Antioxidant mechanism of potato protein hydrolysates against in vitro oxidation of reduced glutathione

Sulphydryl moiety of cysteine is critical for determining peptides' reducing capacity. Glutathione (GSH) oxidation with Fenton's regent, Fe(II)/H2O2, decreased the sulphydryl content from 2406 ± 36 to 787 ± 14 µM/g. Potato proteins hydrolysed with pepsin and pepsin-pancreatin (PPHppc), with different surface and molecular properties, dose-dependently protected GSH sulphydryl group when added prior to oxidation, with PPHppc elevating the sulphydryl to 1186 ± 17 µM/g. PPHppc was found to regenerate two-thirds of oxidised GSH sulphydryl, indicating strong reducing capacity or thiol-disulfide exchange. The activities did not correlate with the hydrolysates' Fe(III) reducing, Fe(II)-chelating capacities, or contents of oxidatively labile and antioxidant amino acid residues. On direct oxidation, free amino content was increased for PPHppc and sulphydryl contents were decreased for both hydrolysates, suggesting oxidative cleavage and direct sulphydryl activity, respectively. The findings suggest complex interactions of the hydrolysates within the oxidative system, and potential supportive role of the peptide product in regenerating GSH from the oxidised state.