3.2.2. Fe(II) chelating capacity
In addition to reducing capacity, potato protein hydrolysates have also been shown to bind divalent prooxidant metal (Wang & Xiong, 2005). Some proteins are capable of chelating metals even when their specific biological function is not related to metal storage or transport; this ability can be attributed to amino acid residues His, Cys, Trp, Asp and Glu, which are known to bind divalent metals if exposed on the protein or polypeptide surface (Elias et al., 2008). In this study, the potato protein hydrolysates showed dose-dependent Fe(II) chelating capacity with PPHppc exhibiting a 2-fold higher activity than PPHp at 1 mg/mL, equivalent to the chelating capacity of 0.12 mM EDTA (Fig. 1B). There are similar amounts of all the possible metal chelating amino acid residues in the protein hydrolysates, except for Trp (Table 1). Interestingly, the hydrolysate (PPHp) with 3-fold higher amount of Trp had lower Fe(II) chelating capacity. The findings suggest complex interactions of the amino acid residues and biological targets, Fe(II) in this case. One possible explanation of our observation is that the smaller peptides in PPHppc were more accessible to the solvent and were available to form metal coordinates (chelate complexes) compared to the larger peptides resulting from only peptic hydrolysis. More plausibly, the weakly positive ζ-potential of PPHp (Table 1), compared to the negative surface of PPHppc, can induce electrostatic repulsion of Fe(II), thereby reducing metal binding to the relevant amino acid residues.