Identification and Quantitation of the Lipation Product 2-Amino-6-(3-methylpyridin-1-ium-1-yl)hexanoic Acid (MP-Lysine) in Peanuts

The lipid peroxidation product acrolein was semiquantitated by GC-MS (EI) in unheated and heated peanut oil, respectively, representing a model system for peanut roasting. Depending on the heating time, acrolein levels significantly increased from 0.2 to 10.7 mg/kg oil. As a result of heating N(α)-acetyl-l-lysine and acrolein, the pyridinium derivative 2-acetamido-6-(3-methylpyridin-1-ium-1-yl)hexanoic acid (MP-acetyl lysine) was identified. In addition, the lysine derivative 2-amino-6-[5-(hydroxymethyl)-3,6-dihydro-2H-pyridin-1-yl]hexanoic acid was identified after reduction and hydrolysis. After preparation of 2-amino-6-(3-methylpyridin-1-ium-1-yl)hexanoic acid (MP-lysine) as reference material, its amounts were quantitated in acrolein-modified peanut proteins by HPLC-ESI-MS/MS after acid hydrolysis, showing that at low acrolein concentrations, the modification of lysine could be entirely explained by the formation of MP-lysine. Furthermore, for the first time, MP-lysine was quantitated in peanut samples in amounts up to 10.2 mg/kg, showing an increase depending on the roasting time. Thus, MP-lysine might represent a marker to evaluate the extent of food protein lipation by acrolein.