Measurement of endogenous leucine enkephalin in canine thalamus by high-performance liquid chromatography and field desorption mass spectrometry

Recent advances in mass spectrometry analysis of neuropeptides

Due to their involvement in numerous biochemical pathways, neuropeptides have been the focus of many recent research studies. Unfortunately, classic analytical methods, such as western blots and enzyme-linked immunosorbent assays, are extremely limited in terms of global investigations, leading researchers to search for more advanced techniques capable of probing the entire neuropeptidome of an organism. With recent technological advances, mass spectrometry (MS) has provided methodology to gain global knowledge of a neuropeptidome on a spatial, temporal, and quantitative level. This review will cover key considerations for the analysis of neuropeptides by MS, including sample preparation strategies, instrumental advances for identification, structural characterization, and imaging; insightful functional studies; and newly developed absolute and relative quantitation strategies. While many discoveries have been made with MS, the methodology is still in its infancy. Many of the current challenges and areas that need development will also be highlighted in this review.

Separation and determination of enkephalin-related peptides using capillary electrophoresis

CZE with UV-absorption detection has been used for the separation and determination of enkephalin-related peptides. The experimental conditions, such as pH and concentration of running buffer, applied voltage, injection method, and time, were investigated in detail. Excellent separation efficiency could be obtained for ten enkephalin-related peptides with a 50 microm (ID) x 58 cm capillary using sodium dihydrogen phosphate as the running buffer (pH 3.11) when 20 kV of applied voltage was used. The concentration detection limits were found to be in the range of 0.31-1.94 microg/mL (defined as S/N = 3). The proposed method has been applied to analyze the spiked cerebrospinal fluid (CSF) sample, and the results showed that CZE is a powerful technique for separation and detection of the above biological peptides.

Absolute Quantification of the Model Biomarker Prostate-Specific Antigen in Serum by LC−MS/MS Using Protein Cleavage and Isotope Dilution Mass Spectrometry

Protein cleavage-isotope dilution mass spectrometry (PC-IDMS) can be used to quantify proteins, with an isotope-labeled analogue of the peptide fragment used as an internal standard. Here, we investigate use of a standard LC-MS/MS platform for quantifying a model biomarker directly from serum by this technique. We synthesized a peptide (IVGGWECEK) identical to the N-terminal tryptic fragment of PSA but with each glycine containing two 13C atoms and one 15N atom. PSA-free human serum was denatured with urea followed by the introduction of PSA standard and the stable isotope labeled internal standard peptide. The sample was then proteolyzed with trypsin and subjected to quantification using LC-MS/ MS on a triple quadrupole mass spectrometer. A linear least squares calibration curve made from five different concentrations of PSA added to serum and digested (each made in triplicate and randomly injected three times) had a mean slope of 0.973 (SE = 0.023), intercept of -0.003 (SE = 0.022), and R2 of 0.971. Recovery of calibrators ranged from 70 to 85% with a mean run-to-run CV of 13% and a mean within-run CV of 5.7%. PC-IDMS is a promising technique for quantifying proteins covering a broad range of applications from standardizing immunoassays to monitoring post-translational modifications to quantifying newly discovered biomarkers prior to the development and implementation of an immunoassay, just to name a few. Issues surrounding the application of PC-IDMS for the absolute quantification of proteins include selection of a proteolytic fragment for quantification that can be cleaved and isolated reproducibly over a broad dynamic range, stable isotope labeled synthetic peptide standards that give consistent results, and LC-MS/MS methods that provide adequate sensitivity and reproducibility without creating impractical analysis times. The results presented here show that absolute quantification can be performed on the model biomarker PSA introduced into denatured serum when analyzed by LC-MS/MS. However, concerns still exist regarding sensitivity compared to existing immunoassays as well as the reproducibility of PC-IDMS performed in different matrixes.

Determination of leucine enkephalin and methionine enkephalin in equine cerebrospinal fluid by microbore high-performance liquid chromatography and capillary zone electrophoresis coupled to tandem mass spectrometry

The performance of microbore high-performance liquid chromatography and capillary zone electrophoresis, both equipped with on-line tandem mass spectrometric detection capability, was evaluated critically for the determination of endogenous amounts of leucine enkephalin and methionine enkephalin in equine cerebrospinal fluid. Using an identical sample clean-up and enrichment procedure, capillary zone electrophoresis-mass spectrometry is limited in its concentration detection capacity owing to its much smaller injection volume. Leucine enkephalin was identified in post-mortem equine cerebrospinal fluid at the 1-5 ng/ml level by liquid chromatography-mass spectrometry.

Profiling of neuropeptides using gradient reversed-phase high-performance liquid chromatography with novel detection methodologies

Biological tissues and fluids are subjected to gradient reversed-phase high-performance liquid chromatography (RP-HPLC) separation and the neuropeptide profile of the collected fractions is obtained by radioreceptor assay (RRA) using a broad-based competing ligand. Radioimmunoassay (RIA) is also used to detect specific neuropeptides in the HPLC-purified fractions. Further confirmation of the identity of the peptides present in the tissue is obtained by mass spectrometry (MS) in the fast atom bombardment (FAB) mode. FAB-MS produces the protonated molecular ion of the peptide and allows direct measurement of underivatized peptides at the nanogram level, with increased molecular specificity. FAB-MS-MS identifies a unique amino acid sequence-determining ion in the mass spectrum of a peptide and offers maximum molecular specificity. This analytical chromatography methodology is applied to the study of the molecular basis of several disease states by monitoring several peptidergic pathways and individual peptides and their metabolic relationships. Molecular mechanisms involved in pain, stress, tumor formation, and neurological studies are studied.

Metabolic profiling of opioid peptides in tooth pulp by HPLC and radioreceptor assay

A analytical system using a combination of gradient RP-HPLC and radioreceptor assay as the HPLC-detector is used to analyze the peptide-rich fraction extracted from a canine tooth pulp homogenate and to provide a metabolic profile of endogenous receptoractive peptides. The gradient RP-HPLC effectively separates the endogenous peptide mixture into a range of hydrophobicities that corresponds to a spectrum of peptide sizes. The receptor preparation is derived from a canine limbic system synaptosome fraction. 3H-DADL serves as the ligand in the RRA. The RP-HPLC/RRA data indicate canine tooth pulp contains a wide range of peptides that interact with the opioid peptide receptor preparation and displace the delta-receptor preferring ligand D2-ala, D5-Leu leucine enkephalin.

Measurement of enkephalin peptides in canine brain regions, teeth, and cerebrospinal fluid with high-performance liquid chromatography and mass spectrometry

Endogenous enkephalin pentapeptides are measured with unambiguous molecular specificity in canine and human tissue and fluid extracts. Both field desorption and fast atom bombardment mass spectrometry have been used to produce a protonated molecular ion of the peptide high-performance liquid chromatography fraction. The protonated molecular ion is subjected to collision-activated dissociation processes and a linked-field scan (B/E) selects a unique amino acid sequence-determining ion for monitoring and measurement. Stable isotope-incorporated peptide internal standards are used for quantification. Endogenous enkephalins are measured in hypothalamus, cerebrospinal fluid, pituitary, caudate nucleus, and tooth pulp extracts. Part-per-billion levels of endogenous peptide are measured.

Preparation of stable isotope-incorporated peptide internal standards for field desorption mass spectrometry quantification of peptides in biologic tissue

Stable isotopes have been incorporated into two opioid pentapeptides, leucine enkephalin and methionine enkephalin, using chemical- and enzymatic-catalyzed reactions. Oxygen-18 from H218O was the stable isotope incorporated. High-performance liquid chromatography separated the chemical reaction mixture into three fractions: hydrolyzed peptide fraction, 18O-incorporated enkephalin, and 18O-incorporated enkephalin ester. Analysis of individual isotopic species (18O2, 18O16O, 16O2) in the latter two fractions was done with field desorption mass spectrometry. Porcine esterase II was used to hydrolyse enkephalin esters and recover stable isotope. Up to 90% 18O2 was incorporated. Yields of 18O2 species for the overall procedure were for leucine enkephalin, 56%, and for methionine enkephalin, 38%. This scheme represents the first fast and facile preparation of a stable isotope-incorporated peptide internal standard for use in measuring peptides in biologic extracts and is readily extended to any peptide with a free carboxyl group. Fast atom bombardment-collision activated dissociation-linked field (B/E) scan mass spectrometry unambiguously locates the two 18O atoms in the carboxyl group of the peptide and not in a peptide amide bond or tyrosine side chain. The two 18O-labeled peptide internal standards were used to measure, in a structurally unambiguous fashion, endogenous leucine enkephalin and methionine enkephalin in thalamus tissue at the ppb level. A microcomputer was used for data acquisition and reduction.

Measurement of leucine enkephalin in caudate nucleus tissue with fast atom bombardment-collision activated dissociation-linked field scanning mass spectrometry

The endogenous amount of the opioid pentapeptide leucine enkephalin was measured in a canine caudate nucleus tissue extract using mass spectral analytical methods which retain absolute molecular specificity. Fast atom bombardment mass spectrometry generation of the protonated molecular ion of leucine enkephalin followed by collision activated dissociation produced amino acid sequence-determining ions. These amino acid sequence-determining ions were analyzed by a linked field (B/E) scan. One amino acid sequence-determining ion was selected to measure endogenous leucine enkephalin. This novel measurement mode offers optimal molecular specificity for quantification of an endogenous amount (451 pmol g-1 tissue) of leucine enkephalin in a biologic tissue extract of canine caudate nucleus.

Field desorption mass spectrometric measurement of picomole amounts of leucine enkephalin in canine spinal cord tissue

Leucine enkephalin is measured in canine spinal cord tissue in a structurally unambiguous manner. A rapid tissue procurement procedure minimizes enkephalin metabolism. High-performance liquid chromatography purification of brian neuropeptides is followed by field desorption mass spectrometric measurement of leucine enkephalin in spinal cord tissue extracts. Quantification is performed at the 70 ng (126 pmol) g-1 of wet weight tissue, or 70 parts per billion level. The higher homolog of leucine enkephalin, 2ala-leucine enkephalin, is utilized as internal standard. Straight-line statistics are obtained for a series of samples to which a peptide standard is added.

Fast atom bombardment-collision activated dissociation-linked field scanning mass spectrometry of the neuropeptide substance P

Amino acid sequence-determining information is obtained from nanomole amounts of the underivatized, biologically important peptide substance P by combining fast atom bombardment, collision activated dissociation, and linked field scanning mass spectrometry. Protonated molecular ions of substance P are produced by fast atom bombardment mass spectrometry, accelerated to high translational energy (8 kV), and transit a collision chamber. Collision activated dissociations occur in the first field-free region. Amino acid sequence-determining ions are collected by scanning the magnetic and electric fields, keeping their ratio constant. In this manner, the precursor-product relationship among ions produced during fragmentation of the protonated molecular ion is firmly established.