cinnamic acid Ki = 0·019 mM, for sorbic acid Ki = 0·15 mM).