Synthesis of mixed-acid phosphatidylcholines and high pressure liquid chromatographic analysis of isomeric lysophosphatidylcholines

A new method for the synthesis of mixed-chain phosphatidylcholines is reported. Silver ion catalyzed acylation of lysophosphatidylcholines by 2-thiopyridyl esters occurs rapidly (10 min) at room temperature in organic solvents. Yields of isomerically pure mixed-chain phosphatidylcholines (greater than 98% isomeric purity) are generally greater than 80%. The reaction proceeds with only 1.5- to 2-fold excess of thiopyridyl ester, thus offering some advantages over existing procedures when precious acylating agents are used. The major disadvantage of the procedure is its sensitivity to water. Phosphatidylcholines having hydroxy fatty acyl groups are prepared by protection of the hydroxyl as the levulinate ester, conversion to the 2-thiopyridyl ester, acylation, and removal of the levulinate with hydrazine. For purification of lysophosphatidylcholines, a reverse-phase high pressure liquid chromatographic method for separation of 1-acylglycerophosphocholines from 2-acylglycerophosphocholines was developed.

Oxidation of arachidonic acid in micelles by superoxide and hydrogen peroxide

Arachidonic acid was co-oxidized by xanthine oxidase. Both superoxide radical and hydrogen peroxide were required for oxidation, as shown by essentially complete inhibition caused by superoxide dismutase or by catalase. Pure arachidonate, free of lipid hydroperoxides, was susceptible to this co-oxidation, and the presence of lipid hydroperoxides did not accelerate the process. The role of trace metals was indicated by the stimulatory effect of EDTA-Fe and by the inhibitory effect of diethylenetriamine pentaacetate. Initiation of arachidonate co-oxidation was due to a potent oxidant generated by the interaction of H2O2 and O2- in the presence of Fe, rather than to either O2- or H2O2 per se. Hence, mannitol, a scavenger of OH ., but not of O2- or H2O2, also inhibited oxidation. Arachidonic acid autoxidation, a much slower process than xanthine oxidase co-oxidation, was barely detectable on the time scale of these observations. Unlike the co-oxidation, autoxidation was autocatalytic and therefore accelerated by hydroperoxide products. Marked quantitative differences in the distribution of isomeric hydroperoxide products of enzymic co-oxidation, as compared to the autoxidation, were noted and their significance was discussed.

Arachidonate metabolism via lipoxygenase and 12L-hydroperoxy-5,8,10,14-icosatetraenoic acid peroxidase sensitive to anti-inflammatory drugs

The enzymes of arachidonate metabolism via the lipoxygenase pathway in human platelet cytosol have been characterized and partially purified. The lipoxygenase activity has a pH optimum of 7.3 and reaches half-maximal activity at an arachidonate concentration of 80 microM. The oxidation of arachidonate by these enzymes is inhibited by reagents that modify sulfhydryl groups. Two separable lipoxygenase activities can be detected by chromatography of platelet cytosol on Sephadex G-150 and of partially purified preparations on DEAE-Sephadex. One of these has an apparent Mr of 100,000. A second enzyme species behaves as a Mr 160,000 entity containing, in addition to lipoxygenase, a peroxidase activity that catalyzes the conversion of 12L-hydroperoxy-5,8,10,14-icosatetraenoic acid (HPETE) to 12L-hydroxy-5,8,10,14-icosatetraenoic acid (HETE). Aspirin, indomethacin, sodium salicylate, phenylbutazone, ibuprofen, naproxen, and sulindac, but not acetaminophen or phenacetin, give rise to increased levels of HPETE in the lipoxygenase pathway. This increase in HPETE levels is the result of the ability of these drugs to inhibit directly the enzymatic conversion of HPETE to HETE.

Monocyclic peroxides as inhibitors of arachidonic acid and prostaglandin endoperoxide analog initiated aggregation of human platelets

Arachidonic acid initiates the irreveresible aggregation of human platelets on conversion to the bicyclic prostaglandin endoperoxides, PGG2 and PGH2. An enzyme in arterial walls catalyzes the conversion of PGG2 and PGH2 to PGX, which inhibits human platelet aggregation. Preincubation with monocyclic peroxides (3-(alpha-hydroxyethyl)-1,2-dioxane, 3-(alpha-hydroxypropyl)-1,2-dioxolane or 3-methyl-3-(hydroxymethyl)-1,2-dioxolane) completely inhibited arachidonic acid initiated aggregation. Similarly, two analogs of PGH2, (15S)-hydroxy-9 alpha, 11 alpha-(epoxymethano)prosta-5Z, 13E-dienoic and (15S)-hydroxy-11 alpha, 9 alpha-(epoxymethano)prosta-5Z, 13E-dienoic acids, initiated irreversible aggregation of platelets. but were completely blocked by the monocyclic peroxides. Aggregation initiated by ADP or epinephrine was also completely inhibited by the cyclic peroxides. Aggregation of human platelets appears initiated through an endoperoxide receptor which can combine with either the natural bicyclic prostaglandin peroxides or the synthetic monocyclic peroxides. Natural inhibitors, such as PGX, may well be monocyclic endoperoxides similar to the compounds studied here.

Cyclic peroxides and the thiobarbituric assay

Several monocyclic peroxide compounds and two acyclic hydroperoxides have been tested for activity in the thiobarbituric acid test. All cyclic peroxides tested which have beta dioxygen functionality on non-tertiary carbons gave positive thiobarbituric acid tests at 532 nm. (epsilon = 10(3) --10(4)) Two acyclic unsaturated hydroperoxides which were derived from gamma-linolenic acid also gave positive thiobarbituric acid tests. In addition to the 532-nm-absorbing species, all peroxidic compounds tested showed a transient absorption at 450 nm during the thiobarbituric acid test. The species responsible for this 450 nm absorption appears to be an intermediate in a series reaction sequence. This intermediate is converted, in time, to the 532-nm-absorbing species. Thiobarbituric acid assay of the crude autoxidation product mixture of gamma-linolenic acid also shows this transient 450-nm-absorbing species. Added ferric ion enhances the 532 nm abosrbance of the thiobarbituric acid assay of cyclic peroxides.

Preparation and purification of lipid hydroperoxides from arachidonic and gamma-linolenic acids

Commercial soybean lipoxygenase may be used under carefully controlled reaction conditions to give high yields of lipid hydroperoxides. Lipid hydroperoxides so derived from gamma-linolenic or arachidonic acid may be purified by high pressure liquid chromatography. Thus, commercial lipoxygenase serves as a viable source for 100 mg quantities of lipid hydroperoxides.