Quantification of desmosine and isodesmosine using MALDI-ion trap tandem mass spectrometry

Desmosine (Des) and isodesmosine (Isodes), cross-linking amino acids in the biomolecule elastin, may be used as biomarkers for various pathological conditions associated with elastin degradation. The current study presents a novel approach to quantify Des and Isodes using matrix-assisted laser desorption ionization (MALDI)-tandem mass spectrometry (MS2) in a linear ion trap coupled to a vacuum MALDI source. MALDI-MS2 analyses of Des and Isodes are performed using stable-isotope-labeled desmosine d(4) (labeled-Des) as an internal standard in different biological fluids, such as urine and serum. The method demonstrated linearity over two orders of magnitude with a detection limit of 0.02ng/L in both urine and serum without enrichment prior to mass spectrometry, and relative standard deviation of <5%. The method is used to evaluate the time-dependent degradation of Des upon UV irradiation (254nm) and found to be consistent with quantification by H-1 NMR. This is the first characterized MALDI-MS2 method for quantification of Des and Isodes and illustrates the potential of MALDI-ion trap MS2 for effective quantification of biomolecules. The reported method represents improvement over current liquid chromatography-based methods with respect to analysis time and solvent consumption, while maintaining similar analytical characteristics.