Journal Highlight: Tryptophan and cysteine oxidation products dominate in α-lactalbumin-derived peptides analyzed with LC–MSn

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  • Published: Oct 23, 2017
  • Author: separationsNOW
  • Channels: HPLC
thumbnail image: Journal Highlight: Tryptophan and cysteine oxidation products dominate in α-lactalbumin-derived peptides analyzed with LC–MS<sup>n</sup>

The oxidation products of tryptophan and cysteine residues in α-lactalbumin, a major milk whey protein, have been identified, which will help to monitor the safety of protein-containing foods, including food supplements for sports nutrition.

Tryptophan and cysteine oxidation products dominate in α-lactalbumin-derived peptides analyzed with LC–MSn

Journal of Food Science, 2017, 82, 2062-2069
Tuuli P. Koivumäki, Göker Gürbüz and I. Marina Heinonen

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+16LAHKALC). 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+16LAHKALC 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. The results contribute to a better understanding of oxidative deterioration of essential amino acids in milk whey protein. Monitoring the formation of the proposed α-lactalbumin oxidation markers could benefit in the evaluation of whey protein nutritional and technological quality. The results may also be applied to safety evaluations of protein-containing foods, including protein-rich food supplements marketed for sports nutrition.

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