Covalent bonding of 4-methylcatechol to β-lactoglobulin results in the release of cysteine-4-methylcatechol adducts after in vitro digestion

Protein-polyphenol adducts are formed upon covalent bonding between oxidized polyphenols and proteins. 4-Methylcatechol (4MC) is a polyphenol with origin in coffee and is oxidized to 4-methylbenzoquinone (4MBQ) under conditions used during food processing. The present study characterizes 4MBQ-induced covalent modifications on β-lactoglobulin (β-LG) from bovine milk, (henceforth β-LQ) and the effect on protein digestibility. Significant thiol and amine loss was found in β-LQ compared to β-LG. Site-specific 4MBQ-induced modifications were identified on Cys, Lys, Arg, His and Trp in β-LQ. No significant differences between β-LG and β-LQ on in vitro digestibility were observed by assessment with SDS-PAGE, degree of hydrolysis and LC-MS/MS unmodified peptide intensities. Cys-4MC adduct (1.7 ± 0.1 µmol/g) was released from β-LQ after in vitro digestion. Thus, it is relevant to investigate how released Cys-4MC adducts are absorbed in vivo in future studies.