Isolation and characterization of a 'quinone-trapping' substance from a crude Carica papaya protein preparation

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 Pt 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 gamma-Glu-Cys. gamma-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.