FAB-MS/MS study of two neuropeptides, dynorphins 1–7 and 1–13

Analysis of neuropeptides by perfusion liquid chromatography/electrospray ion-trap mass spectrometry

Perfusion high-performance liquid chromatography (HPLC) combined with electrospray ion trap mass spectrometry (ITMS) was evaluated for the determination of neuropeptides in plasma. Perfusion HPLC offers the capability of resolving neuropeptides spiked into plasma in 5 min compared to the 30-60 min separations performed on packed capillary C18 columns. Electrospray combined with the ITMS provides the ability to ionize these neuropeptides and mass analyze them with high sensitivity and specificity. Sub-picomole quantities of neuropeptides injected on-column could be specifically detected in a plasma matrix. The electrospray-ITMS mass spectrum of each neuropeptide showed multiply charged ions which could be used to determine or confirm their molecular weights.

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.

Electrospray mass spectrometry for the analysis of opioid peptides and for the quantification of endogenous methionine enkephalin and β-endorphin

Electrospray ionization mass spectrometry was used to characterize several different neuropeptides, whose molecular weights ranged from 555 to 3463 Da, and to quantify endogenous methionine enkephalin (ME) and β -endorphin (β E) extracted from a human pituitary gland. Methionine enkephalin and leucine enkephalin both yield only an [M + H] + ion with electrospray mass spectrometry; the other peptides produce a series of multiply charged even-electron molecular ions of the general nature [M + nH](n)+ in proportion to the number of basic amino acid units present, with no evidence of fragmentation. The electrospray mass spectra are characterized by low background noise. The quantiftcation of ME is based on a comliarison of the ion current due to the [M + H] + ion of native and of a deuterated ME ([(2)H5 s-(4)Phe]-ME) internal standard. The calibration curve is linear in the range of ca. 1-35 pmol synthetic ME. The amounts of ME determined in three separate human pituitary extracts were 9.1, 8.2, and 4.7 pmol/mg protein. The corresponding amount of ME in a canine pituitary was 39.8 pmol/mg protein. To quantify β E, the ion current due to the [M + 5H](5) + ion was monitored and compared to an external calibration curve obtained by analyzing solutions of synthetic β E in the range 5 μmol-50 pmol. The analysis of a human pituitary yielded 660 fmol β E/mg protein.

Proenkephalin A and proopiomelanocortin peptides in human cerebrospinal fluid

Precursors to beta-endorphin (BE) and methionine enkephalin (ME), and proteolytic enzymes that cleave those BE and ME precursors to BE and ME, were determined in several milliliters of human cerebrospinal fluid. Endogenous peptides were purified by reversed-phase high-performance liquid chromatography (HPLC), and were detected with radioreceptor assay (RRA), radioimmunoassay, and mass spectrometry (MS). Total opioid receptor activity measurements and the profile of HPLC-receptor activity of human CSF samples were both used to monitor neuropeptide metabolism. MS data linked the molecular ion of ME to a unique fragment ion. A later-eluting fraction (84 min) in a 90-min HPLC gradient appeared in all HPLC-RRA profiles, contained opioid receptor activity that displaced [3H]etorphine, and the quantitative and qualitative patterns of opioid receptor activity in those profiles both changed within the few minutes that elapsed between acquiring the first and second cerebrospinal fluid samples. That 84-min fraction contained precursors to opioid peptides and was fractionated further with a more shallow 120-min HPLC gradient into three sections that displayed delta-opioid receptor-preferring activity, using [3H]ME as ligand. These three sections were hydrolyzed separately with human cerebrospinal fluid as the source for endogenous neuropeptides to yield products that correlated to immunoreactive BE in section I and immunoreactive ME in section III.

Fast atom bombardment tandem mass spectrometry for amino acid sequence determination in tripeptides

Mass spectral information obtained from the spectra of 26 tripeptides was carefully studied. The data were obtained from their mass, metastable ion and collisional activation spectra. On the basis of the positive ion tandem mass spectra a simple and unambiguous method for the sequence determination of amino acids in tripeptides and Y3" ions is proposed. The use of negative ion spectra for sequence determination in tripeptides is evaluated.

Characterization of beta-endorphin in human pituitary by fast atom bombardment mass spectrometry of trypsin-generated fragments

A novel mass spectrometric method possessing a high level of structural specificity is described for characterization in biological fluids and tissues of endogenous beta-endorphin of the human amino acid sequence (beta h-EP). The method is based upon purification of tissue extracts by an RP-HPLC gradient, followed by trypsinolysis of that particular HPLC fraction corresponding to the elution time of synthetic beta h-EP. The tryptic digest of that endogenous beta h-EP fraction was purified further by a second RP-HPLC gradient. A unique tryptic fragment selected from the second gradient was analyzed by fast atom bombardment mass spectrometry and B/E linked-field scan MS/MS techniques to provide molecular weight and amino acid sequence-determining fragment ion information, respectively, of that fragment. Collectively, these independent analytical methodologies provided unequivocal structure evidence for the presence of endogenous beta h-EP in human pituitary. The method was established first by utilizing synthetic beta h-EP to optimize experimental parameters, and then applied to the analysis of beta h-EP in post-mortem human pituitary extracts. The suitability of the present method for semi-quantitation of tissue extracts is also demonstrated. The corresponding detection limit of the synthetic beta h-EP was 90 fmol, and human pituitary contained 1.5 pmol of beta h-EP mg-1 protein. The method can be extended readily to the analysis of beta-endorphin derived from other species and tissues.