Analysis of endogenous neuropeptides by reversed-phase high-performance chromatography and mass spectrometry

Profiling histidine-containing dipeptides in rat tissues by liquid chromatography/electrospray ionization tandem mass spectrometry

The histidine-containing dipeptides carnosine (CAR) and structurally related anserine (ANS) and homocarnosine (HCAR), widely distributed in vertebrate organisms, have recently been proposed as endogenous quenchers for highly cytotoxic alpha,beta-unsaturated aldehydes generated by peroxidation. A sensitive, selective, specific and rapid liquid chromatographic/electrospray ionization tandem mass spectrometric assay was developed and validated for the simultaneous determination of these peptides in biological matrices in order to establish their plasma/tissue distribution. Samples (plasma or tissue homogenates from male rats) were prepared by protein precipitation with HClO(4) (1 : 1, v/v) containing H-Tyr-His-OH as internal standard. The supernatant was separated on a Phenomenex Sinergy polar-RP column with a mobile phase of water-acetonitrile-heptafluorobutyric acid (9 : 1 : 0.01, v/v/v) at a flow-rate of 0.2 ml min(-1), with a run time of 10 min. Detection was effected on an ion trap mass spectrometer equipped with an electrospray ionization interface operating in positive ionization mode. The acquisitions were in the multiple reaction monitoring mode using the following precursor --> product ion combinations: H-Tyr-His-OH (internal standard) m/z 319 --> 301; CAR m/z 227 --> 210 + 209; ANS m/z 241 --> 224 + 197 + 170; HCAR m/z 241 --> 156. The method was validated over the concentration range 15-1000 nmol g(-1) and the limit of quantification (LOQ) and limit of detection (LOD) were 12.5 and 4.2 pmol injected, respectively. The intra- and inter-day precisions were <10% (< or =17.47% at the LOQ) and the intra- and inter-assay accuracies were within +/-10% for all concentrations. The mapping profile in rat tissue gave the following results: the highest concentrations of CAR and ANS were found in skeletal muscles (soleus, gastrocnemius, tibialis), followed by the heart, cerebellum and brain (ANS below the LOQ). HCAR was found only in the brain and cerebellum. No histidine-containing dipeptides were detectable in plasma, liver, kidney and lung.

Ion-pair reversed-phase liquid chromatography-electrospray mass spectrometry for the analysis of underivatized small peptides

The single run analysis of 23 small peptides (principally glycyl and lysyl dipeptides) is performed by ion-pair reversed-phase liquid chromatography coupled with evaporative light scattering detection or electrospray (tandem) mass spectrometry. Several perfluorinated carboxylic acid homologues are evaluated with an octadecyl silica stationary phase (Supelcosil ABZ+ Plus). Among the perfluorocarboxylic acids tested the nonafluoropentanoic acid and the tridecafluoroheptanoic acid gave the best results. Special attention was paid to the separation of isomer/isobar dipeptides (e.g., Gly-Ile, Gly-Leu, Leu-Gly, as well as Gly-Gln, Gly-Lys, etc.) which is usually necessary in spite of the high specificity of mass spectrometry. Before LC-MS analysis, ion-spray fragmentation as well as optimization of MS parameters of the analysed peptides was investigated. The optimum collision energy of glysyl peptides, Ala-Gln, Asp-Asp and Asp-Asp-Asp (13-18 eV) was different from that of the lysyl peptides, Tyr-Glu and oxidised glutathione (25-32 eV). Limits of detection varied from 0.1 to 1.2 mg l(-1) for simple MS and 0.05 to 25 mg l(-1) for tandem MS.

Electrospray ionization fourier transform mass spectrometry quantification of enkephalin using an internal standard

Fourier transform mass spectrometry (FTMS), long-known for its capabilities in structural characterization of molecules, is an emerging tool in quantification, and quantification methods using external and internal standards with electrospray ionization (ESI) FTMS have recently been demonstrated. Here, commercial ESI-FTMS is used to quantify the opioid pentapeptide methionine enkephalin using an internal standard. Linear working curves over three orders of magnitude are obtained using the internal standard, an improvement of one order of magnitude over the previous external standard ESI-FTMS quantification method for enkephalins. Low coefficients of variation (generally <6%) are observed, and inter-day and intra-day assays are compared and found to possess similar linearity and precision. The high mass accuracy advantage of FTMS can be exploited to give molecular specificity. Efforts to improve mass accuracy using internal mass calibration generally provide mass accuracies within 2.5 ppm.