N-ε-methylated lysine derivatives in food

The proteinogenic amino acid lysine can be methylated enzymatically. In this study, different foods were analysed for their content of N -ε-mono- (MML), N -ε-di- (DML) and N -ε-trimethyllysine (TML). A deuterated internal standard was synthesised and an HPLC-ESI-MS/MS method developed and validated. Following suitable clean-up procedures, the methylated lysine derivatives were quantified with the developed method in the MRM mode. In food samples, the derivatives occur mostly in their protein-bound form. TML is the dominant derivative (up to 337 mg/kg), followed by either MML (up to 201 mg/kg) or DML (up to 325 mg/kg). Especially in foods with an active metabolism like mushrooms or yeast, lysine seems to be more strongly methylated. In addition, meat and soy-based food items contain high amounts of the derivatives. Interestingly, the amounts in different food groups are comparable when related to the protein content. Based on the literature as well as our own data, a dietary intake of about 2–3 mg MML, 1–2 mg DML and 8–13 mg TML is assumed. By eating 150 g of pork meat, 0.86 mg MML, 0.25 mg DML and 1.4 mg TML are ingested. Further studies therefore have to deal with their bioactivity and stability during human digestion. • A method for quantification of methylated lysine derivates in food was developed. • For this occasion, a deuterated standard was synthesised and characterised. • Methylated lysine derivatives were quantified in commercially available food samples. • The methylated derivatives could be detected in all samples. • The derivatives occur mostly in their protein-bound form.