3.3.2. Regenerating effects
Conversely, PPHppc dose-dependently regenerated GSH after oxidation with Fenton's reagent compared to equal amounts of PPHp, which provided unsubstantial increase in sulphydryl content (Fig. 2B). At 1 mg/mL, PPHppc was found to restore about two-thirds the amount of GSH sulphydryl content. This represents a strong reducing capacity (or other mechanisms) of PPHppc and promising physiological role of the hydrolysates in modulating the effects of oxidative stress. However, the Fe(III) reducing capacity of the hydrolysates (Fig. 1A) does not appear to be directly translated in GSH regeneration (GSSG → 2GSH; Fig. 2B) since PPHp showed strong activity in the former and was completely inactive in the latter. This is also surprising since PPHp had higher amount of sulphydryl content compared to PPHppc. The Cys and other relevant amino acid residues of proteins and peptides are known to donate electrons during oxidative stress (Elias et al., 2008), and this was expected to be a contributing mechanism of the potato protein hydrolysates in GSH regeneration. Moreover, the Cys residues can participate in thiol-disulfide exchange reactions (e.g. PCysSH + GSSG → PCysSSG + GSH, where PCysSH is the peptidyl Cys) leading to the formation of mixed disulfides, regeneration of a portion of GSH, depletion of GSSG and, consequently, maintenance of a healthy GSSG/2GSH redox potential (Schafer & Buettner, 2001). Despite the possibilities, it is likely that the Cys residues played a minor role as the sulphydryl group detected in the potato protein hydrolysates (average of 4 µM/g protein) were orders of magnitude lower than that of the GSH oxidation system.