P 072 - Oxidative modifications of α- and β- caseins induced by AAPH-derived peroxyl radicals: Role of tryptophan and tyrosine residues

Abstract

In whole milk, as consequence of lipid-peroxidation, α- and β-caseins (α-Cn and β-Cn) can be oxidized leading to protein aggregation and/or fragmentation. The present work was designed to address the hypothesis that changes in the molecular mass of caseins induced by AAPH-derived peroxyl radicals (ROO • ) would be related to Trp and/or Tyr residue oxidation. SDS-PAGE results showed that the exposition of β-Cn to a low AAPH concentration (20 mM), leaded exclusively to the formation of two fragments. In addition, the kinetic profile of the decrease of β-Cn monomer was in agreement with the consumption of its single Trp residue suggesting that β-Cn is fragmented by a process related to Trp oxidation. By contrast, at a high AAPH concentration (100 mM), β-Cn oxidation showed only the formation of protein aggregates. No relationship was observed between the kinetic profiles of consumption of the monomer and Trp. Nonetheless, consumption of Tyr and Lys residues was evidenced by UPLC-methodology. When α-Cn was exposed to different concentrations of AAPH, only aggregates of proteins were generated and the consumption of Tyr and Lys residues was detected. These results show that both caseins have a different susceptibility to reaction with ROO • . These difference are probably associated with their content of Trp and Tyr residues; α-Cn has 2 Trp, and 10 Tyr, while β-Cn has 1 Trp and 4 Tyr.