Quantitative determination of ɛ-N-carboxymethyl-l-lysine in human plasma by liquid chromatography–tandem mass spectrometry

• LC–MS/MS method was validated to determine the underivatized ɛ-N-carboxymethyl-l-Lysine in human plasma.
• Deuterated CML was used as internal standard.
• The plasma sample is pretreated with trichloroacetic acid for protein precipitation.
• The LLOQ is 10 ng/mL.
• The method was applied in the analysis of Chinese subjects.

ɛ-N-carboxymethyl-l-lysine (CML) is a stable chemical modification of protein lysine residues resulting from glycation and oxidation reactions and a potential biomarker of oxidative stress caused by sugar and lipid oxidation. In this study, a rapid, simple and sensitive method based on liquid chromatography–tandem spectrometry (LC–MS/MS) for the determination of CML in human plasma has been developed and validated. Sample preparation involved protein precipitation using trichloroacetic acid after addition of deuterated CML as internal standard. Chromatography was performed on an amino column by gradient-elution with a mobile phase containing acetonitrile:ultrapure water (80:20, v/v). CML and CML-d2 were detected by multiple reaction monitoring mode with ion pairs 205.0/130.1 and 207.2/84.1 respectively. The assay was linear in the range 10–1000 ng/mL with a lower limit of quantitation (LLOQ) of 10 ng/mL and recovery >90%. Assay validation showed that inter- and intra-day precision and accuracy were satisfactory. The method was applied to compare plasma CML levels in healthy Chinese subjects and patients with diabetes and uremia. In healthy subjects CML concentration (mean ± SD) was 16.6 ± 7.8 ng/mL. CML level in diabetic patients was not significantly different from healthy subjects whereas the level in patients with uremia was significantly higher than both healthy subjects and diabetic patients (P < 0.001). The method will be useful to assess the value of CML as a biomarker of diabetic vascular complications resulting from elevated oxidative stress.

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