INHIBITION STUDIES ON APPLE POLYPHENOL OXIDASE

Inhibition studies by reductant, carboxylic acid, and halide compounds have been carried out on purified apple polyphenol oxidase (PPO). When ascorbic acid, cysteine, or bisulfite was added, a lag period was observed in the color formation whereas oxygen uptake was immediate. The most efficient compound in decreasing the remaining activity was bisulfite followed by cysteine and ascorbic acid. All tested aromatic carboxylic acids were pure competitive inhibitors. For a same substitution, inhibition decreased in the order cinnamic, benzoic, phenylpropionic, and phenylacetic. In each series, inhibition was slightly enhanced by p-hydroxy substitution and greatly decreased by m-methoxy substitution. Inhibition increased as pH was lowered from 5 to 3.6, and the results indicated that the neutral form of the carboxyl group was mainly responsible for inhibition. Sodium halides were inhibitors and the decreasing order was NaF, NaCl, NaBr, and Nal. When pH was varied between 3.5 and 5, fluoride exhibited the same behavior as the carboxylic inhibitors. The Kifor HF was close to 4 μM; thus it was by far the most potent inhibitor of apple PPO. Chloride was a noncompetitive inhibitor, and an equation is given for the variation of the apparent of Kichloride with pH. © 1990, American Chemical Society. All rights reserved.