Stable isotope dilution gas chromatography/mass spectrometry of prostaglandins and leukotrienes
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The group of arachidonic acid metabolites comprising the prostaglandins, thromboxanes, and leukotrienes (eicosanoids) are extremely potent, biologically active compounds. Their properties include proaggregatory anti-aggregatory activity for platelets, chemotactic activity for neutrophils, vasoactive activity, and contractile activity to smooth muscle. In order to determine the role of these substances in pathophysiological conditions, it is essential to have highly sensitive methods available for their analysis.
It is generally accepted that combined gas chromatography/mass spectrometry is the most specific technique available for the quantitative analysis of eicosanoids. However, methods based on electron impact ionization and positive ion chemical ionization are relatively insensitive, and many investigators have preferred the use of less specific but more sensitive methods based on radioimmunoassay. We have explored the use of negative ion chemical ionization mass spectrometry to improve sensitivity coupled with capillary column chromatography to maximize specificity.
Conversion of the terminal carboxyl group (present in all eicosanoids) to the pentafluorobenzyl ester derivative confers excellent electron capturing properties to the molecule. The derivative undergoes highly efficient thermal electron capture in the gas phase, and any fragmentation that occurs subsequently is directed almost entirely away from the analyte molecule. The stabilized carboxylate anion that results carries at least 30% of the total ion current. Using selected ion monitoring techniques it is possible to detect eicosanoids in the range 1–8 pg on column.
This methodology has been applied to the development of stable isotope dilution assays for plasma 6-oxo-prostaglandin (PG) F1α (1) and for the simultaneous analysis of six biologically important PGs in biological fluids (2). In addition, stable isotope dilution techniques have been developed for the analysis of serum thromboxane B2 and serum leukotriene B4 (3). The application of this technology to understanding the role of arachidonic acid metabolism in humans will be discussed.