Abstract
The oxidation of different phenols [4‐methylcatechol, chlorogenic acid, (−)‐epicatechin and (+)‐catechin] by endive polyphenoloxidase (PPO) was investigated in the presence of an extract from Carica papaya. The occurrence of cysteine and another ‘quinone‐trapping’ substance in the extract was demonstrated. The unknown substance was purified, as a 4‐methylcatechol conjugate form, by a combination of Bio‐gel Pz chromatography and semipreparative high‐performance liquid chromatography (HPLC). Use of liquid chromatography/tandem mass spectroscopy (LC–MS/MS) equipment and an amino acid analyser allowed us to identify this agent as a dipeptide cysteine‐glutamic acid, commercially available as γ‐Glu‐Cys. γ‐Glu‐Cys formed one adduct compound with 4‐methylcatechol and chlorogenic acid, and two with the flavan‐3‐ols. The thiol adducts were not substrates for endive PPO but, in the case of the 4‐methylcatechol conjugate, they acted as competitive PPO inhibitors.
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