Synthesis of ethyl arachidonate-19,19,20,20-d4 and ethyl dihomo-gamma-linolenate-19,19,20,20-d4

Asymmetric Synthesis of Methoxylated Ether Lipids: Total Synthesis of n-3 Polyunsaturated Docosahexaenoic Acid-Like Methoxylated Ether Lipid

The first total synthesis of a docosahexaenoic acid (DHA)-like methoxylated ether lipid (MEL) is reported. This compound constitutes an all-cis methylene skipped hexaene framework identical to that present in DHA, the well-known omega-3 polyunsaturated fatty acid. The polyene C22 hydrocarbon chain, bearing a methoxyl group in the 2-position and R-configuration at the resulting chiral center, is attached by an ether linkage to the pro-S hydroxymethyl group (sn-1 position) of a glycerol backbone. The asymmetric synthesis is highly convergent and based on the polyacetylene approach involving iterative copper-promoted coupling reactions of propargyl bromides with terminal alkynes and semihydrogenation of the resulting hexayne. Starting from enantiopure R-solketal and racemic epichlorohydrin, the targeted MEL was accomplished in an 8.2% yield over eight steps (longest linear sequence) involving an enantio- and diastereopure glyceryl glycidyl ether key C6-building blocks from which the polyynes were constructed.

Synthesis and Functional Assessment of a Novel Fatty Acid Probe, ω-Ethynyl Eicosapentaenoic Acid Analog, to Analyze the in Vivo Behavior of Eicosapentaenoic Acid

Eicosapentaenoic acid (EPA) is an ω-3 polyunsaturated fatty acid that plays various beneficial roles in organisms from bacteria to humans. Although its beneficial physiological functions are well-recognized, a molecular probe that enables the monitoring of its in vivo behavior without abolishing its native functions has not yet been developed. Here, we designed and synthesized an ω-ethynyl EPA analog (eEPA) as a tool for analyzing the in vivo behavior and function of EPA. eEPA has an ω-ethynyl group tag in place of the ω-methyl group of EPA. An ethynyl group has a characteristic Raman signal and can be visualized by Raman scattering microscopy. Moreover, this group can specifically react in situ with azide compounds, such as those with fluorescent group, via click chemistry. In this study, we first synthesized eEPA efficiently based on the following well-known strategies. To introduce four C-C double bonds, a coupling reaction between terminal acetylene and propargylic halide or tosylate was employed, and then, by simultaneous and stereoselective partial hydrogenation with P-2 nickel, the triple bonds were converted to cis double bonds. One double bond and an ω-terminal C-C triple bond were introduced by Wittig reaction with a phosphonium salt harboring an ethynyl group. Then, we evaluated the in vivo function of the resulting probe by using an EPA-producing bacterium, Shewanella livingstonensis Ac10. This cold-adapted bacterium inducibly produces EPA at low temperatures, and the EPA-deficient mutant (ΔEPA) shows growth retardation and abnormal morphology at low temperatures. When eEPA was exogenously supplemented to ΔEPA, eEPA was incorporated into the membrane phospholipids as an acyl chain, and the amount of eEPA was about 5% of the total fatty acids in the membrane, which is comparable to the amount of EPA in the membrane of the parent strain. Notably, by supplementation with eEPA, the growth retardation and abnormal morphology of ΔEPA were almost completely suppressed. These results indicated that eEPA mimics EPA well and is useful for analyzing the in vivo behavior of EPA.

The questionable role of a microsomal delta8 acyl-coA-dependent desaturase in the biosynthesis of polyunsaturated fatty acids

Several experimental approaches were used to determine whether rat liver and testes express an acyl-CoA-dependent delta8 desaturase. When [1-14C]5,11,14-eicosatrienoic acid was injected via the tail vein, or directly into testes, it was incorporated into liver and testes phospholipids, but it was not metabolized to other labeled fatty acids. When [1-14C]11,14-eicosadienoic acid was injected, via the tail vein or directly into testes, or incubated with microsomes from both tissues, it was only metabolized to 5,11,14-eicosatrienoic acid. When ethyl 5,5,11,11,14,14-d6-5,11,14-eicosatrienoate was fed to rats maintained on a diet devoid of fat, it primarily replaced esterified 5,8,11-eicosatrienoic acid, but not arachidonic acid. No labeled linoleate or arachidonate were detected. Dietary ethyl linoleate and ethyl 19,19,20,20-d4-1,2-13C-11,14-eicosadienoate were about equally effective as precursors of esterified arachidonate. The doubly labeled 11,14-eicosadienoate was metabolized primarily by conversion to 17,17,18,18-d4-9,12-ocatdecadienoic acid, followed by its conversion to yield esterified arachidonate, with a mass four units greater than endogenous arachidonate. In addition, the doubly labeled substrate gave rise to a small amount of arachidonate, six mass units greater than endogenous arachidonate. No evidence was obtained, with the radiolabeled substrates, for the presence of a delta8 desaturase. However, the presence of an ion, six mass units greater than endogenous arachidonate when doubly labeled 11,14-eicosadienoate was fed, suggests that a small amount of the substrate may have been metabolized by the sequential use of delta8 and delta5 desaturases.

Analysis of the acyl-CoAs that accumulate during the peroxisomal beta-oxidation of arachidonic acid and 6,9,12-octadecatrienoic acid

The biosynthesis of 4,7,10,13,16-22:5 and 4,7,10,13,16,19-22:6 requires that when 6,9,12,15,18-24:5 and 6,9,12,15,18,21-24:6 are produced in microsomes they must move to peroxisomes for partial beta-oxidation. When the 24-carbon acids were incubated with peroxisomes, 22-carbon acids with their first double bond at position 4 accumulated as did those with their first two double bonds at the 2-trans-4-cis-positions (D. L. Luthria, S. B. Mohammed, and H. Sprecher, J. Biol. Chem. 271, 16020-16025, 1996; and B. S. Mohammed, D. L. Luthria, S. P. Baykousheva, and H. Sprecher, Biochem. J., 326, 425-430, 1997). In the study reported here we analyzed the acyl-CoAs that accumulated when peroxisomes were incubated with 5,8,11,14-20:4 and 6,9,12-18:3, a metabolite that would be produced via one cycle of arachidonate degradation via the pathway requiring both NADPH-dependent 2,4-dienoyl-CoA reductase and delta 3,5, delta 2,4-dienoyl-CoA isomerase. With both substrates the acyl-CoAs of 2-trans-4-10:2, 4-10:1, 2-trans-4,7,10-16:4, and 4,7,10-16:3 accumulated. These results further establish that the reductase catalyzes a control step in the peroxisomal degradation of unsaturated fatty acids. It was not possible to detect any 18- or 12-carbon acyl-CoA when arachidonate was the substrate, nor did any 12-carbon catabolite accumulate from 6,9,12-18:3. The fractional amount of 5,8-14:2 and arachidonate catabolized via the pathway using only the enzymes of saturated fatty acid degradation versus the pathway that also uses the reductase and the isomerase could thus not be estimated.

High-resolution nuclear magnetic resonance spectroscopy--applications to fatty acids and triacylglycerols

During the past two decades, nuclear magnetic resonance spectroscopy (NMR) has played an ever-increasing role in the structural determination of fatty acids, fatty acid derivatives and analogues, and in the analysis of the structures of triacylglycerols including the quantitative analysis of lipid mixtures. This article discusses some of the results obtained through the application of the NMR technique to lipid molecules and reviews the literature. To maintain brevity, this article does not cover the underlying theory of NMR spectroscopy as numerous books devoted to modern NMR spectroscopy have been published.

Regulation of the biosynthesis of 4,7,10,13,16-docosapentaenoic acid

It is now established that fatty acid 7,10,13,16-22:4 is metabolized into 4,7,10,13,16-22:5 as follows: 7,10,13,16-22:4-->9,12,15, 18-24:4-->6,9,12,15,18-24:5-->4,7,10,13,16-22:5. Neither C24 fatty acid was esterified to 1-acyl-sn-glycero-3-phosphocholine (1-acyl-GPC) by microsomes, whereas the rates of esterification of 4, 7,10,13,16-22:5, 7,10,13,16-22:4 and 5,8,11,14-20:4 were respectively 135, 18 and 160 nmol/min per mg of microsomal protein. About four times as much acid-soluble radioactivity was produced when peroxisomes were incubated with [3-14C]9,12,15,18-24:4 compared with 6,9,12,15,18-24:5. Only [1-14C]7,10,13,16-22:4 accumulated when [3-14C]9,12,15,18-24:4 was the substrate, but both 4,7,10,13,16-22:5 and 2-trans-4,7,10,13,16-22:6 were produced from [3-14C]6,9,12,15, 18-24:5. When the two C24 fatty acids were incubated with peroxisomes, microsomes and 1-acyl-GPC there was a decrease in the production of acid-soluble radioactivity from [3-14C]6,9,12,15, 18-24:5, but not from [3-14C]9,12,15,18-24:4. The preferential fate of [1-14C]4,7,10,13,16-22:5, when it was produced, was to move out of peroxisomes for esterification into the acceptor, whereas only small amounts of 7,10,13,16-22:4 were esterified. By using 2H-labelled 9,12,15,18-24:4 it was shown that, when 7,10,13,16-22:4 was produced, its primary metabolic fate was degradation to yield esterified arachidonate. Collectively, the results show that an inverse relationship exists between rates of peroxisomal beta-oxidation and of esterification into 1-acyl-GPC by microsomes. Most importantly, when a fatty acid is produced with its first double bond at position 4, it preferentially moves out of peroxisomes for esterification to 1-acyl-GPC by microsomes, rather than being degraded further via a cycle of beta-oxidation that requires NADPH-dependent 2,4-dienoyl-CoA reductase.