Tryptophan and Cysteine Oxidation Products Dominate in α-Lactalbumin-Derived Peptides Analyzed with LC-MSn.

ABSTRACT

α-Lactalbumin (α-La), a major milk whey protein, is comprised of several amino acids prone to metal-catalyzed oxidation (MCO) typical in processing and during storage of foods. New tools are needed for the detection of characteristic oxidation products especially from tryptophan and cysteine that often remain unrecognized when using the traditional methods of carbonyl formation monitoring. In this study, the oxidative changes in α-La were investigated through tryptic digestion and collection of 3 descriptive peptides fitted into a metal-catalyzed oxidation (Fenton reaction) model. The peptide samples were oxidized at +37 °C for 14 d and explored with liquid chromatography-quadrupole ion trap-mass spectrometer (LC-MSn ). The fractionated α-La peptides were valyl-glycyl-isoleucyl-asparaginyl-tyrosyl-tryptophyl-leucyl-alanyl-histidyl-lysine (VGINYWLAHK), leucyl-aspartyl-glutaminyl-tryptophyl-leucyl-cysteinyl-glutamyl-lysine (LDQWLCEK), and tryptophyl+16 -leucyl-alanyl-histidyl-lysyl-alanyl-leucyl-cysteine (W+16 LAHKALC). Oxidation of several amino acids, such as cysteine, histidine, lysine, and tryptophan was observed. In the peptide LDQWLCEK, cysteine was rapidly trioxidized to sulfonic acid, followed by other amino acid side chains as secondary oxidation sites. Tryptophan oxidation was more pronounced in the peptides W+16 LAHKALC and VGINYWLAHK, and also formation of the harmful N-formylkynurenine was observed. As a conclusion, several stable and promising oxidation markers are proposed for α-La, which could be implemented in the evaluation of quality and safety of dairy protein-containing products.