Antioxidant/prooxidant effects of alpha-tocopherol, quercetin and isorhamnetin on linoleic acid peroxidation induced by Cu(II) and H2O2

The peroxidation of linoleic acid (LA) in the presence of copper(II) (Cu(II)) ions alone and with alpha-tocopherol (alpha-TocH) was investigated in aerated and incubated emulsions at 37 degrees C and pH 7. Additionally, the effects of quercetin (QR) and its O-methylated derivative, isorhamnetin (IR), as potential antioxidant protectors were studied in the (Cu(II) + TocH)-induced LA peroxidation system. Cu(II)-induced LA peroxidation followed pseudo-first-order kinetics with respect to primary (hydroperoxides) and secondary (aldehydes- and ketones-like) oxidation products, which were determined by ferric thiocyanate and thiobarbituric acid-reactive substances methods, respectively. As opposed to the concentration-dependent (at 0.6 and 10.0 mu M) prooxidative action of alpha-TocH in the absence of QR and IR, the latter two compounds showed antioxidant effect over TocH. The peroxidation of LA in the presence of Cu(II)-H2O2 combination alone and with TocH, QR and IR were also investigated in aerated and incubated emulsions, where the latter three compounds exhibited antioxidant effects

Polar paradox revisited: analogous pairs of hydrophilic and lipophilic antioxidants in linoleic acid emulsion containing Cu(II)

Background Literature data are scarce on the activities of analogous pairs of hydrophilic and lipophilic antioxidants related to the polar paradox' distinguishing antioxidants based on their partitioning between lipids and water. The peroxidation of linoleic acid (LA) in the presence of either Cu(II) ions alone or Cu(II) ions combined with Trolox (TR), ascorbic acid (AA), hydroquinone (HQ) and gallic acid (GA), as hydrophilic antioxidants, or with -tocopherol (TocH), ascorbyl palmitate (AP), tert-butyl hydroquinone (TBHQ) and propyl gallate (PG), as their respective lipophilic analogues, was investigated in aerated and incubated emulsions at 37 degrees C and pH 7. Results LA peroxidation induced by Cu(II) followed pseudo-first-order kinetics with respect to the formation of primary (hydroperoxides) and secondary (aldehyde- and ketone-like) oxidation products, which were determined by ferric thiocyanate (Fe(III)-SCN) and thiobarbituric acid-reactive substances (TBARS) methods respectively. With the exception of TocH at certain concentrations, the tested compounds showed antioxidant behaviour depending on their polarities. The results were evaluated in the light of structure-activity relationships and the polar paradox. Conclusion The results of this study partly confirm the hypothesis that the polar paradox experiences limitations in oil-in-water emulsions and that its validity is also dependent on the concentrations of the antioxidants employed. (c) 2013 Society of Chemical Industr

Protection of ascorbic acid from copper(II)-catalyzed oxidative degradation in the presence of flavonoids: quercetin, catechin and morin

Protection of ascorbic acid (AA) (vitamin C) from Cu(II)-catalyzed autoxidation is an important aspect of antioxidant chemistry. The autoxidation of AA in the absence and presence of Cu(II) ions was investigated in aerated solution at room temperature and I = 0.1 ionic strength (KNO3); the effects of three different flavonoids of similar structure (quercetin, morin and catechin) and their mixtures on the AA system were studied. The concentration of unoxidized AA remaining in solution was measured with the modified cupric ion reducing antioxidant capacity spectrophotometric method. The Cu(II)-catalyzed oxidation at pH 4.5 followed first-order kinetics with respect to AA concentration. Catalytic autoxidation of AA was inhibited to a greater extent by stable quercetin and morin complexes of Cu(II) than by catechin complex. The inhibitive effectiveness order of mixtures gives information about possible synergistic or antagonistic combinations of flavonoid antioxidants, which should be further confirmed with other antioxidant tests

The Protective Effect of Amino Acids on the Copper(II)-Catalyzed Autoxidation of Ascorbic Acid

Protection of ascorbic acid (AA: vitamin C) from Cu(II)-catalyzed autoxidation is an important aspect of antioxidant chemistry. The autoxidation of AA in the absence and presence of Cu(II) ions was investigated in aerated solution at room temperature and I = 0.1 ionic strength (KNO3). Effects of eight biochemically important amino acids (alanine, glycine, aspartic acid, asparagine, glutamic acid, glutamine, phenylalanine, and histidine) on this system were studied. The concentration of unoxidized AA remaining in solution was measured with the CUPRAC (cupric ion reducing antioxidant capacity) spectrophotometric method without interference from the amino acids tested. The autoxidation rate constants of AA decreased with increasing amino acid concentration for fixed Cu, and increased with Cu(II) for fixed amino acid concentrations. Catalytic autoxidation of AA was inhibited by stable Cuamino acid complexes; histidine, having the highest conditional stability constant for its Cu-complex, showed the strongest inhibitive effect. Since amino acids are food and drug compatible compounds, they can be used in commercial products to increase the stability of vitamin C

Antioxidant and prooxidant effects of alpha-tocopherol in a linoleic acid-copper(II)-ascorbate system

The peroxidation of linoleic acid (LA) in the absence and presence of either Cu(II) ions alone or Cu(II)-ascorbate combination was investigated in aerated and incubated emulsions at 37 degrees C and pH 7. LA peroxidation induced by either copper(II) or copper(II)-ascorbic acid system followed pseudo-first order kinetics with respect to primary (hydroperoxides) and secondary (aldehydes- and ketones-like) oxidation products, detected by ferric-thiocyanate and TBARS tests, respectively. -Tocopherol showed both antioxidant and prooxidant effects depending on concentration and also on the simultaneous presence of Cu(II) and ascorbate. Copper(II)-ascorbate combinations generally led to distinct antioxidant behavior at low concentrations of -tocopherol and slight prooxidant behavior at high concentrations of -tocopherol, probably associated with the recycling of tocopherol by ascorbate through reaction with tocopheroxyl radical, while the scavenging effect of -tocopherol on lipid peroxidation was maintained as long as ascorbate was present. On the other hand, in Cu(II) solutions without ascorbate, the antioxidant behavior of tocopherol required higher concentrations of this compound because there was no ascorbate to regenerate it. Practical applications: Linoleic acid (LA) peroxidation induced by either copper(II) or copper(II)-ascorbic acid system followed pseudo-first order kinetics with respect to primary (hydroperoxides) and secondary (e.g., aldehydes and ketones) oxidation products. -Tocopherol showed both antioxidant and prooxidant effects depending on concentration and also on the simultaneous presence of Cu(II) and ascorbate. The findings of this study are believed to be useful to better understand the actual role of -tocopherol in the preservation of heterogenous food samples such as lipid emulsions. Since -tocopherol (vitamin E) is considered to be physiologically the most important lipid-soluble chain-breaking antioxidant of human cell membranes, the results can be extended to in vivo protection of lipid oxidation

Antioxidant protective effect of flavonoids on linoleic acid peroxidation induced by copper(II)/ascorbic acid system

Antioxidants are compounds that can delay or inhibit lipid oxidation. The peroxidation of linoleic acid (LA) in the absence and presence of Cu(II) ion-ascorbate combinations was investigated in aerated and incubated emulsions at 37 degrees C and pH 7. LA peroxidation induced by copper(II)-ascorbic acid system followed first order kinetics with respect to hydroperoxides concentration. The extent of copper-initiated peroxide production in a LA system assayed by ferric thiocyanate method was used to determine possible antioxidant and prooxidant activities of the added flavonoids. The effects of three different flavonoids of similar structure, i.e. quercetin (QR), morin (MR) and catechin (CT), as potential antioxidant protectors were studied in the selected peroxidation system. The inhibitive order of flavonoids in the protection of LA peroxidation was: morin > catechin >= quercetin, i.e. agreeing with that of formal reduction potentials versus NHE at pH 7, i.e. 0.60, 0.57 and 0.33 V for MR, CT, and QR. respectively. Morin showed antioxidant effect at all concentrations whereas catechin and quercetin showed both antioxidant and prooxidant effects depending on their concentrations. The structural requirements for antioxidant activity in flavonoids interestingly coincide with those for Cu(II)-induced prooxidant activity, because as the reducing power of a flavonoid increases, Cu(II)-Cu(I) reduction is facilitated that may end up with the production of reactive species. The findings of this study were evaluated in the light of structure-activity relationships of flavonoids, and the results are believed to be useful to better understand the actual conditions where flavonoids may act as prooxidants in the preservation of heterogeneous food samples containing traces of transition metal ions. (C) 2011 Elsevier Ireland Ltd. All rights reserved