Analysis of methionine enkephalin in human pituitary by multi-dimensional reversed-phase high-performance liquid chromatography, radioreceptor assay, radioimmunoassay, fast atom bombardment mass spectrometry, and mass spectrometry-mass spectrometry

Analysis of protein-protein interactions in MCF-7 and MDA-MB-231 cell lines using phthalic acid chemical probes

Phthalates are a class of plasticizers that have been characterized as endocrine disrupters, and are associated with genital diseases, cardiotoxicity, hepatotoxicity, and nephrotoxicity in the GeneOntology gene/protein database. In this study, we synthesized phthalic acid chemical probes and demonstrated differing protein–protein interactions between MCF-7 cells and MDA-MB-231 breast cancer cell lines. Phthalic acid chemical probes were synthesized using silicon dioxide particle carriers, which were modified using the silanized linker 3-aminopropyl triethoxyslane (APTES). Incubation with cell lysates from breast cancer cell lines revealed interactions between phthalic acid and cellular proteins in MCF-7 and MDA-MB-231 cells. Subsequent proteomics analyses indicated 22 phthalic acid-binding proteins in both cell types, including heat shock cognate 71-kDa protein, ATP synthase subunit beta, and heat shock protein HSP 90-beta. In addition, 21 MCF-7-specific and 32 MDA-MB-231 specific phthalic acid-binding proteins were identified, including related proteasome proteins, heat shock 70-kDa protein, and NADPH dehydrogenase and ribosomal correlated proteins, ras-related proteins, and members of the heat shock protein family, respectively.

Practical aspects of in vivo detection of neuropeptides by microdialysis coupled off-line to capillary LC with multistage MS

A method using capillary liquid chromatography-triple-stage mass spectrometry (LC-MS(3)) to determine endogenous opioid peptides in microdialysis samples collected in vivo was developed, validated, and applied to measurements in the rat striatum. Peptides in dialysate rapidly degraded when stored at room temperature or -80 degrees C. Adding acetic acid to a final concentration of 5% stabilized the peptides for 5 days allowing storage of fractions and off-line measurements which proved more convenient and reliable than previously used on-line methods. Study of the effect of dialysis flow rate from 0.2 to 2 microL/min and column inner diameter (i.d.) from 25 to 75 microm on the relative signal obtained for peptides revealed that lowest flow rates and smallest column i.d. gave the highest relative signal. The method was tested for 10 different neuropeptides and limits of detection (LODs) were from 0.5 to 60 pM (4 microL samples) for most. beta-Endorphin had an LOD of 5 nM when detected directly, but it could be quantitatively determined by detecting a characteristic peptide produced by tryptic digestion with an LOD of 3 pM. This approach may prove useful for other large neuropeptides as well. The method was used to determine met-enkephalin, leu-enkephalin, dynorphin A(1-8), and beta-endorphin in vivo. Endomorphin 1 and 2 were below the detection limit of the method in vivo. Quantitative determination of leu-enkephalin using external calibration was verified by standard addition experiments. The improvements over previous approaches using capillary LC-MS(n) make in vivo neuropeptide monitoring more practical and feasible for a variety of neuropeptides.

Capillary electrophoretic competitive immunoassay with laser-induced fluorescence detection for methionine-enkephalin

Immunoassays are commonly used in bioresearch for the detection and quantification of small proteins and macromolecules in biological fluids and other complex matrices. In this report, a competitive immunoassay using capillary electrophoresis (CE) with laser-induced fluorescence was developed for methionine-enkephalin (ME). The method is based on the competitive reaction between the ME and fluorescein conjugated ME (ME-F) with anti-ME antibody, capillary electrophoresis separation of the ME-antibody bound and free ME-F, followed by the laser-induced fluorescence detection of the fluorescent species. With the optimized separation conditions, it was possible to separate the antibody bound and free fluorescien conjugated ME by a capillary electrophoresis-laser-induced fluorescence (CE-LIF) analysis using an uncoated fused-silica capillaries. The results concluded that the assay specificity, selectivity and accuracy were excellent.

Capillary liquid chromatography with MS3 for the determination of enkephalins in microdialysis samples from the striatum of anesthetized and freely-moving rats

In vivo microdialysis sampling was coupled to capillary liquid chromatography (LC)/electrospray ionization quadrupole ion trap mass spectrometry (MS) to monitor [Met]enkephalin and [Leu]enkephalin in the striatum of anesthetized and freely-moving rats. The LC system utilized a high-pressure pump to load 2.5 microl samples and desalt the 25 microm i.d. by 2 cm long column in 12 min. Samples were eluted with a separate pump at approximately 100 nl min(-1). A rapid gradient effectively separated the endogenous neuropeptides in 4 min. A comparison was made for operating the mass spectrometer in the MS2 and MS3 modes for detection of the peptides. In standard solutions, the detection limits were similar at 1-2 pM (2-4 amol injected); however, the reproducibility was improved with MS3 as the relative standard deviation was <5% compared with 20% for MS2 for 60 pM samples. For dialysate solutions, reconstructed ion chromatograms and tandem mass spectra had much higher signal-to-noise ratios in the MS3 mode, resulting in more confident detection at in vivo concentrations. The method was successfully used to monitor the peptides under basal conditions and with stimulation of peptide secretion by infusion of elevated K+ concentration.

Gradient liquid chromatography of leucine-enkephalin peptide and its metabolites with electrochemical detection using highly boron-doped diamond electrode

Boron-doped diamond thin film (BDD) electrodes have been used to study the oxidation reactions and to detect leucine-enkephalinamide (LEA) and its metabolites, tyrosine (T), tyrosyl-alanine (TA), tyrosyl-alanine-glycine (TAG) and leucine-enkephalin (LE) using cyclic voltammetry (CV), flow-injection analysis (FIA), and gradient liquid chromatography (LC) with amperometric detection. At diamond electrodes, well-defined and highly reproducible cyclic voltammograms were obtained with signal-to-background (S/B) ratios 5-10 times higher than those observed for glassy carbon (GC) electrodes. The analytical peaks of LC for LEA and its metabolites were well resolved. No deactivation of BDD electrodes was found after several experiments with standard as well as plasma samples, indicating high stability of the electrode. Calibration curves were linear over a wide range from 0.06 to 30 microM with regression coefficients of 0.999 for all compounds. The limits of detection obtained based on a signal-to-noise ratio of 3:1 were 3, 2.2, 2.7, 20 and 11 nM for T, TA, TAG, LE and LEA, respectively. These values were at least one order lower than those obtained at GC electrodes, which has given limits of detection of 22.88, 20.64, 89.57, 116.04 and 75.67 for T, TA, TAG, LE and LEA, respectively. Application of this method to real samples was demonstrated and validated using rabbit serum samples. This work shows the promising use of conducting diamond as an amperometric detector in gradient LC, especially for the analysis of enkephalinamide and its metabolites.

Capillary LC-MS2 at the attomole level for monitoring and discovering endogenous peptides in microdialysis samples collected in vivo

Fused-silica capillary LC columns (25-microm i.d.) with 3-microm-i.d. integrated electrospray emitters interfaced to a quadrupole ion trap mass spectrometer were evaluated for high-sensitivity LC-MS2. Column preparation involved constructing frits by in situ photopolymerization of glycidyl methacrylate and trimethylolpropane trimethacrylate, preparing the electrospray emitter by pulling the column outlet to a fine tip with a CO2 laser puller, and slurry-packing the column with 5-microm reversed-phase particles. Large-volume injections were facilitated by an automated two-pump system that allowed high-flow rates for sample loading and low-flow rates for elution. Small electrospray emitters, low elution flow rates, and optimization of gradient steepness allowed a detection limit of 4 amol, corresponding to 2 pM for 1.8 microL injected on-column, for a mixture of peptides dissolved in artificial cerebral spinal fluid. The system was coupled on-line to microdialysis sampling and was used to monitor and discover endogenous neuropeptides from the globus pallidus of anesthetized male Sprague-Dawley rats. Time-segmented MS2 scans enabled simultaneous monitoring of Met-enkephalin, Leu-enkephalin, and unknown peptides. Basal dialysate levels of Met-enkephalin and Leu-enkephalin were 60 +/- 30 and 70 +/- 20 pM while K+-stimulated levels were 1,900 +/- 500 and 1,300 +/- 300 pM, respectively (n = 7). Data-dependent and time-segmented MS2 scans revealed several unknown peptides that were present in dialysate. One of the unknowns was identified as peptide I(1-10) (SPQLEDEAKE), a novel product of preproenkephalin A processing, using MS2, MS3, and database searching.

Matrix-assisted laser desorption/ionization mass spectrometric quantification of the mu opioid receptor agonist DAMGO in ovine plasma

The synthetic opioid peptide analog Tyr-D-Ala-Gly-N-methyl-Phe-Gly-ol (DAMGO), which is a mu opioid receptor-selective agonist, was quantified in ovine plasma samples with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS), using delayed extraction and a reflectron. The internal standard was pentadeuterated DAMGO. Timed-ion selection was used to select the precursor ion. The analysis of the post-source decay fragments improved the detection sensitivity, and the use of the precursor-product ion relationship optimized the specificity. For plasma samples, the inter-assay variability of this method was 6.4% (n = 79) and the intra-assay variability was 6.0% (n = 10). The variability for controls was 3.4% (n = 43). The profile of DAMGO amount versus time was determined in sheep plasma, and the corresponding pharmacokinetic data were calculated.

Mass spectrometric analysis of neuropeptidergic systems in the human pituitary and cerebrospinal fluid

Neuropeptidergic systems have been studied in human tissues and fluids, which include the pituitary and lumbar cerebrospinal fluid, respectively. This paper reviews the qualitative and quantitative mass spectrometric analytical data obtained from three areas of study. Methionine enkephalin (ME) and beta-endorphin (BE) were quantified in the human pituitary by liquid secondary ion mass spectrometry (LSI MS)-tandem mass spectrometry. Corresponding stable isotope-incorporated synthetic peptide internal standards were used. Proenkephalin A and proopiomelanocortin produce ME and BE, respectively. The analysis of neuropeptides in macroadenomas demonstrated a decrease in both of those neuropeptidergic systems relative to controls. An analysis of prolactin-secreting microadenomas showed an increase in the proenkephalin A system. Mass spectrometry was also used to detect opioid peptide-containing proteins in the pituitary. Enzymes that process the precursors of proenkephalin A and tachykinin (substance P) neuropeptides were studied in human lumbar cerebrospinal fluid. Electrospray ionization mass spectrometry was used to characterize the molecular mass of each peptide product.

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.

Quantitative analysis of methionine enkephalin and beta-endorphin in the pituitary by liquid secondary ion mass spectrometry and tandem mass spectrometry

This manuscript reviews the use of an off-line combination of liquid chromatography (LC) and mass spectrometry (MS) to quantify endogenous neuropeptides in biological tissues and fluids, and tandem MS (MS/MS) to optimize the molecular specificity of the quantification of native peptides. Reversed-phase high-performance liquid chromatography (RP-HPLC) was used to purify selected endogenous neuropeptides from biological tissues and fluids. Liquid secondary ion MS (LSI-MS), also known as fast atom bombardment (FAB), is used to desorb and to ionize the peptide. The corresponding stable isotope-incorporated synthetic peptide of each peptide is used as the internal standard (I.S.) for quantification. The measurement of methionine enkephalin (ME) and of beta-endorphin1-31 (BE) in the human pituitary is described. This analytical method offers the highest molecular specificity for the measurement of a fully post-translationally modified peptide.

Beta-endorphin-containing proteins in the human pituitary

Two new proopiomelanocortin (POMC)-derived beta-endorphin (BE)-containing proteins were detected in the human pituitary, using HPLC, trypsin digestion, and a high sensitivity search with liquid secondary ion mass spectrometry (LSIMS) for the protonated molecule ion, (M + H)+, of tryptic peptides that are unique to BE. Proteins were extracted from pituitary tissues and were purified by solid phase extraction (SPE) chromatography and RP-HPLC. Each HPLC fraction was treated with trypsin, and each unseparated peptide mixture was analyzed by LSIMS to detect the two selected marker peptides (BE 20-24 and BE 10-19) that have excellent LSIMS desorption-ionization properties. The detection of both of those peptides indicated the presence of BE-containing proteins in two HPLC fractions (number 47 and 51). Tandem MS determined the amino acid sequence of the marker peptide BE 20-24 (NAIIK), and those sequence data optimized the specificity of the method. The two new BE-containing proteins derive from the C-terminal region of POMC, and were minor components in the two HPLC fractions. The major component in fraction 51 derived from the vasopressin-neurophysin 2-copeptin precursor.

Mass spectrometry, high performance liquid chromatography, and brain peptides

This paper is a personal recollection of some of the events and research that surrounded the amino acid sequence determination of the hypothalamic releasing factor, TRF (now known as TRH), by mass spectrometry (MS), and the development of reverse phase high performance liquid chromatography (RP-HPLC) MS and tandem MS (MS/MS) methods for the qualitative and quantitative analysis of native opioid neuropeptides in human pituitary tissue extracts.

Peptide quantification by tandem mass spectrometry

This manuscript reviews the literature on the mass spectrometry (MS) and tandem mass spectrometry (MS/MS) quantification of biologically important peptides that have been extracted from tissues. The most important aspect of this quantification process is the use of MS/MS to link the protonated molecule ion, (M + H)(+) , of the peptide with one or more of its amino acid sequence-determining fragment ions. The actual name of a peptide cannot be used in any study until the amino acid sequence of that peptide has been firmly established. This article reviews the analytical data obtained from the measurement of opioid peptides in human pituitary tissues. For example, the proopiomelanocortin (POMC)-derived beta-endorphin (BE) and the proenkephalin-derived methionine enkephalin (ME) opioid peptides have been quantified. The biogenesis of opioid neuropeptides is briefly reviewed; critical aspects of pituitary neuropeptides are discussed, including their localization and regulation, and their role in tumor formation; other analytical methods used to detect and measure neuropeptides are mentioned, including radioimmunoassay (RIA), radioreceptorassay (RRA), in situ hybridization, mRNA, and cDNA methods; and the MS and MS/MS methods are described. The use of stable isotope-incorporated synthetic peptide internal standards is described. Data are presented on the measurement of BE and ME in control pituitaries and in pituitary tumors (PRL-secreting and nonsecreting tumors). A significant alteration in the POMC peptide BE was found between the control and tumor tissues. That difference suggests that the POMC neuropeptidergic system had been down-regulated in those tumors. © 1997 John Wiley & Sons, Inc.

Preparative capillary zone electrophoresis of synthetic peptides conversion of an autosampler into a fraction collector

Preparative capillary zone electrophoresis of three synthetic peptides was performed either manually or automatically by simple manipulations of a commercial electropherograph that is equipped only with an autosampler without any built-in fraction collection capability. Manual fraction collection was achieved by replacing the outlet (cathode) beaker with a microcentrifuge tube, and automatic fraction collection was accomplished by converting the electropherograph's autosampler into a fraction collector. The latter was easily achieved mainly by the use of an extension wire, which completed the electrical circuit and facilitated fraction collection either at a specified time or within fixed time intervals.

Optimization of the loading limit for capillary zone electrophoresis of synthetic opioid and tachykinin peptides: a study of the interactions among the amount of peptide, resolution, saturation, injection volume and capillary diameter

The upper loading limit in the capillary zone electrophoresis (CZE) of a mixture of fourteen synthetic opioid peptides and one tachykinin peptide was determined for capillaries of 50-, 75-, 100-, 130-, 150-, and 200-microns internal diameter (I.D.) to optimize the separation of these neuropeptides. The loading limit is an important parameter for preparative post-CZE studies. Electrophoretic resolution is illustrated for each capillary I.D. as the injection volume and amount of injected peptide increase. Loading limit is evaluated, based on the considerations of resolution, saturation, and current-stability. Among the capillaries studied, the capillary with 100 microns I.D. gives the optimal loading limit (39-78 pmol of each peptide).

Characterization of an opioid peptide-containing protein and of bovine α-lactalbumin by electrospray ionization and liquid secondary ion mass spectrometry

Mass spectrometry methods have been used to characterize two proteins: an opioid peptide-containing protein extracted from bovine pituitary, and bovine α-lactalbumin (BAL). A protein that contains β-endorphin was found in bovine pituitary, and that protein was characterized with electrospray ionization mass spectrometry (ESIMS), gel permeation chromatography, reversed-phase high performance liquid chromatography (RP-HPLC), radioimmunoassay, trypsinolysis, and liquid secondary ion mass spectrometry (LSIMS).BAL is a protein that was used as a model to develop analytical methods to study opioid peptide-containing proteins. Commercial BAL was purified by RP-HPLC, and its molecular weight (M.W.) was determined by ESIMS. The shift in mass observed following dithiothreitol (DTT) reduction estimated the number of disulfide bonds.For all of the data obtained for BAL with or without RP-HPLC separation, ESIMS determined the M.W. of the peptides produced by trypsin treatment of BAL, and LSIMS selected a precursor ion, the protonated molecule ion [M + H](+), of a tryptic peptide, which was analyzed by tandem mass spectrometry. Following DTT reduction, ESIMS and LSIMS detected each peptide that contained disulfide bonds in that mixture of tryptic peptides.

Capillary zone electrophoresis of synthetic opioid and tachykinin peptides

Capillary zone electrophoresis was performed on fourteen synthetic opioid peptides and one tachykinin peptide (substance P = SP) at pH values of 2.5, 5.5 and 8.5 to rationalize the electrophoretic behavior of these neuropeptides. A plot of the theoretical q/M(r)2/3 values (where q = peptide charge) calculated across the pH range of 1 to 10 for these peptides was used to understand and to predict their separation. The experimentally determined electrophoretic mobilities (mu) were correlated to the estimate of the relative mu predicted by q/M(r)2/3 and by [ln (q + 1)]/n0.43 (where n = number of constituent amino acids), where q values were calculated using amino acid pKa values for an isolated amino acid and for an amino acid in a peptide. In general, relatively high correlations were obtained with either mathematical expression and with both sets of amino acid pKa values for data at a single pH value. However, the combination of the former expression and the adjusted pKa values gave the best prediction of electrophoretic behavior when data for the three pH values were correlated across these different separation conditions.

Capillary zone electrophoresis of two synthetic neuropeptides: examination of detectability and resolution as a function of peptide concentration and buffer concentration

The capillary zone electrophoretic behavior of substance P and methionine enkephalin, when the amount of peptide was increased at a constant injection volume, was examined using a 50-microns I.D. capillary. Peak-shape distortion (asymmetry) increased when the concentration of these two peptides exceeded 1% of the buffer concentration. Increasing the buffer concentration increased the peak height and decreased the peak width, leading to higher detectability and resolution, respectively. Peak area versus analyte concentration remained linear, even under the overloading conditions that distorted the peak shape.

Detection of neurotensin in tissues by capillary zone electrophoresis

An electrophoretic method for detecting neurotensin in tissues was developed. Homogenates of rat duodenum and adrenal glands were first extracted by solid-phase extraction and purified by reversed-phase high-performance liquid chromatography. The neurotensin-rich fraction was further analyzed by capillary zone electrophoresis (CZE) using a commercial instrument with UV detection. A minor peak was detected as neurotensin and its identity was further confirmed by performing several CZE analyses at different pH values. Such an approach could be used to analyze with good molecular specificity the neuropeptides in some human tissues.

Mass spectrometric analysis of opioid and tachykinin neuropeptides in non-secreting and ACTH-secreting human pituitary adenomas

In a study to test the hypothesis that defects in the metabolism of neuropeptides might be a contributing factor to human anterior pituitary tumor formation, the proenkephalin A, proopiomelanocortin (POMC), and tachykinin systems, which produce methionine enkephalin (ME), beta-endorphin (BE), and substance P (SP), respectively, were measured in patients who had a wide variety of pituitary tumors. Mass spectrometry was used to optimize the level of molecular specificity of the ME and BE analytical measurements, and radioimmunoassay was used to measure SP-like immunoreactivity (SP-li). Compared to data obtained from pituitaries from post-mortem controls, the non-secreting tumors contained a significantly lower amount of the POMC neuropeptide, BE. The lower ME level was not significant. However, two adrenocorticotrophic hormone (ACTH)-secreting tumors contained ME, BE, and SP-li amounts that were much higher than both the controls and nonsecreting tumors. These data suggest that a hypometabolism of the POMC precursor may be operating in non-secreting tumors, and that a hypermetabolism of the proenkephalin A, POMC, and tachykinin precursors may be operating in two ACTH-secreting tumors. These data demonstrate that mass spectrometry plays a critical role in the study of human pituitary tumors.

Determination of vasoactive intestinal peptide in rat brain by high-performance capillary electrophoresis

A high-performance capillary electrophoresis (HPCE) method for determining vasoactive intestinal peptide (VIP) in rat brain was developed. Cerebral cortex was first extracted by solid-phase extraction and purified by reversed-phase high-performance liquid chromatography. The VIP-rich fraction was further analysed by capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography using a commercial HPCE instrument with UV detection. The identity of the peak of endogenous VIP was confirmed by performing multiple CZE analyses at different pH values. This HPCE method allows VIP to be detected and measured with good molecular specificity and could represent a reference method to validate data obtained by radioimmunoassay.