Mass spectrometric measurement of β-endorphin and methionine enkephalin in human pituitaries. Tumors and post-mortem controls

Monitoring neuropeptide-specific proteases: processing of the proopiomelanocortin peptides adrenocorticotropin and alpha-melanocyte-stimulating hormone in the skin

The neuroendocrine precursor protein proopiomelanocortin (POMC) and its derived neuropeptides are involved in a number of important regulatory processes in the central nervous system as well as in peripheral tissues. Despite its important role in controlling the local activation of melanocortin (MC) receptors, the extracellular proteolytic processing of POMC peptides has received little attention. The mechanisms relevant for controlling the bioavailability of adrenocorticotropin and melanocyte-stimulating hormones for the corresponding MC receptors in the skin by specific peptidases such as neprilysin (neutral endopeptidase; NEP) or angiotensin-converting enzyme (ACE) have been addressed in a number of recent investigations. This review summarizes the current body of knowledge concerning the qualitative and quantitative POMC peptide processing with respect to the action and specificity of NEP and ACE and discusses relevant recent analytical methodologies.

Proteolytic 18O labeling for comparative proteomics: model studies with two serotypes of adenovirus

A new method for proteolytic stable isotope labeling is introduced to provide quantitative and concurrent comparisons between individual proteins from two entire proteome pools or their subfractions. Two 18O atoms are incorporated universally into the carboxyl termini of all tryptic peptides during the proteolytic cleavage of all proteins in the first pool. Proteins in the second pool are cleaved analogously with the carboxyl termini of the resulting peptides containing two 16O atoms (i.e., no labeling). The two peptide mixtures are pooled for fractionation and separation, and the masses and isotope ratios of each peptide pair (differing by 4 Da) are measured by high-resolution mass spectrometry. Short sequences and/or accurate mass measurements combined with proteomics software tools allow the peptides to be related to the precursor proteins from which they are derived. Relative signal intensities of paired peptides quantify the expression levels of their precursor proteins from proteome pools to be compared, using an equation described in the paper. Observation of individual (unpaired) peptides is mainly interpreted as differential modification or sequence variation for the protein from the respective proteome pool. The method is evaluated here in a comparison of virion proteins for two serotypes (Ad5 and Ad2) of adenovirus, taking advantage of information already available about protein sequences and concentrations. In general, proteolytic 18O labeling enables a shotgun approach for proteomic studies with quantitation capability and is proposed as a useful tool for comparative proteomic studies of very complex protein mixtures.

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.

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.

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.

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.

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.