Effect of alpha-tocopherol on the volatile thermal decomposition products of methyl linoleate hydroperoxides

Tocopherols as antioxidants in lipid-based systems: The combination of chemical and physicochemical interactions determines their efficiency

Lipid oxidation is a major concern in the food, cosmetic, and pharmaceutical sectors. The degradation of unsaturated lipids affects the nutritional, physicochemical, and organoleptic properties of products and can lead to off-flavors and to the formation of potentially harmful oxidation compounds. To prevent or slow down lipid oxidation, different antioxidant additives are used alone or in combination to achieve the best possible efficiency with the minimum possible quantities. In manufactured products, that is, heterogeneous systems containing lipids as emulsions or bulk phase, the efficiency of an antioxidant is determined not only by its chemical reactivity, but also by its physical properties and its interaction with other compounds present in the products. The antioxidants most widely used on the industrial scale are probably tocopherols, either as natural extracts or pure synthetic molecules. Considerable research has been conducted on their antioxidant activity, but results regarding their efficiency are contradictory. Here, we review the known mechanisms behind the antioxidant activity of tocopherols and discuss the chemical and physical features that determine their efficacy. We first describe their chemical reactivity linked with the main factors that modulate it between efficient antioxidant capacity and potential prooxidant effects. We then describe their chemical interactions with other molecules (phenolic compounds, metals, vitamin C, carotenes, proteins, and phospholipids) that have potential additive, synergistic, or antagonist effects. Finally, we discuss other physical parameters that influence their activity in complex systems including their specific interactions with surfactants in emulsions and their behavior in the presence of association colloids in bulk oils.

Thermal conversions of fatty acid peroxides to cyclopentenones: a biomimetic model for allene oxide synthase pathway

The trimethylsilyl (TMS) peroxides of linoleic acid 9(S)-hydroperoxide (TMS or Me esters) were subjected to gas chromatography-mass spectrometry (GC-MS) analyses. The cyclopentenones, trans- and cis-10-oxo-11-phytoenoic acid (10-oxo-PEA, Me or TMS esters) were first time detected as the products of TMS-peroxide thermal conversions. The major products were ketodienes, epoxyalcohols, hemiacetals and decadienals. For further study of thermal cyclopentenone formation, 9(S)- or 13(S)-hydroperoxides of linoleic acid (Me esters) were sealed in ampoules and heated at 230 °C for 15 or 30 min. The products were separated by HPLC. The cyclopentenone fractions were collected and analyzed by GC-MS. Trans-10-oxo-PEA (Me) and 10-oxo-9(13)-PEA (Me) were formed during the thermal conversion of 9-hydroperoxide (Me ester). Similarly, the cyclopentenones trans-12-oxo-PEA (Me) and 12-oxo-9(13)-PEA (Me) were detected after the heating of 13-hydroperoxide (Me ester). Thermal formation of cyclopentenones can be considered as a biomimetic model of AOS pathway, providing new insights into the mechanisms of allene oxide formation and cyclization.

Methodology for measuring malonaldehyde as a product of lipid peroxidation in muscle tissues: A review

The purpose of this review is to summarize concerns regarding the formation and quantification of malonaldehyde as a product of lipid peroxidation in muscle tissues. The spectrophotometric thiobarbituric acid (TBA) method is the most frequently used test for malonaldehyde quantification, especially in muscle tissues, as a marker of lipid peroxidation. However, the TBA method has been criticized as lacking specificity and adequate sensitivity towards malonaldehyde. High performance liquid and gas chromatographic methods offer better specificity and sensitivity for malonaldehyde detection. The TBA method, however, may be preferred over the chromatographic method because of its simplicity, especially when a large number of samples need to be analyzed in a short period of time on a daily basis. In addition, the TBA method has been correlated with other objective and subjective methods of measuring lipid peroxidation and its specificity can be improved with the use of a solid phase extraction C(18) cartridge.

Addition products of alpha-tocopherol with lipid-derived free radicals

The addition products of alpha-tocopherol with lipid-derived free radicals have been reviewed. Free radical scavenging reactions of alpha-tocopherol take place via the alpha-tocopheroxyl radical as an intermediate. If a suitable free radical is present, an addition product can be formed from the coupling of the free radical with the alpha-tocopheroxyl radical. The addition products of alpha-tocopherol with lipid-peroxyl radicals are 8a-(lipid-dioxy)-alpha-tocopherones, which are hydrolyzed to alpha-tocopherylquinone. On the other hand, the carbon-centered radicals of lipids prefer to react with the phenoxyl radical of alpha-tocopherol to form 6-O-lipid-alpha-tocopherol under anaerobic conditions. The addition products of alpha-tocopherol with peroxyl radicals (epoxylinoleoyl-peroxyl radicals) produced from cholesteryl ester and phosphatidylcholine were detected in the peroxidized human plasma using a high-sensitive HPLC procedure with postcolumn reduction and electrochemical detection. Thus, the formation of these addition products provides us with much information on the antioxidant function of vitamin E in biological systems.

Synthesis of dihydroperoxides of linoleic and linolenic acids and studies on their transformation to 4-hydroperoxynonenal

The cytotoxic aldehydes 4-hydroxynonenal, 4-hydroperoxynonenal (4-HPNE), and 4-oxononenal are formed during lipid peroxidation via oxidative transformation of the hydroxy or hydroperoxy precursor fatty acids, respectively. The mechanism of the carbon chain cleavage reaction leading to the aldehyde fragments is not known, but Hock-cleavage of a suitable dihydroperoxide derivative was implicated to account for the fragmentation [Schneider, C., Tallman, K.A., Porter, N.A., and Brash, A.R. (2001) Two Distinct Pathways of Formation of 4-Hydroxynonenal. Mechanisms of Nonenzymatic Transformation of the 9- and 13-Hydroperoxides of Linoleic Acid to 4-Hydroxyalkenals, J. Biol. Chem. 275, 20831-20838]. Both 8,13- and 10,13-dihydroperoxyoctadecadienoic acids (diHPODE) could serve as precursors in a Hock-cleavage leading to 4-HPNE via two different pathways. Here, we synthesized diastereomeric 9,12-, 10,12-, and 10,13-diHPODE using singlet oxidation of linoleic acid. 8,13-Dihydroperoxyoctadecatrienoic acid was synthesized by vitamin E-controlled autoxidation of gamma-linolenic acid followed by reaction with soybean lipoxygenase. The transformation of these potential precursors to 4-HPNE was studied under conditions of autoxidation, hematin-, and acid-catalysis. In contrast to 9- or 13-HPODE, neither of the dihydroperoxides formed 4-HPNE on autoxidation (lipid film, 37 degrees C), regardless of whether the free acid or the methyl ester derivative was used. Acid treatment of 10,13-diHPODE led to the expected formation of 4-HPNE as a significant product, in accord with a Hock-type cleavage reaction. We conclude that, although the suppression of 4-H(P)NE formation from monohydroperoxides by alpha-tocopherol indicates peroxyl radical reactions in the major route of carbon chain cleavage, the dihydroperoxides previously implicated are not intermediates in the autoxidative transformation of monohydroperoxy fatty acids to 4-HPNE and related aldehydes.

Thermal conversions of trimethylsilyl peroxides of linoleic and linolenic acids

The trimethylsilyl (TMS) peroxides/esters of the fatty acid hydroperoxides (9S,10E,12Z)-9-hydroperoxy-10,12-octadecadienoic acid (9-HPOD) and (9Z,11E,13S,15Z)-13-hydroperoxy-9,11,15-octadecatrienoic acid (13-HPOT) were subjected to gas chromatography-mass spectrometry and products formed by thermal rearrangements were identified. The main products were decadienals and the TMS derivatives of 13-oxo-9,11-tridecadienoic acid, epoxyalcohols, hemiacetals, and ketodienes. Oxy radicals as well as epoxyallylic radicals served as intermediates in the formation of these compounds. The thermal TMS peroxide conversions documented provided biomimetic models for enzymatic conversions of fatty acid hydroperoxides and also offered a method to generate an array of oxylipin derivatives of value as reference compounds in GC-MS studies.

Synthesis of Phospholipids Bearing a Conjugated Oxo-polyunsaturated Fatty Acid Residue

2-(15′-Oxo-5’,8’,11’,13′-eicosatetraenoyl)-1-stearoyl- sn-glycerol(3)phosphocholine (APC-CO) 1 and 2 and 2-(13′-oxo-9’,11′-octadecadienoyl)-1-stearoyl- sn-glycero(3)phosphocholine (LPC-CO) 3 are synthesized and an analytical system established for the determination of geometrical isomers at the 13’ position of APC-CO.

Reaction of alpha-tocopherol in heated bulk phase in the presence of methyl linoleate (13S)-hydroperoxide or methyl linoleate

alpha-Tocopherol and methyl (9Z,11E)-(S)-13-hydroperoxy-9,11-octadecadienoate (13-MeLOOH) were allowed to stand at 100 degrees C in bulk phase. The products were isolated and identified as methyl 13-hydroxyoctadecadienoate (1), stereoisomers of methyl 9,11,13-octadecatrienoate (2), methyl 13-oxo-9,11-octadecadienoate (3), epoxy dimers of methyl linoleate with an ether bond (4), a mixture of methyl (E)-12,13-epoxy-9-(alpha-tocopheroxy)-10-octadecenoates and methyl (E)-12,13-epoxy-11-(alpha-tocopheroxy)-9-octadecenoates (5), a mixture of methyl 9-(alpha-tocopheroxy)-10,12-octadecadienoates and methyl 13-(alpha-tocopheroxy)-9,11-octadecadienoates (6), alpha-tocopherol spirodiene dimer (7), and alpha-tocopherol trimer (8). alpha-Tocopherol and 13-MeLOOH were dissolved in methyl myristate, and the thermal decomposition rate and the distributions of reaction products formed from alpha-tocopherol and 13-MeLOOH were analyzed. alpha-Tocopherol disappeared during the first 20 min, and the main products of alpha-tocopherol were 5 and 6 with the accumulation of 1-4 which were the products of 13-MeLOOH. The results indicate that the alkyl and alkoxyl radicals from the thermal decomposition of 13-MeLOOH could be trapped by alpha-tocopherol to produce 5 and 6. The reaction products of alpha-tocopherol during the thermal oxidation of methyl linoleate were compounds 6 and 7. Since the radical flux during the autoxidation might be low, the excess alpha-tocopheroxyl radical reacted with each other to form 7.

The chemistry and antioxidant properties of tocopherols and tocotrienols

This article is a review of the fundamental chemistry of the tocopherols and tocotrienols relevant to their antioxidant action. Despite the general agreement that alpha-tocopherol is the most efficient antioxidant and vitamin E homologue in vivo, there was always a considerable discrepancy in its "absolute" and "relative" antioxidant effectiveness in vitro, especially when compared to gamma-tocopherol. Many chemical, physical, biochemical, physicochemical, and other factors seem responsible for the observed discrepancy between the relative antioxidant potencies of the tocopherols in vivo and in vitro. This paper aims at highlighting some possible reasons for the observed differences between the tocopherols (alpha-, beta-, gamma-, and delta-) in relation to their interactions with the important chemical species involved in lipid peroxidation, specifically trace metal ions, singlet oxygen, nitrogen oxides, and antioxidant synergists. Although literature reports related to the chemistry of the tocotrienols are quite meager, they also were included in the discussion in virtue of their structural and functional resemblance to the tocopherols.

Effect of alpha-tocopherol and Trolox on the decomposition of methyl linoleate hydroperoxides

To clarify the mechanisms of antioxidant action, the effect of alpha-tocopherol and its water-soluble carboxylic acid derivative, Trolox, was studied on the decomposition of methyl linoleate hydroperoxides (MeLoOOH). Decomposition rate and the distribution of autoxidation products formed from MeLoOOH were followed by analyzing the volatile and non-volatile products by static headspace gas chromatography and normal-phase high-performance liquid chromatography, respectively. Both alpha-tocopherol and Trolox markedly inhibited the decomposition of MeLoOOH in a concentration-dependent way. In the absence of antioxidants, MeLoOOH was completely decomposed after incubation for 48 h at 60 degrees C, and in the presence of equal molar concentration of antioxidants only 6-7% of initial MeLoOOH was decomposed even after 280 h of incubation. MeLoOOH produced 1.2% methyl linoleate hydroxy compounds (MeLoOOH) in the presence of alpha-tocopherol and 3.8% in the presence of Trolox. Both antioxidants inhibited the formation of volatile decomposition products and the formation of ketodiene compounds. The hydroxy compounds may be formed by the reaction of alkoxy radical and hydrogen donating antioxidants. Conversion of MeLoOOH into stable MeLoOH demonstrated that the antioxidants alpha-tocopherol and Trolox trap alkoxyl radicals by H-donation.

Malondialdehyde and thiobarbituric acid-reactivity as diagnostic indices of lipid peroxidation and peroxidative tissue injury

Increasing appreciation of the causative role of oxidative injury in many disease states places great importance on the reliable assessment of lipid peroxidation. Malondialdehyde (MDA) is one of several low-molecular-weight end products formed via the decomposition of certain primary and secondary lipid peroxidation products. At low pH and elevated temperature, MDA readily participates in nucleophilic addition reaction with 2-thiobarbituric acid (TBA), generating a red, fluorescent 1:2 MDA:TBA adduct. These facts, along with the availability of facile and sensitive methods to quantify MDA (as the free aldehyde or its TBA derivative), have led to the routine use of MDA determination and, particularly, the "TBA test" to detect and quantify lipid peroxidation in a wide array of sample types. However, MDA itself participates in reactions with molecules other than TBA and is a catabolic substrate. Only certain lipid peroxidation products generate MDA (invariably with low yields), and MDA is neither the sole end product of fatty peroxide formation and decomposition nor a substance generated exclusively through lipid peroxidation. Many factors (e.g., stimulus for and conditions of peroxidation) modulate MDA formation from lipid. Additional factors (e.g., TBA-test reagents and constituents) have profound effects on test response to fatty peroxide-derived MDA. The TBA test is intrinsically nonspecific for MDA; nonlipid-related materials as well as fatty peroxide-derived decomposition products other than MDA are TBA positive. These and other considerations from the extensive literature on MDA. TBA reactivity, and oxidative lipid degradation support the conclusion that MDA determination and the TBA test can offer, at best, a narrow and somewhat empirical window on the complex process of lipid peroxidation. The MDA content and/or TBA reactivity of a system provides no information on the precise structures of the "MDA precursor(s)," their molecular origins, or the amount of each formed. Consequently, neither MDA determination nor TBA-test response can generally be regarded as a diagnostic index of the occurrence/extent of lipid peroxidation, fatty hydroperoxide formation, or oxidative injury to tissue lipid without independent chemical evidence of the analyte being measured and its source. In some cases, MDA/TBA reactivity is an indicator of lipid peroxidation; in other situations, no qualitative or quantitative relationship exists among sample MDA content, TBA reactivity, and fatty peroxide tone. Utilization of MDA analysis and/or the TBA test and interpretation of sample MDA content and TBA test response in studies of lipid peroxidation require caution, discretion, and (especially in biological systems) correlative data from other indices of fatty peroxide formation and decomposition.