Heavy metal complexation of thiol-containing peptides from soy glycinin hydrolysates

Many thiol-containing molecules show heavy metal complexation ability and are used as antidotes. In this study, the potential function associated with thiol-containing peptides (TCPs) from soy protein hydrolysates as natural detoxicants for heavy metals is reported. TCPs enriched by Thiopropyl-Sepharose 6B covalent chromatography had different molecular weight distributions as well as different numbers of proton dissociable groups, depending on the proteases and degree of hydrolysis. The major contribution of sulfhydryl groups was confirmed by the largest pH decrease between 8.0 and 8.5 of the pH titration curves. The complexation of TCPs with heavy metals was evaluated by stability constants (βn) of TCP-metal complexes whose stoichiometry was found to be 1:1 (ML) and 1:2 (ML2). TCPs from degree of hydrolysis of 25% hydrolysates gave high affinities towards Hg2+, Cd2+, and Pb2+ (giving similar or even bigger lgβ values than that of glutathione). A significantly positive correlation was found between the logarithm of stability constants for ML2 (lgβ2) and the sulfhydryl group content. Molecular weight distribution of TCPs affected the complexation with Pb2+ notably more than Hg2+ and Cd2+. These results suggest that soy TCPs have the potential to be used in the formulation of functional foods to counteract heavy metal accumulation in humans.