Covalent bonding of 4-methylcatechol to ß-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 ß-lactoglobulin (ß-LG) from bovine milk, (henceforth ß-LQ) and the effect on protein digestibility. Significant thiol and amine loss was found in ß-LQ compared to ß-LG. Site-specific 4MBQ-induced modifications were identified on Cys, Lys, Arg, His and Trp in ß-LQ. No significant differences between ß-LG and ß-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 ß-LQ after in vitro digestion. Thus, it is relevant to investigate how released Cys-4MC adducts are absorbed in vivo in future studies.

Non-enzymatic transformations of dietary 2-hydroxyalkenyl and aromatic glucosinolates in the stomach of monogastrics.

Monogastric animals exhibit different biological responses to structurally diverse glucosinolates and their transformation products, depending on the dietary levels. The transformations of 2-hydroxyalkenyl and aromatic glucosinolates were examined in vitro under gastric conditions, ex vivo in ligated porcine stomachs and in vivo in a rat model. Intact glucosinolates were almost completely transformed in vitro within 1 h at pH 3 (73-88%) and at pH 5 (97-100%) upon addition of Fe2+ ranging from two-fold molar excess. Glucosinolate transformations reached 78-99% when incubated ex vivo in ligated porcine stomachs. Rat in vivo feeding trials showed major reductions (81-84%) in the intact glucosinolate contents upon passage through the gastrointestinal (GI) tract. Non-enzymatic transformations of glucosinolates occur in the stomach, where pH and the level of Fe2+ are primary determinants. This is the first study to show a complex formation between iron-progoitrin and iron-sinalbin, facilitating the transformation into nitriles and thionamides.