Determination of platelet-activating factor and alkyl-ether phospholipids by gas chromatography-mass spectrometry via direct derivatization

An improved assay for platelet-activating factor using HPLC-tandem mass spectrometry

We describe an improved assay for platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) using HPLC-tandem mass spectrometry (LC-MS/MS). The present method can readily detect as little as 1 pg (1.9 fmol) of PAF, a significant improvement over previously described LC-MS/MS methods, and gives a linear response up to 1,000 pg of PAF. Our method also overcomes the artifacts from isobaric lipids that have limited the usefulness of certain existing LC-MS/MS assays for PAF. In the course of these studies, we detected three novel lipid species in human neutrophils. One of the novel lipids appears to be a new molecular species of PAF, and the other two have chromatographic and mass spectrometric properties consistent with stearoyl-formyl-glycerophosphocholine and oleoyl-formyl-glycerophosphocholine. These observations identify previously unknown potential interferences in the measurement of PAF by LC-MS/MS. Moreover, our data suggest that the previously described palmitoyl-formyl-glycerophosphocholine is not unique but rather is a member of a new and poorly understood family of formylated lipids.

Derivatization of phospholipids

Phospholipids are major components of biological membranes. Without chemical derivatization, it is difficult to identify and quantitate phospholipids in biological samples. Chemical derivatization can improve both the selectivity and sensitivity of the analytes. This paper gives a full review, through March, 2002, of derivatization methods used for phospholipids in HPLC, CE and GC as well as the spray reagent used for TLC in the early days.

High-performance liquid chromatography determination of bis(monoacylglycerol) phosphate and other lysophospholipids

Bis(monoacylglycerol) phosphate (BMP) is a very minor component of the phospholipid (PL) fraction in rat uterine stromal cell cultures (U(III) cells). Under several culture conditions, including the addition of (n-3) or (n-6) polyunsaturated fatty acids, BMP selectively accumulates docosahexaenoic acid (DHA). We have recently described the structure of this PL, but its biological function is still largely unknown, except for a role in late endosomes trafficking. In order to further investigate this function, we have developed a sensitive assay for accurate determination of BMP in small biological samples. Total PL from cells, labeled or not with trace amount of [3H]DHA, were extracted and PL classes separated by thin-layer chromatography. After extraction of the gel corresponding to the BMP area, a known amount of an internal standard was added. The free hydroxyl groups of PL were totally derivatized with naproxen. Derivatized PL were separated by normal-phase high-pressure liquid chromatography and quantified using UV absorption at 231 nm. Since the sensitivity of the proposed method was about 0.1 nmol for BMP, samples of only 3 x 10(5) cells were required. The BMP level was found to be 616 +/- 46 pmol for 10(6) control cells. It was increased threefold in starved cells and significantly increased in cells cultured in the presence of exogenous phosphatidylglycerol.

The sensitivity of bovine corneal epithelial lyso-PAF acetyltransferase to cyclooxygenase and lipoxygenase inhibitors is independent of arachidonate metabolites

Lyso-platelet-activating factor (PAF) acetyltransferase is a critical regulatory step in PAF synthesis. PAF accumulates in the cornea in response to injury and is a potent inflammatory mediator that stimulates corneal cyclooxygenase but not lipoxygenase reactions. 12(S)-hydroxyeicosatetraenoic (HETE) acid, a major lipoxygenase product of mammalian corneas, is also generated during injury. In the bovine corneal epithelium, both PAF acetyltransferase and 12-lipoxygenase are microsomal enzymes. A potential interaction between these two lipid mediators was, therefore, examined. PAF acetyltransferase activity was assayed by determining radioactivity in the trichloroacetic acid-precipitated complex of [3H]PAF bound to albumin, formed after incubation of corneal epithelial microsomes with lyso-PAF and [3H]acetyl CoA. Lipoxygenase metabolism by bovine corneal epithelial microsomes was also studied. While 12(S)-HETE did not activate lyso-PAF acetyltransferase, PAF synthesis was decreased when microsomes were treated with lipoxygenase inhibitors. The IC50 values for nordihydroguaiaretic acid (NDGA), and baicalein were 75 and 105 microM, respectively. The IC50 value for CDC, a more potent inhibitor of platelet 12-lipoxygenase, was greater than 200 microM. The results indicate that concentrations suppressing lyso-PAF acetyltransferase activity exceed those required to inhibit lipoxygenases from bovine corneal epithelia. Similarly, concentrations of aspirin and indomethacin, cyclooxygenase inhibitors that decrease PAF formation, were greater than those reported to block prostaglandin generation. A number of other compounds, some structurally similar to the lipoxygenase inhibitors that suppress lyso-PAF acetyltransferase, and others unrelated chemically but known as anti-oxidant and cationic-chelators, also inhibited lyso-PAF acetyltransferase. This suggests that lyso-PAF acetyltransferase activity is inhibited by mechanisms independent of lipoxygenase and cyclooxygenase metabolites.

Molecular heterogeneity of PAF in normal human mixed saliva: quantitative mass spectral analysis after direct derivatization of PAF with pentafluorobenzoic anhydride

Platelet-activating factor (PAF), a family of phospholipid autacoids with potent pro-inflammatory activities, is present in saliva. The current study has quantitated various species of PAF isolated from normal human mixed saliva. Choline-containing, sn-2 acetylated phospholipids with sn-1 ether- or ester-linked fatty alcohol/acid moieties (alkyl-PAF or acyl-PAF, respectively) were evaluated after direct derivatization with pentafluorobenzoic (PFB) anhydride. Individual species of PFB-derivatized PAF were separated by gas chromatography prior to mass spectral analysis; quantitative estimates of six different species of PAF in saliva were made by comparison to corresponding authentic, synthetic PAF standards. In each saliva sample, all six species of PAF were readily detected by this facile procedure. The predominant PAF was 1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine or 16:0-alkyl-PAF (0.75 +/- 0.09 pmol/ml saliva; mean +/- S.E.; n = 5) which represented only 30.4 +/- 1.5% of the total PAF. Substantial amounts of 18:1- and 18:0-alkyl-PAF and 16:0-acyl-PAF were also identified (0.52 +/- 0.07, 0.35 +/- 0.06, and 0.35 +/- 0.02 pmol/ml saliva, respectively). In summary, mass spectrometric analysis of PAF after direct derivatization with PFB anhydride has revealed that at least six different species of PAF are present in normal human mixed saliva. This structural diversity may represent an important aspect of homeostasis in the healthy oral cavity.