Gas chromatographic-mass spectrometric characterization of N-methylated basic amino acids in human urine

ESI-MS/MS analysis of protonated N-methyl amino acids and their immonium ions

Methylation is one of the important posttranslational modifications of biological systems. At the metabolite level, the methylation process is expected to convert bioactive compounds such as amino acids, fatty acids, lipids, sugars, and other organic acids into their methylated forms. A few of the methylated amino acids are identified and have been proved as potential biomarkers for several metabolic disorders by using mass spectrometry-based metabolomics workstation. As it is possible to encounter all the N-methyl forms of the proteinogenic amino acids in plant/biological systems, it is essential to have analytical data of all N-methyl amino acids for their detection and identification. In earlier studies, we have reported the ESI-MS/MS data of all methylated proteinogenic amino acids, except that of mono-N-methyl amino acids. In this study, the N-methyl amino acids of all the amino acids (1-21; including one isomeric pair) were synthesized and characterized by ESI-MS/MS, LC/MS/MS, and HRMS. These data could be useful for detection and identification of N-methyl amino acids in biological systems for future metabolomics studies. The MS/MS spectra of [M + H]⁺ ions of most N-methyl amino acids showed respective immonium ions by the loss of (H₂ O, CO). The other most common product ions detected were [MH-(NH₂ CH₃ ]⁺ , [MH-(RH)]⁺ (where R = side chain group) ions, and the selective structure indicative product ions due to side chain and N-methyl group. The isomeric/isobaric N-methyl amino acids could easily be differentiated by their distinct MS/MS spectra. Further, the MS/MS of immonium ions inferred side chain structure and methyl group on α-nitrogen of the N-methyl amino acids.

Rapid high-performance liquid chromatography of 3-methylhistidine in human urine

An internally standardized method for the determination of 3-methylhistidine in human urine is presented. This methylated amino acid and the chemically analogous internal standard 3-ethylhistidine were isolated from human urine specimens using small columns of cation-exchange resin. Quantification was accomplished by high-performance liquid chromatography using post-column derivatization with o-phthalicdicarboxaldehyde-2-mercaptoethanol followed by fluorometric detection. Sample-to-sample and day-to-day reproducibility were shown to have respective relative standard deviations of 2 and 5% for a human urine specimen containing 250 nmol/ml 3-methylhistidine when using 250 microliter urine per analysis. The chromatographic separation was evaluated in terms of various peak descriptors (capacity factor and retention time) and "Chromatographic Figures of Merit" (peak symmetry and chromatographic efficiency). The utility of the method was demonstrated by its successful application to 1000 human urine specimens.

Drug-protein conjugates--X. The role of protein conjugation in the disposition of dinitrofluorobenzene

The metabolism and irreversible protein binding of 2,4-[3,5-3H]dinitrofluorobenzene (3H-DNFB), a model chemically reactive compound, were studied in the rat. 3H-DNFB given intravenously (5 micrograms, 5 mg or 25 mg per kg) to anaesthetized cannulated rats was rapidly metabolized via the mercapturic acid pathway. The metabolites were extensively eliminated in bile and urine: predominantly as the glutathione conjugate and mercapturate in bile, and as the mercapturate in urine. Only ca. 3-10% of the doses remained in the liver, kidneys, spleen, heart and lungs at 3 hr. Dinitrophenyl mercapturate was the principal urinary metabolite in conscious rats dosed i.v. (5 mg or 25 mg per kg). Only 15-25% of the radiolabelled material in liver and kidney at 3 hr was irreversibly bound to protein, but 45-99% of that in the other organs and 49-88% in plasma was irreversibly bound. Preliminary evidence for the metabolism of 3H-DNFB (5 mg/kg and 25 mg/kg doses) to N2-acetyl-N6-DNP-lysine, a novel conjugate and metabolite of dinitrophenylated proteins in vivo, is presented.