Mass spectrometric analysis of leukotriene A4 and other chemically reactive metabolites of arachidonic acid

The biosynthesis of prostaglandins and leukotrienes proceeds through the formation of chemically reactive intermediates leukotriene A4 (LTA4) and prostaglandin H2 (PGH2) which in aqueous solutions have chemical half-lives of 3 s and 3 min, respectively. Prostacyclin (PGI2) is another chemically reactive prostanoid that has a chemical half-life of 3-4 min. The recent development of reversed phase HPLC stationary phases that are stable to elevated pH (pH 10-12) without significant column damage has permitted direct analysis of these acid-sensitive eicosanoids. Using electrospray ionization, molecular anions [M - H]- of these compounds were observed at m/z 317 for LTA4 and m/z 351 for both PGH2 and PGI2. The mechanism of formation of ions derived from collisional activation of LTA4 was studied using stable isotope labeled and chemical analogs of LTA4 and found to involve formation of highly conjugated anions at m/z 261 and 163. The collisional activation of the molecular anion of PGH2 yielded a product ion spectrum identical to that observed for the isomeric prostaglandins PGE2 and PGD2. However, it was possible to baseline separate PGE2, PDG2, and PGH2 by reversed phase HPLC using basic HPLC mobile phases. The collisional activation of PGI2 led to a family of abundant ions including highly conjugated carbon-centered and oxygen-centered radical species (m/z 245 and 205) likely derived from the attack of the carboxylate anion on the cyclic enolether of PGI2 as well as the most abundant product ion (m/z 215) which formed following loss of neutral hexanal and water. The structures of these product ions were consistent with high resolution measurements measured in a quadrupole time-of-flight mass spectrometer.

Cyclooxygenase-2, prostaglandin synthases, and prostaglandin H2 metabolism in traumatic brain injury in the rat

Inflammatory mediators are important in traumatic brain injury (TBI). The objective of the present study was to investigate the expression of cyclooxygenase-2 (COX-2), prostaglandin E (PGE) and PGD synthases, and PGH2 metabolism in two rat models of TBI. Fluid percussion injury (FPI) resulted in bilateral induction of COX-2 mRNA in the dentate gyri and the cortex, whereas controlled cortical contusion injury (CCC) induced COX-2 mRNA in the ipsilateral dentate gyrus and intensely in the cortex as judged by in situ hybridization. The induction subsided within 24 h. COX-2 immunoreactivity was detectable in these areas and persisted in the ipsilateral cortex for at least 72 h after CCC. Regions with COX-2 induction co-localized with TUNEL staining, suggesting a link between COX-2 expression and cell damage. COX-2 forms PGH2, which can be isomerized to PGD2, PGE2, and PGF2alpha by enzymatic and non-enzymatic mechanisms. In situ hybridization showed that mRNA of PGD synthase and microsomal PGE synthase were present in the choroid plexus. The microsomal PGE synthase was induced bilaterally after FPI and unilaterally after CCC. Liquid chromatography-mass spectrometry showed that low speed supernatant of normal and traumatized cortex and hippocampus transformed PGH2 to PGD2 as main product. PGD2 was dehydrated in brain homogenates to biological active compounds, for example, 15-deoxy-delta12,14-PGJ2. Thus COX-2 increases in certain neurons following TBI without neuronal induction of PGD and microsomal PGE synthases, suggesting that PGH2 may decompose to PGD2 and its dehydration products by nonenzymatic mechanisms or to PGD2 by low constitutive levels of PGD synthase.

Simultaneous HPLC determination of metabolites of the inflammatory cascade

OBJECTIVE

A model approach is presented to the in vivo inflammation cascade, in which the activities of key enzymes (phospholipase A2 [3.1.1.4], prostaglandin synthase [EC 1.14.99.1], and lipoxygenases [EC 1.13.11.12]) are determined simultaneously in a single in vitro assay.

METHODS AND RESULTS

Detection of phospholipids (up to 50 pM) and arachidonic acid (up to 33 pM) is attained with high sensitivity and without radiolabelling using a SEDERE light scattering detector.

CONCLUSIONS

Thus, in combination with a diode array UV-detector, lipids, prostaglandins, HETEs and other metabolites of the inflammation cascade can be determined with high efficiency using a reversed phase-high performance liquid chromatograph equipped with two highly sensitive detectors in series.

Endothelium-dependent contractions are associated with both augmented expression of prostaglandin H synthase-1 and hypersensitivity to prostaglandin H2 in the SHR aorta

Prostaglandin H2 (PGH2 [endoperoxide]) is an immediate product of prostaglandin H (PGH) synthase activity (cyclooxygenase) and a likely candidate to mediate endothelium-dependent contractions evoked by acetylcholine in the aorta of the spontaneously hypertensive rat (SHR). Experiments were designed to investigate whether or not endothelium-dependent contractions were associated with an increased expression of PGH synthase, an augmented acetylcholine-induced release of PGH2, and/or a hypersensitivity of the smooth muscle to endoperoxides in SHR aorta compared with normotensive Wistar-Kyoto (WKY) aorta. In SHR aorta, endothelium-dependent contractions to acetylcholine were abolished by tenidap (10(-8) mol/L), a preferential PGH synthase-1 inhibitor, but slightly impaired by NS-398 (10(-6) mol/L), a preferential PGH synthase-2 inhibitor. PGH synthase-1 expression, which was evaluated by both reverse transcriptase-polymerase chain reaction and Western blotting, was about twofold greater in preparations with endothelium from SHR than from WKY rats. There was no difference in PGH synthase-1 expression between preparations with and those without endothelium in both strains. In SHR but not WKY aortas, acetylcholine (10(-5) mol/L, 5 minutes) caused a significant endothelium-dependent release of PGH2 as measured by gas chromatography/mass spectrometry. PGH2 evoked more potent contractions in rings without endothelium from SHR than from WKY rats, whereas the thromboxane analogue U46619 and prostaglandin F2 alpha caused a comparable response in both preparations. These results show that endothelium-dependent contractions to acetylcholine in SHR aorta are associated with a greater expression of PGH synthase-1, a significant release of PGH2, and a hypersensitivity of the smooth muscle to the endoperoxide.

Characterization of thromboxane A2/prostaglandin H2 binding sites in guinea pig cardiac membrane preparations

Thromboxane A2 (TXA2) and prostaglandin H2 (PGH2) induce platelet aggregation and are potent vasoconstrictors, and they have been implicated in coronary vasospasm and myocardial infarction. The TXA2 mimetic [1S-(1 alpha, 2 beta (5Z), 3 alpha (1E,3S*), 4 alpha)]-7-[3-(3-hydroxy-4-(4'- iodophenoxy)-1-butenyl)-7-oxabicyclo-[2.2.1]heptan-2-yl]-5-h eptenoic acid (IBOP) was used to characterize binding to microsomal membrane preparations from saline-perfused guinea pig atria (GPA) and ventricles (GPV). [125I]IBOP bound to GPA and GPV in a protein-dependent and saturable manner, although total binding was two-fold greater and non-specific binding was proportionately less in GPA compared to GPV. Analysis of equilibrium binding data indicated one class of binding sites in both GPA and GPV with Kd values of 333 +/- 117 and 645 +/- 187 pM, respectively, which were in close agreement with kinetically determined Kd values of 226 and 882 pM, respectively. Bmax values of GPA and GPV of 57 +/- 5.6 and 24 +/- 4.3 fmol/mg protein were significantly different (P < 0.01). Ki values (from IC50s) were determined for various TXA2/PGH2 analogues and prostaglandins in competition binding assays with [125I]IBOP. The rank order for ability to inhibit binding in GPA was U46619 = SQ29548 > I-PTA-OH > PGF2 alpha = PGE2. In GPV, the rank order was U46619 = SQ29548 > PGF2 alpha = I-PTA-OH = PGE2. [125I]IBOP binding to GPA and GPV was completely displaced by the TXA2/PGH2 agonist U46619 and by the TXA2/PGH2 antagonist SQ29548.(ABSTRACT TRUNCATED AT 250 WORDS)

Ammonia (NH3 and N2H3) direct chemical ionization mass spectrometry of underivatized prostaglandin-H2 and other selected stable prostaglandins

A study of the positive and negative ion, ammonia (NH3 and N2H3) direct chemical ionization mass spectrometry of highly purified prostaglandin endoperoxide (PGH2) is presented. The positive ion spectra were characterized by an intense [M + NH4]+ adduct at m/z 370 and several fragment ions, most notably a [M + NH4-H2O]+ ion at m/z 352 and an ion at m/z 298, assigned as the [M + NH4-72]+ ion of 12-hydroxy-5,8,10-heptadecatrienoic acid formed form PGH2 in the spectrometer. The negative ion spectra of PGH2 were characterized by a base peak at m/z 352 [M]- and by an ion at m/z 334 corresponding to the loss of water from the parent ion. A combination of negative ion and deuterated ammonia reagent gas was used in making assignments and in demonstrating that the spectra observed were due to intact PGH2 and its stable PGE2 and PGD2 isomers formed in the spectrometer. In addition, use of the latter reagent gas was shown to clearly distinguish between several arachidonic acid metabolites, differing in their number of exchangeable protons. Furthermore, preliminary results with several stable prostaglandins indicate that the spectra are sensitive to different functional groups that are present. Consequently, it would appear that negative and positive ion, NH3 (N2H3) direct chemical ionization mass spectrometry would be useful in the analysis of labile arachidonic acid metabolites, without the need for prior derivatization.

Use of the rabbit transverse stomach-strip to identify and assay prostacyclin, PGA2, PGD2 and other prostaglandins

The preparation and use of the rabbit transverse stomachstrip as an isolated assay tissue for prostaglandins is described. The tissue is relaxed by low concentrations of PGE2, PGA2 and PGD2 and contracted by prostacyclin (PGI2) and PGF2alpha. The decomposition product of prostacyclin, 6-oxo-PGF1alpha, has little effect, whereas the endoperoxide PGH2 and thromboxane A2 cause a small contraction. Arachidonic acid also contracts the tissue and this response is abolished by indomethacin. This tissue also responds to low concentrations of histamine, acetylcholine, noradrenaline and bradykinin. In conjunction with other bioassay tissues such as the rat stomach strip and rabbit coeliac artery, the rabbit transverse stomach strip provides a convenient method of detection of identification of known arachidonic acid metabolites.