Abstract: The inhibition mechanism of L-cysteine on polyphenoloxidase (PPO) was studied by spectrophotometry and gel filtration chromatography. The results from PPO oxidized product analysis by spectrophotometer implied that the sulfhydryl compound has the maximal absorbance at 295 nm with catechol as the substrate, which may be used as the assay wavelength. As quinone reacts with L-cysteine, the absorbance at 295 nm increases rapidly, while the absorbance at 410 nm decreases, When the PPO was treated with 50 mmol/L L-cysteine for 30 min, the enzyme activity did not decrease at all, after being isolated by gel filtration chromatography. The results indicated that L-cysteine inhibites the browning reaction due to the formation of a colourless compound, but does not involve a direct inhibition on PPO active site or form a covalently-bounded complex between PPO and L-cysteine. The PPO measurement method can be modified in terms of this inhibition mechanism.

Key words: polyphenoloxidase, L-cysteine, inhibition