Quantitation of the fatty acid composition of phosphatidic acid by capillary gas chromatography electron-capture detection with picomole sensitivity

LC/ESI-MS/MS detection of FAs by charge reversal derivatization with more than four orders of magnitude improvement in sensitivity

Quantitative analysis of fatty acids (FAs) is an important area of analytical biochemistry. Ultra high sensitivity FA analysis usually is done with gas chromatography of pentafluorobenzyl esters coupled to an electron-capture detector. With the popularity of electrospray ionization (ESI) mass spectrometers coupled to liquid chromatography, it would be convenient to develop a method for ultra high sensitivity FA detection using this equipment. Although FAs can be analyzed by ESI in negative ion mode, this method is not very sensitive. In this study, we demonstrate a new method of FA analysis based on conversion of the carboxylic acid to an amide bearing a permanent positive charge, N-(4-aminomethylphenyl)pyridinium (AMPP) combined with analysis on a reverse-phase liquid chromatography column coupled to an ESI mass spectrometer operating in positive ion mode. This leads to an ∼60,000-fold increase in sensitivity compared with the same method carried out with underivatized FAs. The new method is about 10-fold more sensitive than the existing method of gas chromatography/electron-capture mass spectrometry of FA pentafluorobenzyl esters. Furthermore, significant fragmentation of the precursor ions in the nontag portion improves analytical specificity. We show that a large number of FA molecular species can be analyzed with this method in complex biological samples such as mouse serum.

Randomized, placebo-controlled trial of lisofylline for early treatment of acute lung injury and acute respiratory distress syndrome

OBJECTIVE

To determine whether the administration of lisofylline (1-[5R-hydroxyhexyl]-3,7-dimethylxanthine) would decrease mortality in patients with acute lung injury (ALI) or acute respiratory distress syndrome (ARDS).

DESIGN

A prospective, randomized, double-blind, placebo-controlled, multicenter study.

SETTING

Intensive care units at 21 hospitals at the ten centers constituting the ARDS Clinical Trials Network.

PATIENTS

A total of 235 patients who met eligibility criteria were enrolled in the study (116 into the lisofylline group, 119 into the placebo group).

INTERVENTIONS

Patients were randomized to receive either lisofylline or placebo. The dose of lisofylline was 3 mg/kg with a maximum dose of 300 mg intravenously every 6 hrs. The intravenous solution of study drug was administered over 10 mins every 6 hrs. Dosing was continued for 20 days or until the patient achieved 48 hrs of unassisted breathing.

MEASUREMENTS AND MAIN RESULTS

The trial was stopped by the Data Safety Monitoring Board for futility at the first scheduled interim analysis. The patient groups had similar characteristics at enrollment. No significant safety concerns were associated with lisofylline therapy. There was no significant difference between groups in the number of patients who had died at 28 days (31.9% lisofylline vs. 24.7% placebo, p = .215). There was no significant difference between the lisofylline and placebo groups in terms of resolution of organ failures, ventilator-free days, infection-related deaths, or development of serious infection during the 28-day study period. The median number of organ failure-free days for the five nonpulmonary organ failures examined (cardiovascular, central nervous system, coagulation, hepatic, and renal) was not different between the lisofylline and placebo groups. Although lisofylline has been reported to decrease circulating free fatty acid levels, we did not find any such treatment effect compared with placebo.

CONCLUSIONS

In this study, there was no evidence that lisofylline had beneficial effects in the treatment of established ALI/ARDS.

A frontal variant of Alzheimer's disease exhibits decreased calcium-independent phospholipase A2 activity in the prefrontal cortex

A frontal variant of Alzheimer's disease (AD) has recently been identified on neuropathological and neuropsychological grounds (Johnson, J.K., Head, E., Kim, R., Starr, A., Cotman, C.W., 1999. Clinical and pathological evidence for a frontal variant of Alzheimer Disease. Arch. Neurol. 56, 1233-1239). Frontal AD differs strikingly from typical AD by the occurrence of neurofibrillary tangle densities in the frontal cortex as high or higher than in the entorhinal cortex. Since cerebrocortical membranes are commonly abnormal in Alzheimer's disease (AD), we assayed frontal AD cases for enzymes regulating membrane phospholipid composition. We specifically measured activity of phospholipase A2s (PLA2s) in dorsolateral prefrontal and lateral temporal cortices of frontal AD cases (n=12), which have respectively high and low densities of neurofibrillary tangles. In neither cortical area was Ca(2+)-dependent PLA2 activity abnormal compared to controls (n=12). In contrast, a significant 42% decrease in Ca(2+)-independent PLA2 activity was found in the dorsolateral prefrontal, but not the lateral temporal, cortex of the frontal AD cases. Similarly, the dorsolateral prefrontal cortex, but not the lateral temporal cortex of the frontal AD cases suffered a 42% decrease in total free fatty acid content, though neither that decrease nor those in any one species of free fatty acid was significant. The observed biochemical changes probably occurred in neurons given (a) our finding that PLA2 activity of cultured human NT2 neurons is virtually all Ca(2+)-independent and (b) the finding of others that nearly all Ca(2+)-independent PLA2 in brain gray matter is neuronal. The decrease in Ca(2+)-independent PLA2 activity is not readily attributable to Group VI or VIII iPLA2s since neither NT2N neurons nor our brain homogenates were greatly inhibited by drugs potently suppressing those iPLA2s. Decreased Ca(2+)-independent PLA2 activity in frontal AD may reflect a compensatory response to pathologically accelerated phospholipid metabolism early in the disorder. That could cause an early elevation of prefrontal free fatty acids, which can stimulate polymerization of tau and thus promote the prefrontal neurofibrillary tangle formation characteristic of frontal AD.

Preferential distribution of long chain polyunsaturated fatty acids in phospatidyl ethanolamine fraction of guinea pig alveolar apical membranes

We investigated the fatty acid distribution in guinea pig alveolar apical membranes at different developmental stages. Fatty acid composition of the purified membranes isolated from guinea pig fetuses (at 65 day, term=68 day), neonates (day 1) and adult males was determined. The levels of arachidonic acid (AA) and docosahexaenoic acid (DHA) were higher in the adult guinea pig alveolar apical membrane phosphatidylethanolamine (PE) fraction (9. 3+/-2.2 and 2.9+/-1.0%, respectively) while in other phospholipids (PL) fractions their levels were low or absent (P<0.01). Furthermore, levels of AA and DHA in the PE fraction of apical membrane increased significantly from fetal (6.6+/-3.0 and 0.8+/-0.4%, respectively) to neonatal life (10.3+/-1.5 and 3.0+/-0.8%, respectively). Increase in the level of DHA (almost four-fold) was much more pronounced than that of AA (P<0.05). As for guinea pig alveolar membranes, EPA and AA were mostly present in the PE fraction in pulmonary adenocarcinoma derived cells (A549 cells), a parallel model of type II pneumocytes, with the levels of AA around three-fold greater than that of EPA, Binding of radiolabelled fatty acids to A549 cells showed no significant differences between the maximum uptake achieved for different fatty acids (AA, 1.7+/-0.2, EPA, 2.3+/-0.3, LA, 1.7+/-0.2, OA, 2.0+/-0.2nmol/mg protein, P>0.5). Once the fatty acids were taken up by these cells AA was mostly identifiable in the monoacylglycerol (MAG) fraction, whereas EPA was equally distributed between the MAG and PL fractions. Oleic acid was mainly present in the triglyceride (TAG) fraction whereas LA was evenly distributed between the TAG, MAG, and PL fractions. Our data demonstrate a preferential distribution of AA and DHA in PE fractions of alveolar apical membranes during development.

High-resolution separation and quantification of neutral lipid and phospholipid species in mammalian cells and sera by multi-one-dimensional thin-layer chromatography

An improvement of current methods is needed for simple, rapid, and precise quantification of cellular lipids, including rare species of biologically active cellular lipids, such as phosphatidic acid (PA) and diradylglycerol (DG). In addition, further analysis of hydrolyzed acyl chains from these species by methods such as gas chromatography requires complete separations. Methods have been developed for the quantification of neutral lipids and several phospholipids extracted from mammalian cells and sera. Lipid masses were determined for the major classes of the neutral, nonpolar lipids, and of the phospholipids. The lipid classes were separated by a multistep thin-layer chromatography (TLC) procedure in different solvent systems, a method which we have designated as multi-one-dimensional thin-layer chromatography (MOD-TLC). Resolved lipid bands were visualized by the lipophilic dye primulin (direct yellow 59) and scanned by an automated laser-fluorescence detector. The mass of each band was determined by comparing band intensities of unknown samples to dilution curves of authentic standards. With modifications in solvent mixtures and length of separation times, the majority of biological lipids could be resolved and quantified with MOD-TLC methods. Since the detection method is nondestructive, purified lipids could then be recovered by scraping the visualized bands and extracting the lipids from the silica. The structural identities of the recovered lipids were confirmed by fast-atom bombardment and electrospray mass spectrometry. Extracted lipids were also hydrolyzed to release acyl chains and acyl chain species were determined in comparison to authentic standards by gas chromatography. PA and DG levels in ECV.304 cells were found to be 4. 6 and 3.3%, respectively, of PC levels, with a PA/DG ratio of 1.4, which is in accord with published experience using other methods and different cell types. PA in human serum was detected at 0.6% of PC, indicating the sensitivity of the technique. In contrast to two-dimensional thin-layer chromatography, which allows for good resolution of some lipid species, but cannot be used to analyze more than a single experimental point per plate, MOD-TLC allows for direct comparative analysis of multiple samples on a single TLC plate, while still providing good resolution for the quantification of most major classes of lipid species.

Electron-capture detection: difluorobenzyl and related electrophores

Difluorobenzyl derivatives (several isomers were tested) of 4-hydroxyacetophenone were synthesized and found to have similar properties (retention and response) by both reversed-phase HPLC and GC-ECD relative to each other, and also relative to that of a corresponding conventional pentafluorobenzyl derivative. The same was true for a representative difluorobenzyl derivative of thymine and 1-naphthoic acid. Overall, the responses by GC-ECD for the same core structure were only about two- to four-fold lower for a difluorobenzyl compared to a corresponding pentafluorobenzyl derivative. This makes a difluorobenzyl derivative attractive as an HPLC-UV retention marker, and sometimes as a substitute for a pentafluorobenzyl derivative (to help overcome an interference) in a method based on detection by electron capture. We also observed, somewhat as an aside, that the GC-ECD response of the benzyl derivative of 4-hydroxyacetophenone was only seven-fold lower than that of the corresponding pentafluorobenzyl derivative, and that this former benzyl derivative gave a 2.10(4) higher response than acetophenone. Thus, replacing the ring hydrogen atoms of a benzyl group with fluorine atoms had a relatively small impact on both the hydrophobicity and electron capture properties of the compounds tested here.

General strategies in chromatographic analysis of lipids

Lipid extracts of natural sources contain a large number of lipid classes and molecular species. Completely reproducible samples are obtained only with great care and skill. Analytical methods other than chromatography and/or mass spectrometry are of little use for resolution and identification of lipid molecules even in simple mixtures. The analytical information desired governs the selection of the chromatographic and mass spectrometric method, which determine the sample preparation and derivative needed. Usually a combination of chromatographic methods is necessary to identify specific species of lipids. The recent development of soft ionization techniques, that are readily interfaced with mass spectrometers, have greatly simplified the sample preparation and have largely eliminated the need for derivatization. Because these techniques require expensive equipment and dedicated operators, the methods selected must be consistent with the true analytical needs and the available resources. Although personal preference cannot be eliminated entirely, the general strategies outlined below should help to reduce the number of possibilities facing a lipid analyst to a few practical choices.

Fatty acid profiles of lipid samples

Most lipids are best characterized by their fatty acids which differ in chain length, the degree of unsaturation, configuration and position of the double bonds, and the presence of other functionalities. The fatty acid profiles are currently most frequently determined by capillary gas chromatography of the methyl esters which are prepared by a variety of methods. These are discussed with an emphasis on more recent developments, along with the stationary phases used for the separations and the methods employed for identification of the fatty acids. HPLC is applied less frequently for ascertaining fatty acid profiles than GC, but a very large number of derivatives for ultraviolet and fluorescence detection have been proposed. This method continues to evoke increasing interest, particularly in conjunction with fluorescence detection. This technique enables attainment of greater sensitivities than with standard GC methods employing flame ionization detection. Extensive applications of it to the analysis of free fatty acids in blood and other biomedical samples are clearly discernible. Other methods, including supercritical fluid chromatography, have found only limited application for fatty acid profiling.

Diacylglycerol hydrolysis to arachidonic acid is necessary for insulin secretion from isolated pancreatic islets: sequential actions of diacylglycerol and monoacylglycerol lipases

Arachidonic acid has been implicated as a second messenger in insulin secretion on the basis of (1) mobilization of intracellular Ca2+ from the endoplasmic reticulum of islets and (2) amplification of voltage-dependent Ca2+ entry. The insulin secretagogues D-glucose and the muscarinic agonist carbachol both increase unesterified arachidonic acid accumulation in isolated islets. We now show that diacylglycerol, a product of phospholipase C action, is a major source of free arachidonic acid in islets. Diacylglycerol hydrolysis in islets occurs through a two-step process. In the first step, the sn-1 bond of 1-stearoyl-2-arachidonyl-sn-glycerol is hydrolyzed by a diacylglycerol lipase, giving rise to 2-arachidonyl-sn-glycerol. Next, the sn-2 bond of 2-arachidonyl-sn-glycerol is hydrolyzed by a monoacylglycerol lipase, which is the rate-limiting step, releasing unesterified arachidonic acid. Both diacylglycerol lipase and monoacylglycerol lipase are highly enriched in the plasma membrane of beta-cells. Diacylglycerol lipase activity in islet homogenates is selectively inhibited in a dose-dependent manner by the compound RHC-80267, a specific diacylglycerol lipase inhibitor. RHC-80267 inhibits glucose- and carbachol-induced insulin release from intact islets in a dose-dependent manner that parallels its inhibition of diacylglycerol lipase activity. Importantly, RHC-80267, at concentrations that almost completely inhibit diacylglycerol lipase activity and glucose- and carbachol-induced insulin secretion by islets, markedly inhibits glucose- and carbachol-induced increases in islet arachidonic acid levels, as measured by gas chromatography with electron-capture detection of its pentafluorobenzyl esters. RHC-80267 did not significantly affect islet glucose oxidation, phospholipase C, monoacylglycerol lipase, or phospholipase A2. Since glucose and carbachol are known to stimulate phospholipase C, our observations indicate that diacylglycerol is an important source of arachidonic acid and other free fatty acids in islets. Furthermore, production of arachidonic acid from the hydrolysis of diacylglycerol is essential for glucose- and carbachol-induced insulin secretion.