Rat testis lipoxygenase-like enzyme. Characterization of products from linoleic acid.

The linoleate oxidation products of the affinity chromatography-purified lipoxygenase-like enzyme isolated from rat testes microsomes were characterized. Three types of reaction products separated by thin-layer chromatography were generally present: polar byproducts (A and B) and hydroperoxides. The methyl hydroxystearates obtained from the enzymically produced hydroperoxides were analysed by gas-liquid chromatography and showed a ratio of 67% 13-hydroxy isomer to 33% 9-hydroxy isomer. The major polar byproduct was analysed by infrared spectra, nuclear magnetic resonance and mass spectrometry (of the toluene-p-sulphonyl derivative) and its structure was established as 13-hydroxy-12-oxo-octadec-cis-9-enoic acid. The possibility of the existence of a linoleate hydroperoxide isomerase in the affinity-purified preparation is discussed.     

Characterization of fluorescent products from reaction of methyl linoleate hydroperoxides with adenine in the presence of Fe2+ and ascorbic acid

The structures of fluorescent products formed in the reaction of methyl linoleate hydroperoxides with adenine, FeSO4 and ascorbic acid were investigated to elucidate the mechanism of interaction. The fluorescent products consisted of at least four major components (I-IV), which could be separated by thin-layer chromatography and high-performance liquid chromatography. Both 2-octenal and 2,4-decadienal, degradation products of methyl linoleate hydroperoxides, reacted with adenine to produce a fluorescent product similar to one of the major compounds (II) formed in the reaction of methyl linoleate hydroperoxides. Spectroscopic data suggest that I and III are the same type of compounds, which have closed ring structures with alpha, beta-unsaturated carbonyl groups between the amino group at the 6-position and the nitrogen at the 1-position of adenine. Component II has a closed ring structure at the same site as I and III, and the presence of an ether linkage was suggested. On the basis of these structures, the involvement of 3-nonenal, methyl 12-oxo-9-dodecenoate and 2-octenal was suggested in the interaction of the methyl linoleate hydroperoxides decomposition products and adenine or DNA in the presence of FeSO4 and ascorbic acid.     

Crystallization and positional specificity of hydroperoxidation of Fusarium lipoxygenase.

A lipoxygenase obtained from the fungus Fusarium oxysporum was purified and crystallized. Using the purified enzyme, the positional specificity of linoleate peroxidation was studied. Linoleate hydroperoxides were converted into the corresponding trimethylsilyl derivative by reduction, catalytic hydrogenation and treatment with hexamethyldisilazane/trimethylchlorosilane/pyridine and then analyzed by combined gas-liquid chromatography-mass spectrometry. Fusarium lipoxygenase was found to produce 9- or 13-hydroperoxy-octadecadienoates from linoleate. The ratio of 9- to 13-hydroperoxides produced by the enzyme was also determined by high performance liquid chromatography of their methyl esters. When the enzymic reaction proceeded at pH 9.0 and 12.0, the ratio of 9- to 13-hydroperoxide isomers was 70 : 30 and 56 : 44, respectively. With the use of the heavy isotope of oxygen (18O2), atoms of oxygen introduced into hydroperoxides were found to be derived from the gaseous phase and not from the aqueous phase.     

Thin-layer chromatography blotting for the fluorescence detection of phospholipid hydroperoxides and cholesteryl ester hydroperoxides

A blotting technique was developed to specifically detect lipid hydroperoxides in thin-layer chromatography. Phosphatidylcholine hydroperoxides and cholesteryl linoleate hydroperoxides ranging from 0.1 to 0.5 nmol, which were prepared by reaction with soybean lipoxygenase, were visualized as fluorescent spots on the blotted membrane by immersing the plate into a blotting solvent containing 0.01% (w/v) diphenyl-1-pyrenylphosphine. This technique was applied successfully to monitor lipid peroxidation in human low-density lipoprotein in vitro.     

Water soluble products of UV-irradiated autoxidized linoleic and linolenic acids

The water-soluble products of the UV-initiated autoxidation of linoleic and linolenic acids emulsified in water were separated into volatile and relatively involatile components, each of which reacted with both thiobarbituric acid (TBA) and peroxidase. The volatile TBA-reactive compound is probably malonaldehyde and the volatile peroxidase-reactive compound is hydrogen peroxide. Additional compounds which absorb UV light were present in the volatile fraction. After thin-layer chromatography of the involatile fraction, reactivity toward TBA and peroxidase was found in the same spot. Approximate molar yields of hydrogen peroxide, malonaldehyde, "hydroperoxides", and other TBA-reactive compounds were estimated. The ratio of "hydroperoxide" to TBA reactivity was lower for linoleic than for linolenic acid. The mass of relatively involatile compounds was about 20 times greater than that predicted from either peroxidase or TBA assays of water extracts of oxidized linolenic acid. The properties of the water extract were similar to those shown by others for the products of prolonged autoxidation (without UV-irradiation) of emulsified methyl linoleate.     

Purification to homogeneity and characterization of major fatty acid ethyl ester synthase from human myocardium

Non-oxidative metabolism of ethanol via fatty acid ethyl ester synthase is present in those extrahepatic organs most commonly damaged by alcohol abuse. DEAE-cellulose chromatography of human myocardial cytosol at pH 8.0 separated synthase I, minor and major activities, eluting at conductivities of 5, 7 and 11 mS, respectively. The major synthase was purified 8900-fold to homogeneity by sequential gel permeation, hydrophobic interaction, and anti-human albumin affinity-chromatographies with an overall yield of 25%. SDS-PAGE showed a single polypeptide with a molecular mass of 26 kDa and gel permeation chromatography under nondenaturing conditions indicated a molecular mass of 54 kDa for the active enzyme. The purified enzyme catalyzed ethyl ester synthesis at the highest rates with unsaturated octadecanoic fatty acid substrates (Vmax = 100 and 65 nmol/mg/h for oleate and linoleate, respectively). Km values for oleate, linoleate, arachidonate, palmitate and stearate were 0.22 mM, 0.20 mM, 0.13 mM, 0.18 mM and 0.12 mM, respectively. Thus, human heart fatty acid ethyl ester synthase (major form) is a soluble dimeric enzyme comprised or two identical, or nearly identical, subunits (Mr = 26000).     

High-performance liquid chromatography with mercury cathode electrochemical detection: application to lipid hydroperoxide analysis

Lipid hydroperoxide species can be analyzed with high sensitivity and specificity, using reversed-phase high-performance liquid chromatography with reductive mode electrochemical detection on a mercury drop cathode [HPLC-ED(Hg)]. The purpose of this study was to examine different variables in the operation of HPLC-ED(Hg) and to select optimal conditions for the analysis of several biologically relevant peroxides, including species derived from cholesterol, cholesteryl linoleate, oleate, linoleate, and two synthetic phosphatidylcholines. Parameters such as operating potential and mobile-phase solvent proportions, electrolyte composition, and ionic strength were evaluated for each peroxide class. Under optimal conditions, we have achieved baseline separation of four cholesterol hydroperoxide species, not only from one another, but also from phospholipid hydroperoxides; detection limits were <0.3 pmol and <30 pmol for the cholesterol and phospholipid hydroperoxides, respectively.     

Selective quantification of arachidonic acid hydroperoxides and their hydroxy derivatives in reverse-phase high performance liquid chromatography

For the quantification of lipid hydroperoxides by high performance liquid chromatography (HPLC), it has been necessary to improve the detection system specific to the hydroperoxy group. We first developed a technique which combined detection by uv absorption due to conjugated diene and detection based on electrochemical (EC) reduction in reverse-phase HPLC for the selective determination of arachidonic acid hydroperoxides (hydroperoxyeicosatetraenoic acid, HPETE) and its reduced derivative, hydroxyeicosatetraenoic acid (HETE). 15-HPETE was quantified selectively by EC detection, although both 15-HPETE and 15-HETE were detected by uv absorption and were hardly resolved in the chromatogram. Isomers in HPETE obtained from autoxidized arachidonic acid were partially separated in the chromatogram and seem to have been quantified similarly to 15-HPETE. The application of this analytical system to the analysis of 15-HPETE added in human plasma has demonstrated that the recovery of HPETE extracted from human plasma is much lower than that from normal saline and that HPETE is reduced to HETE by incubation at 37 degrees C. The fact that a high concentration of glutathione accelerated this reduction may indicate that human plasma possesses a glutathione-dependent HPETE-reducing ability as a defense system against excess accumulation of lipid hydroperoxides. Blood plasma effectively suppressed the decomposition of HPETE induced by ferrous ion indicating the presence of factors which prevent the action of ferrous ion on HPETE.     

Inhibition of lipid peroxidation by casein. Evidence of molecular encapsulation of 1,4-pentadiene fatty acids

The capability of cyclodextrins to form molecular inclusion complexes with linoleate appeared in a lipoxygenase-linoleate model reaction as inhibition of oxygenation. The inhibited rates were established instantaneously upon addition of the complexant and maintained until linoleate was exhausted. Total cessation of the reaction was not obtained with cyclodextrins. All these features were reproduced also in casein-inhibited reaction mixtures. Both casein and cyclodextrins protected linoleate also against autoxidation although they did not change free radical generation by xanthine oxidase or Fe2+ reactions. Since neither of the inhibitors affected the enzyme directly, casein may also act by forming linoleate complexes which via a standing equilibrium reduce the oxidizable monomer fatty acids and cause substrate-limited reaction rates. Comparisons at acidic and alkaline pH, in the presence of increasing amounts of the complexants, detergent and hydroperoxides supported this view.     

Oxylipin formation in Nostoc punctiforme (PCC73102)

The dioxygenation of polyunsaturated fatty acids is mainly catalyzed by members of the lipoxygenase enzyme family in flowering plants and mosses. Lipoxygenase products can be metabolized further and are known as signalling substances that play a role in plant development as well as in plant responses to wounding and pathogen attack. Apart from accumulating data in mammals, flowering and non-flowering plants, information on the relevance of lipid peroxide metabolism in prokaryotic organisms is scarce. Thus we aimed to isolate and analyze lipoxygenases and oxylipin patterns from cyanobacterial origin. DNA isolated from Nostoc punctiforme strain PCC73102 yielded sequences for at least two different lipoxygenases. These have been cloned as cDNAs and named NpLOX1 and NpLOX2. Both proteins were identified as linoleate 13-lipoxygenases by expression in E. coli. NpLOX1 was characterized in more detail: It showed a broad pH optimum ranging from pH 4.5 to pH 8.5 with a maximum at pH 8.0 and alpha-linolenic acid was the preferred substrate. Bacterial extracts contain more 13-lipoxygenase-derived hydroperoxides in wounded than in non-wounded cells with a 30-fold excess of non-esterified over esterified oxylipins. 9-Lipoxygenase-derived derivatives were not detectable. 13-Lipoxygenase-derived hydroperoxides in esterified lipids were present at almost equal amounts compared to non-esterified hydroperoxides in non-wounded cells. These results suggest that 13-lipoxygenases acting on free fatty acids dominate in N. punctiforme strain PCC73102 upon wounding.     

Fluorescence formation from the interaction of DNA with lipid oxidation degradation products

To clarify the mechanism of fluorescence formation between DNA and lipid degradation products in the presence of ferric chloride and ascorbic acid, a number of carbonyl compounds and decomposition products of pure methyl linolenate hydroperoxides were examined. Keto derivatives of methyl ricinoleate, linoleate, and oleate, alkanals and 2-alkenals produced little or no fluorescence with DNA in the presence of ferric chloride-ascorbic acid. 2,4-Alkadienals were more active and 2,4,7-decatrienal was the most active. Mixtures of volatile aldehydes prepared from linolenate hydroperoxide decomposed either thermally or with iron and ascorbate had the same activity as 2,4,7-decatrienal. Higher molecular-weight products from the decomposition of methyl linolenate hydroperoxides showed relatively low activity. beta-Carotene, alpha-tocopherol and other antioxidants effectively reduced the amount of fluorescence formed by linolenate hydroperoxides. The results suggest that, in addition to hydroperoxide decomposition products, singlet oxygen and/or free radical species contribute significantly to the fluorescence formed from the interaction of methyl linolenate hydroperoxides with DNA in the presence of ferric chloride and ascorbic acid.     

Ascorbic acid and copper in linoleate oxidation. I. Measurement of oxidation by ultraviolet spectrophotometry and the thiobarbituric acid test

The UV absorption method and the thiobarbituric acid (TBA) test for oxidation of an aqueous suspension of linoleate were compared. The absorption method depends on the formation of hydroperoxides having conjugated double bonds that absorb strongly at 233 nm. The absorption at 233 nm increased markedly during oxidation of linoleate catalyzed by either ascorbic acid or cupric ions. The concentration of ascorbic acid in the reaction mixture was also measured by UV absorption at 265 nm and pH 7.0. Color development in the TBA test also increased markedly with the extent of oxidation of linoleate. When ascorbic acid was the catalyst, UV absorption detected early stages of oxidation with greater sensitivity than the TBA test. The reverse was true when Cu(++) was the catalyst. In general, the relation between the two procedures will depend on whether copper is present when the TBA test is made.     

The specificity of lipoxygenase-catalyzed lipid peroxidation and the effects of radical-scavenging antioxidants

The oxidation of low density lipoprotein (LDL) by lipoxygenase has been implicated in the pathogenesis of atherosclerosis. It has been known that lipoxygenase-mediated lipid peroxidation proceeds in general via regio-, stereo- and enantio-specific mechanisms, but that it is sometimes accompanied by a share of random hydroperoxides as side reaction products. In this study we investigated the oxidation of various substrates (linoleic acid, methyl linoleate, phosphatidylcholine, isolated LDL, and human plasma) by the arachidonate 15-lipoxygenases from rabbit reticulocytes and soybeans aiming at elucidating the effects of substrate, lipoxygenase and reaction milieu on the contribution and mechanism of random oxidation and also the effect of antioxidant. The specific character of the rabbit 15-lipoxygenase reaction was confirmed under all conditions employed here. However, the specificity by soybean lipoxygenase was markedly dependent on the conditions. When phosphatidylcholine liposomes and LDL were oxygenated by soybean lipoxygenase, the product pattern was found to be exclusively regio-, stereo-, and enantio-random. When free linoleic acid was incorporated into PC liposomes and oxidized by soybean lipoxygenase, the free acid was specifically oxygenated, whereas esterified linoleate gave random oxidation products exclusively. Radical-scavenging antioxidants such as alpha-tocopherol, ascorbic acid and 2-carboxy-2,5,7,8-tetramethyl-6-chromanol selectively inhibited the random oxidation but did not influence specific product formation. It is assumed that the random reaction products originate from free radical intermediates, which have escaped the active site of the enzyme and thus may be accessible to radical scavengers. These data indicate that the specificity of lipoxygenase-catalyzed lipid oxidation and the inhibitory effects of antioxidants depend on the physico-chemical state of the substrate and type of lipoxygenase and that they may change completely depending on the conditions.     

Studies on the role of vitamin E in the oxidation of blood components by fatty hydroperoxides.

Studies are reported on the oxidation of vitamin E and changes in lipid and fatty acid composition of rat blood components incubated in vitro with hydroperoxides prepared from autoxidized methyl linoleate. Red blood cells, plasma, serum, and hemoglobin free stroma were incubated at 37 °C with suspensions of linoleate hydroperoxide in Tris buffer at pH 7.4. The RBC were destroyed and substances with excitation-fluorescent properties were produced. Phosphatidylethanolamine, vitamin E and unsaturated fatty acids were oxidized in the reaction. Among the reaction products were substances that gave a positive thiobarbituric acid value, tocoquinone, and an unidentified substance isolated in the nonsaponifiable fraction of the lipid extract of the hemolyzed red cells. The reaction of linoleate hydroperoxide with stroma was similar to that with red blood cells and the same products were observed. In contrast there was little reaction of linoleate hydroperoxide with vitamin E or lipids of the serum or plasma in the absence of red blood cells. The destruction of the red blood cells appeared to be closely related to the oxidation of vitamin E indicating that the strong antioxygenic action of vitamin E in vivo was due to its particular form or structural orientation in the red cell membrane.     

Determination of thiobarbituric acid-reactive substances in oxidized lipids by high-performance liquid chromatography with a postcolumn reaction system

A new high-performance liquid chromatography procedure with a postcolumn reaction system for determination of free malondialdehyde (MDA) and other thiobarbituric acid-reactive substances (TBA-RS) in oxidized lipids in vitro has been developed. Using this procedure, both thermally oxidized methyl linoleate and the degradation products of methyl linoleate hydroperoxides revealed many kinds of lipophilic TBA-RS, but no free MDA was detected on the high-performance liquid chromatography. Similarly, oxidized methyl arachidonate also produced certain kinds of TBA-RS in the lipophilic phase and a small amount of free MDA in the hydrophilic phase. These results indicate that lipophilic TBA-RS produced in oxidized lipids in vitro are major TBA-RS and that the production of free MDA is small, even though the degree of lipid oxidation has previously been estimated as an MDA equivalent measured by the TBA colorimetric test.     

Regioisomeric distribution of cholesteryl linoleate hydroperoxides and hydroxides in plasma from healthy humans provides evidence for free radical-mediated lipid peroxidation in vivo

We have previously reported the detection of cholesteryl ester hydroperoxides, consisting mainly of cholesteryl linoleate hydroperoxides (Ch18:2-OOH), at nm levels in plasma from healthy humans (Y. Yamamoto and E. Niki, 1989. Biochem. Biophys. Res. Commun. 165: 988-993). To elucidate their production mechanism in vivo, we examined the distribution of Ch18:2-O(O)H regioisomers in blood plasma from nine healthy young subjects using a sequential method consisting of methanol/hexane extraction in the presence of antioxidant, reductant, and internal standard, solid phase extraction to remove unoxidized cholesteryl linoleate, purification by reversed-phase high-performance liquid chromatography (HPLC), and detection by normal phase HPLC. Furthermore, we confirm that little artifactual oxidation of cholesteryl linoleate occurred during analytical procedures indicated by the absence of oxidation products of cholesteryl 11Z,14Z-eicosadienoate (Ch20:2) when provided as an exogenous substrate to the experimental procedure. We detected nm levels of all free radical-mediated oxidation products, 13ZE-, 13EE-, 9-EZ-, and 9-EE-forms of Ch18:2-O(O)H, in blood plasma, whereas the 13ZE-isomer resulting from enzymatic 15-lipoxygenase oxidation was not evident as a major product. These results indicate that free radical chain oxidation of lipids occurs even in healthy young individuals.     

Chlorophyll-sensitized Photooxidation Products of Spinach Galactolipids

Spinach monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) were oxidized with singlet molecular oxygen by the use of chlorophyll a as the photosensitizer. The oxidation products were separated from the unoxidized MGDG and DGDG by reverse-phase high performance liquid chromatography (HPLC). The products separated by HPLC were identified to be mono- and di-hydroperoxides formed by ¹O₂ oxidation of the 16:3 or 18:3 component of MGDG and DGDG. Each unsaturated fatty acid moiety in the MGDG and DGDG produced isomeric hydroperoxides in a manner similar to the corresponding fatty acid methyl ester.     

Metabolism of linoleic and arachidonic acids in VX2 carcinoma tissue: Identification of monohydroxy octadecadienoic acids and monohydroxy eicosatetraenoic acids

The metabolism of arachidonic and linoleic acids by VX₂ carcinoma tissue was determined. Prostaglandin E₂ was the major metabolic product of arachidonic acid in the neoplastic tissue. Minor products accounting for 3– 8% of arachidonic acid metabolism were 11-hydroxy-5, 8, 12, 14-eicosatetraenoic acid (11-HETE) and 15-hydroxy-5, 8, 11, 13-eicosatetraenoic acid (15-HETE). Linoleic acid was converted to a mixture of 9-hydroxy-10, 12-octadecadienoic acid (9-HODD) and 13-hydroxy-9, 11-octadecadienoic acid (13-HODD). The conversion of linoleic acid to monohydroxy C-18 fatty acids varied from 40–80% 9-HODD and 20–60% 13-HODD in tumor tissue harvested from different animals. The quantity of monohydroxy C-18 fatty acids biosynthesized by VX₂ carcinoma tissue from endogenous linoleic acid equals or exceeds that of prostaglandin E₂ biosynthesis from endogenous arachidonic acid. The presence of a hydroxyl group adjacent to a conjugated diene suggest that the monohydroxy C-18 and monohydroxy C-20 fatty acids were formed via the action of lipoxygenase-like enzymes. These lipoxygenase-like reactions are inhibited by indomethacin in a concentration-dependent fashion similar to the inhibition of prostaglandin E₂ biosynthesis. The enzymes catalyzing the lipoxygenase-like reactions of linoleic and arachidonic acids are localized in the microsomal fraction of VX₂ carcinoma tissue. These data suggest that the lipoxygenase-like reactions are catalyzed by fatty acid cyclooxygenase and that there are two major pathways of fatty acid cyclooxygenase metabolism of polyenoic fatty acids in the neoplastic tissue. One pathway involves the formation of prostaglandin E₂ via cyclic endoperoxy intermediates. The second pathway involves the formation of monohydroxy C-18 fatty acids from linoleic acid via lipoxygenase-like reactions.     

Purification and properties of a glucoamylase fraction from the culture filtrate of Rhizopus nodosus

A glucoamylase was isolated from the culture filtrate of Rhizopus nodosus and was separated from the acid lipase by DEAE-cellulose chromatography at pH 8.0 It was purified by Concanavalin A Sepharose 4B affinity chromatography followed by CM-Sephadex chromatography 387 fold with 30.7% yield. The homogeneity of the enzyme were confirmed by polyacrylamide gel electrophoresis and immunological studies. The different physico-chemical properties of the enzyme were studied. The molecular weight of the enzyme was found to be 71,000. Ethylenediaminetetraacetic acid had no effect on the enzyme whereas Hg2+ partially inhibited the enzyme activity. Tryptophan residues were found to be essential for the enzyme activity.     

Purification, product characterization and kinetic properties of lipoxygenase from olive fruit (Olea europaea L.).

Lipoxygenase from olive fruit was purified to homogeneity for the first time after differential centrifugations and by hydrophobic chromatography. The enzyme had a molecular mass of 98 kDa and exhibited a maximal activity at pH 6. Lipoxygenase had a better affinity for linoleic acid (Km=82.44 microM) than for linolenic acid (Km = 306.26 microM). It is inhibited by linoleate:oxygen oxidoreductase (LOX) inhibitors like nordihydroguaiaretic acid (NDGA) or propyl gallate. The reaction product was 13-hydroperoxy octadecadienoic acid when linoleic acid was used as substrate.