[50] Mass spectrometry of lipids and steroids

GC-MS structural characterization of fatty acids from marine aerobic anoxygenic phototrophic bacteria

The FA composition of 12 strains of marine aerobic anoxygenic phototrophic bacteria belonging to the genera Erythrobacter, Roseobacter, and Citromicrobium was investigated. GC-MS analyses of different types of derivatives were performed to determine the structures of the main FA present in these organisms. All the analyzed strains contained the relatively rare 11-methyloctadec-12-enoic acid, and three contained 12-methyl-octadec-11-enoic acid, which has apparently never been reported before. High amounts of the very unusual octadeca-5,11-dienoic acid were present in 9 of the 12 strains analyzed. A FA containing a furan ring was detected in three strains. Analytical data indicated that this FA was 10,13-epoxy-11-methyloctadeca-10,12-dienoic acid. A very interesting enzymatic peroxidation of the allylic carbon 10 of cis-vaccenic acid was observed in three strains. Deuterium labeling and GC-MS analyses enabled us to demonstrate that this enzymatic process involves the initial dioxygenase-mediated formation of 10-hydroperoxyoctadec-11(cis)-enoic acid, which is then isomerized to 10-hydroperoxyoctadec-11(trans)-enoic acid and converted to the corresponding hydroxyacids and oxoacids. Different biosynthetic pathways were proposed for these different compounds.

Electron ionization mass spectral fragmentation of C19 isoprenoid aldehydes and carboxylic acid methyl and trimethylsilyl esters

The electron ionization (EI) mass spectra of saturated and alpha,beta-unsaturated C(19) isoprenoid aldehydes and carboxylic acid methyl and trimethylsilyl esters are reported. Different pathways are proposed in order to explain the main fragmentations observed. The conjugated double bond migrates more or less readily before gamma-hydrogen rearrangement according to the structure of the considered compound. Configurations of the double bond of alpha,beta-unsaturated C(19) isoprenoid aldehydes and fatty acid methyl and trimethylsilyl esters can be easily determined thanks to the peaks at m/z 97, 127 and 185, respectively, which are much more abundant in the mass spectra of the Z isomers owing to the formation of a cyclic ion. In the case of trimethylsilyl esters, subsequent fragmentation of the cyclic ion at m/z 185 affords two other diagnostic ions at m/z 95 and 169.

Regiospecific enzymatic oxygenation of cis-vaccenic acid during aerobic senescence of the halophilic purple sulfur bacterium Thiohalocapsa halophila

A regiospecific oxygenation of the allylic carbon 10 of cis-vaccenic acid has been observed in senescent cells of the halophilic purple sulfur bacterium Thiohalocapsa halophila incubated under aerobic conditions in darkness. The results obtained strongly suggest that these enzymatic processes involve the initial dioxygenase-mediated formation of 1 O-hydroperoxyoctadec-cis-11-enoic acid, which is not accumulated in the cells of T. halophila owing to its high cytotoxic properties. Deuterium labeling and GC-MS analyses enabled us to demonstrate that subsequent enzymatic conversions of this allylic hydroperoxide involved reduction, cleavage, isomerization, and saturation reactions. Some of the specific oxidation products thus formed could constitute potential T. halophila biomarkers.

Sphingolipids of a cestode Metroliasthes coturnix

Sphingolipids of Metroliasthes coturnix were studied. The cestode contained no detectable amounts of sphingomyelin. The major glycosphingolipids found were monogalactosylceramide, galactosyl-alpha-1-4-galactosylceramide, galactosyl-beta-1-6-galactosyl-beta-1-6-galactosylceramide and galactosyl-beta-1-6-galactosyl-beta-1-6-galactosyl-beta-1-6-galactosylce ramide. Their ceramides were mostly composed of C18-20 sphinganine or 4-D-hydroxysphinganine, which is N-acylated by extraordinarily long normal and 2-hydroxy acids such as C26 acid (range C16 to C30) except tetragalactosylceramide. The structure of glycosphingolipids was confirmed after their chromatographic separation into each molecular species using a novel analytical device, fast atom bombardment-mass spectrometry linked with reversed-phase high-performance liquid chromatography.

Synthesis and characterization of cytochrome P-450 epoxygenase metabolites of eicosapentaenoic acid

Eicosapentaenoic acid, a major component of fish oil extracts, was recently shown to be metabolized to vicinal diol regioisomers by renal and hepatic cytochrome P-450 epoxygenases. The diol products were also found in the urine of people ingesting fish oil. In the present report, we developed a chemical method of making milligram amounts of the epoxide intermediates and their diol products. Eicosapentaenoic acid was reacted with 0.1 eq. m-chloroperoxybenzoic acid for 15 min. After normal- and reverse-phase high performance liquid chromatography plus capillary gas chromatography and electron impact mass spectrometry, the products were identified as 17,18-cis-epoxy-eicosa-5,8,11,14-tetraenoic, 14,15-cis-epoxy-eicosa-5,8,11,17-tetraenoic, 11,12-cis-epoxy-eicosa-5,8,14,17-tetraenoic, 8,9-cis-epoxy-eicosa-5,11,14,17-tetraenoic and 5,6-cis-epoxy-eicosa-8,11,14,17-tetraenoic acids. The total epoxide yield from eicosapentaenoate was 10% per incubation. By reincubating (cycling) the unused substrate 10-20 times, the total epoxide yield could be increased to 66-88%. Epoxide regioisomers were not produced in equal amounts; epoxygenation was facilitated as the distance between the target double bond and carbomethoxy group increased. Upon hydrolyzing the epoxides, the gas-chromatographic retention times and mass spectra of the diol products were found to match those of biological metabolites. In addition, each of these standards was rapidly and quantitatively synthesized in an amount (milligram) adequate for biological tests.

Novel branched-chain fatty acids in certain fish oils

Methyl-branched fatty acids, which are usually minor components (equal or less than 0.1%) in fish oils, were concentrated in the non-urea-complexing fraction along with polyunsaturated fatty acids during the enrichment of omega-3 fatty acids from certain fish oils via the urea complexation process. The methyl-branched fatty acids in the omega-3 polyunsaturated fatty acid concentrates, which were prepared from three fish body oils, were characterized by gas chromatography and gas chromatography/mass spectrometry. Among the major branched-chain fatty acids expected and identified were the known isoprenoid acids--mainly 4,8,12-trimethyltridecanoic, pristanic, and phytanic--and the well-known iso and anteiso structures. Two novel phytol-derived multimethyl-branched fatty acids, 2,2,6,10,14-pentamethylpentadecanoic and 2,3,7,11,15-pentamethylhexadecanoic, were identified in redfish (Sebastes sp.) oil. These two fatty acids were absent in oils from menhaden (Brevoortia tyrannus) and Pacific salmon (mixed, but mostly from sockeye, Oncorhynchus nerka). The major branched-chain fatty acid in the salmon oil, 7-methyl-7-hexadecenoic acid, was also present to a moderate extent in menhaden oil. A novel vicinal dimethyl-branched fatty acid, 7,8-dimethyl-7-hexadecenoic was detected in all of the fish oils examined, but was most important in the salmon oil. Three monomethyl-branched fatty acids, 11-methyltetradecanoic acid, and 11- and 13-methylhexadecanoic, hitherto undescribed in fish lipids, were also detected in salmon, redfish and menhaden oils.

Amphotericin B-induced damage of Trypanosoma cruzi epimastigotes

Amphotericin B (AmB) autoxidation resulted in oxygen consumption, superoxide anion formation and production of thiobarbituric acid (TBA)-reactive material (malondialdehyde). Malondialdehyde formation increased after incubation of the drug with ascorbate-ADP-FeCl3. Growth of Trypanosoma cruzi epimastigotes in the presence of AmB induced a decrease in the free fatty acid content of the cells (57% in control cells vs. 7% in AmB-treated cells), and in the proportion of unsaturated fatty acids as well as cell killing. No changes were detected on sterol content. No evidence was found for lipid peroxidation as a mechanism of cell injury by this antibiotic.

Studies on steroids. CCXXXVIII. Determination of bile acids in liver tissue by gas chromatography-mass spectrometry with negative ion chemical ionization detection

A method for the determination of bile acids in 2-10 mg of human liver tissue by gas chromatography (GC) in combination with negative ion chemical ionization (NICI) mass spectrometry is described. Unconjugated, glycine- and taurine-conjugated bile acids labelled with 18O and 2H were used as internal standards. The preparation of these compounds was attained by the exchange reaction of the carbonyl group with H218O, followed by metal hydride reduction. Bile acids in solubilized liver tissue were extracted with a Sep-Pak C18 cartridge, separated into the unconjugated, glycine- and taurine-conjugated fractions by ion-exchange chromatography on piperidinohydroxypropyl-Sephadex LH-20 and then derivatized to the pentafluorobenzyl ester-dimethylethylsilyl ethers. Subsequent resolution of each fraction into lithocholate, deoxycholate, chenodeoxycholate, ursodeoxycholate and cholate was attained by GC on a cross-linked 5% phenylmethyl silicone fused-silica capillary column where bile acids were monitored with a characteristic carboxylate anion [M - 181]-in the NICI mode using isobutane as a reagent gas. The newly developed method was applied to the quantitation of bile acids in liver tissue with satisfactory sensitivity and reliability.

Fatty acid profiles of sebaceous triglycerides by capillary gas chromatography with mass-selective detection

Fatty acid methyl esters prepared from the triglyceride fraction of skin surface lipids from six adult human males were chromatographed on a 50-m fused-silica column coated with the highly polar cyanopropylpolysiloxane phase. This permitted the resolution of double-bond positional and geometric isomers. By means of mass-selective detection, 33 saturated and 14 unsaturated fatty acid chain types were analysed. Interpretation of the mass spectra combined with precise calculation of equivalent chain length values permitted the identification of 22 saturated and all of the unsaturated chain types. Quantification by integration of total-ion and selected-ion chromatograms revealed marked variation in the triglyceride fatty acid composition between different subjects. The greatest variation was observed in the concentrations of even-carbon-numbered iso-branched acids, which ranged from 1.5 to 11% of the saturated and from 1.9 to 12.7% of the monounsaturated acids. The anteiso chain structures constituted 4-9% of the saturated and 3-6% of the unsaturated members. Fatty acids with 4-methyl branch showed the least variation, in the range 5.7-7.4%. Other methyl-branched acids made up 4-10% of the saturated group, but were not detected in the unsaturated acids fraction. Two 18:1 fatty acids were identified (a delta 8 and a delta 9), which possibly have different anatomical origins. Similarly, two 18:2 fatty acids (linoleic and a 2,3-dimethyl derivative) were identified. A 2-methyl C17 acid, probably of bacterial origin, was detected.

Modulation of bone metabolism by two chemically distinct lipopolysaccharide fractions from Bacteroides gingivalis

Two separate species of lipopolysaccharide (LPS) from Bacteroides gingivalis 381 have been isolated. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated not only the heterogeneity of each species, but also that they represented high- and low-molecular-weight LPS entities. Although both contained the same carbohydrate and fatty acid components, the proportions of these differed between the LPS species. The direct effects of these two species in modulation of bone resorption and bone collagen and noncollagen protein synthesis have been examined. In a bone resorption assay, these two species stimulated 45Ca release from prelabeled fetal rat bones in a concentration range of 0.5 to 3.0 micrograms/ml. The two LPS species also elicited a 30 to 40% reduction in collagen protein formation at 10 micrograms/ml. Responses of the same order of magnitude were observed with LPS from Salmonella minnesota at 10 micrograms/ml. The higher-molecular-weight LPS species also significantly inhibited noncollagen protein formation. This is the first report that LPS from B. gingivalis 381, a suspected periodontal pathogen, inhibits bone collagen formation and, in conjunction with the bone resorption potency, further implicates LPS in alveolar bone loss associated with periodontal disease.

Differences in membrane unsaturated fatty acids and electron spin resonance in different types of myeloid leukemia cells

The fatty acid composition and some physical properties of intact cells and isolated plasma membranes of two types of mouse myeloid leukemia cell clone grown in culture have been examined. One clone type, MGI+D+, can be induced by the macrophage and granulocyte-inducing protein (MGI) to differentiate into mature macrophages and granulocytes. The other clone type, MGI+D-, could not be induced to differentiate into mature cells. A two-fold increase in the ratio of saturated fatty acid to unsaturated fatty acid was found in the MGI+D- compared to the MGI+D+ clones. The MGI+D- clones produced an unusual polyunsaturated C20:5 fatty acid at 28 degrees C, whereas the MGI+D+ clones did not grow at this temperature. The cells and their isolated plasma membranes were studied by electron spin resonance. The motion of the 5-nitroxide stearate spin label was found to be higher in the intact cells and in the membranes of MGI+D- clones than of the MGI+D+ clones. The cells of MGI+D+ clones showed a similar freedom of motion to normal myeloblasts from the bone marrow. The results indicate that myeloid leukemia cells which differ in their competence to be induced to differentiate into mature cells have different physical properties of their plasma membranes and that this is correlated with their fatty acid acyl chain composition.

Thermal regulation of the fatty acid composition of lipopolysaccharides and phospholipids of Proteus mirabilis

The fatty acid composition of the lipid A moiety of the lipopolysaccharide and phospholipid fractions of Proteus mirabilis changed significantly on varying the growth temperature. A decrease in the growth temperature from 43 degrees C to 15 degrees C resulted in a decrease in the palmitic acid content of the lipopolysaccharide from 19.4% of total fatty acids at 43 degrees C to 1.4% at 15 degrees C, and by the appearance of an unsaturated fatty acid residue, hexadecenoic acid. Changes in the 3-hydroxy-myristic acid content of the lipid A were minimal. The decrease in the growth temperature also resulted in a decrease in the saturated fatty acid content of the phospholipid fraction, which was accompanied by an increase in their fluidity, as measured by the freedom of motion of spin-labeled fatty acids incorporated into dispersions made of the phospholipids. Nevertheless, the fluidity obtained with membrane phospholipids extracted from the cells grown at various temperatures were essentially the same when fluidity was determined at the growth temperature, supporting the hypothesis that variations in the fatty acid composition of membrane phospholipids serve to produce membranes having a constant fluidity at different temperatures of growth.