AAPH-induced Oxidation Research Articles

Dichloro-4-quinolinol-3-carboxylic acid: Synthesis and antioxidant abilities to scavenge radicals and to protect methyl linoleate and DNA

5,7-, 5,8-, 6,8-, 7,8-Dichloro-4-quinolinol-3-carboxylic acid (5,7-, 5,8-, 6,8-, 7,8-DCQA) together with 7-chloro-4-quinolinol-3-carboxylic acid (7-CQA) and 4-quinolinol-3-carboxylic acid (QA) were synthesized to investigate the antioxidant properties. 5,7-DCQA exhibited the highest ability to scavenge 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) cationic radical (ABTS +. ), 2,2′-diphenyl-1-picrylhydrazyl (DPPH) and galvinoxyl radicals. 6,8-DCQA possessed the highest efficacy to protect methyl linoleate against 2,2′-azobis(2-amidinopropane)dihydrochloride (AAPH)-induced oxidation. 5,7-, 5,8-DCQA and QA were able to retard the β-carotene-bleaching in β-carotene-linoleic acid emulsion. In addition, 5,8- and 6,8-DCQA efficiently protected DNA against hydroxyl radical (.OH)-mediated oxidation, and 5,8-DCQA and 7-CQA were active to protect DNA against AAPH-induced oxidation. Furthermore, only 7-CQA can protect DNA against Cu 2+/glutathione (GSH)-mediated oxidation. Dichloro-4-quinolinol-3-carboxylic acids were potent to be antiradical drugs, and were worthy to be researched pharmacologically.

Phenolic and Enolic Hydroxyl Groups in Curcumin: Which Plays the Major Role in Scavenging Radicals?

The aim of this work is to clarify the antioxidant abilities of phenolic and enolic hydroxyl groups in curcumin. 1,7-bis(4-benzyloxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione (BEC), 1,7-bis(4-hydroxy-3-methoxyphenyl)heptane-3,5-diol (OHC), 1,7-bis(4-hydroxy-3-methoxyphenyl)heptane-3,5-dione (THC), and 1,7-bis(3,4-dihydroxyphenyl)-1,6-heptadiene-3,5-dione (BDC) are synthesized to determine the antioxidant activities by using antiradical assays against 2,2'-diphenyl-1-picrylhydrazyl (DPPH) radical, galvinoxyl radical, and 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate) cation radical (ABTS*+) and by protecting DNA and erythrocyte against 2,2'-azobis(2-amidinopropane hydrochloride) (AAPH) induced oxidation. The phenolic hydroxyl is the main group for curcumin to trap DPPH, galvinoxyl, and ABTS*+ radicals. The conjugative system between enolic and phenolic hydroxyl groups is beneficial for curcumin to protect erythrocytes against hemin-induced hemolysis and to protect DNA against AAPH-induced oxidation, but is not beneficial for curcumin to protect erythrocytes against AAPH-induced hemolysis. More hydroxyl groups enhance the antioxidant effectiveness of curcumin in the experimental systems employed herein. Therefore, curcumin acts as an antioxidant through the phenolic hydroxyl group.

Captopril and 6-mercaptopurine: Whose SH possesses higher antioxidant ability?

Antioxidant capacities of captopril (CAP), 6-mercaptopurine (6-MP) and 9-(β- d-ribofuranosyl)-6-mercaptopurine (6-MPR) were investigated by interacting them with 2,2′-diphenyl-1-picrylhydrazyl (DPPH), galvinoxyl radical, and 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) cation radical (ABTS + ), and by protecting DNA and erythrocyte against 2,2′-azobis(2-amidinopropane hydrochloride) (AAPH) induced oxidation. It was found that CAP possessed the highest ability to donate the hydrogen atom in –SH to DPPH and galvinoxyl, while 6-MPR had the strongest ability to reduce ABTS + . In the process of protecting DNA and erythrocytes against AAPH-induced oxidation, CAP can trap 0.5 and 1.3 radicals, 6-MP can trap 0.6 and 2.2, and 6-MPR can trap 1.0 and 3.0 radicals, respectively. CAP can also protect erythrocytes against hemin-induced hemolysis.

Indole and its alkyl‐substituted derivatives protect erythrocyte and DNA against radical‐induced oxidation

The antioxidant properties of 1,2,3,4-tetra-hydrocarbazole, 6-methoxy-1,2,3,4-tetrahydrocar-bazole (MTC), 2,3-dimethylindole, 5-methoxy-2,3-dimethylindole, and indole were investigated in the case of hemolysis of human erythrocytes and oxidative damage of DNA induced by 2,2'-azobis(2-amidinopropane hydrochloride) (AAPH), respectively. The aim of this work was to explore the influence of methoxy, methyl, and cyclohexyl substituents on the antioxidant activities of indole derivatives. These indole derivatives were able to protect erythrocytes and DNA in a concentration-dependent manner. The alkyl-substituted indole can protect erythrocytes and DNA against AAPH-induced oxidation. Especially, the structural features of cyclohexyl and methoxy substituents made MTC the best antioxidant among the indole derivatives used herein. Finally, the interaction between these indole derivatives and 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate) radical cation and 2,2'-diphenyl-1-picrylhydrazyl, respectively, provided direct evidence for these indole derivatives to scavenge radicals and emphasized the importance of electron-donating groups for the free radical-scavenging activity of indole derivatives.

Inhibition of free radical-induced erythrocyte hemolysis by 2- O-substituted ascorbic acid derivatives

Inhibitory effects of 2- O-substituted ascorbic acid derivatives, ascorbic acid 2-glucoside (AA-2G), ascorbic acid 2-phosphate (AA-2P), and ascorbic acid 2-sulfate (AA-2S), on 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH)-induced oxidative hemolysis of sheep erythrocytes were studied and were compared with those of ascorbic acid (AA) and other antioxidants. The order of the inhibition efficiency was AA-2S≥Trolox=uric acid≥AA-2P≥AA-2G=AA>glutathione. Although the reactivity of the AA derivatives against AAPH-derived peroxyl radical (ROO ) was much lower than that of AA, the derivatives exerted equal or more potent protective effects on AAPH-induced hemolysis and membrane protein oxidation. In addition, the AA derivatives were found to react per se with ROO , not via AA as an intermediate. These findings suggest that secondary reactions between the AA derivative radical and ROO play a part in hemolysis inhibition. Delayed addition of the AA derivatives after AAPH-induced oxidation of erythrocytes had already proceeded showed weaker inhibition of hemolysis compared to that of AA. These results suggest that the AA derivatives per se act as biologically effective antioxidants under moderate oxidative stress and that AA-2G and AA-2P may be able to act under severe oxidative stress after enzymatic conversion to AA in vivo.

Anti-oxidant and anti-atherogenic properties of liposomal glutathione: Studies in vitro, and in the atherosclerotic apolipoprotein E-deficient mice

Liposomal glutathione, but not the control liposomes (with no glutathione), dose-dependently inhibited copper ion-induced low density lipoprotein (LDL) and HDL oxidation. As peroxidase activity was found to be present in both LDL and HDL, it has contributed to the anti-oxidative effects of liposomal glutathione. In-vitro, no significant effect of liposomal glutathione on J774 A.1 macrophage cell-line oxidative stress and on cellular cholesterol metabolism was observed. In contrast, in the atherosclerotic apolipoprotein E-deficient (E 0) mice, consumption of liposomal glutathione (12.5 or 50 mg/kg/day, for 2 months), but not control liposomes, resulted in a significant reduction in the serum susceptibility to AAPH-induced oxidation by 33%. Liposomal glutathione (50 mg/kg/day) consumption also resulted in an increment (by 12%) in the mice peritoneal macrophages (MPM) glutathione content, paralleled by a significant reduction in total cellular lipid peroxides content (by 40%), compared to placebo-treated mice MPM. MPM paraoxonase 2 activity was significantly increased by 27% and by 121%, after liposomal glutathione consumption (12.5 or 50 mg/kg/day, respectively). Analyses of cellular cholesterol fluxes revealed that, liposomal glutathione (12.5 mg/kg/day) consumption, decreased the extent of oxidized-LDL (Ox-LDL) uptake by 17% and the cellular cholesterol biosynthesis rate, by 34%, and stimulated HDL-induced macrophage cholesterol efflux, by 19%. Most important, a significant reduction in macrophage cholesterol mass (by 24%), and in the atherosclerotic lesion area (by 30%) was noted. We thus conclude that liposomal glutathione possesses anti-oxidative and anti-atherogenic properties towards lipoproteins and macrophages, leading to attenuation of atherosclerosis development.

Antioxidant activity of olive phenols: mechanistic investigation and characterization of oxidation products by mass spectrometry

In this work, the antioxidant activity of olive phenols is first characterized by their stoichiometries n(tot)(number of radicals trapped per antioxidant molecule) and their rate constants for the first H-atom abstraction k(1) by the stable radical DPPH. It appears that oleuropein, hydroxytyrosol and caffeic acid have the largest k(1) values, whereas dihydrocaffeic acid, an intestinal metabolite of caffeic acid, is the best antioxidant in terms of n(tot). For phenols with a catechol moiety n(tot) is higher than two, implying an antioxidant effect of their primarily formed oxidation products. A HPLC-MS analysis of the main products formed in the AAPH-induced oxidation of olive phenols reveals the presence of dimers and trimers. With hydroxytyrosol and dihydrocaffeic acid, oligomerization can take place with the addition of water molecules.The antioxidant activity of olive phenols is then evaluated by their ability to inhibit the AAPH-induced peroxidation of linoleic acid in SDS micelles. It is shown that olive phenols and quercetin act as retardants rather than chain breakers like alpha-tocopherol. From a detailed mechanistic investigation, it appears that the inhibition of lipid peroxidation by olive phenols can be satisfactorily interpreted by assuming that they essentially reduce the AAPH-derived initiating radicals. Overall, olive phenols prove to be efficient scavengers of hydrophilic peroxyl radicals with a long lasting antioxidant effect owing to the residual activity of some of their oxidation products.

Selenoprotein P Protects Low-density Lipoprotein Against Oxidation

Selenoprotein P (SeP) is an extracellular glycoprotein with 8–10 selenocysteines per molecule, containing approximately 50% of total selenium in human serum. An antioxidant function of SeP has been postulated. In the present study, we show that SeP protects low-density lipoproteins (LDL) against oxidation in a cell-free in-vitro system. LDL were isolated from human blood plasma and oxidized with CuCl2, 2,2′-azobis(2-amidinopropane) (AAPH) or peroxynitrite in the presence or absence of SeP, using the formation of conjugated dienes as parameter for lipid peroxidation. SeP delayed the CuCl2- and AAPH-induced LDL oxidation significantly and more efficiently than bovine serum albumin used as control. In contrast, SeP was not capable of inhibiting peroxynitrite-induced LDL oxidation. The protection of LDL against CuCl2- and AAPH-induced oxidation provides evidence for the antioxidant capacity of SeP. Because SeP associates with endothelial membranes, it may act in vivo as a protective factor inhibiting the oxidation of LDL by reactive oxygen species.

The relationship between the antioxidant and the antibacterial properties of galloylated catechins and the structure of phospholipid model membranes.

The effects of four catechins, (+)-catechin (C), (-)-epicatechin (EC), (-)-epicatechin gallate (ECG), and (-)-epigallocatechin gallate (EGCG), on the physical properties of phospholipid model membranes and the correlation to their antioxidant and antibacterial capacities have been studied by using differential scanning calorimetry (DSC), fluorescence spectroscopy, infrared spectroscopy (IR), AAPH-induced oxidation, and leakage experiments. DSC data revealed that galloylated catechins, especially ECG, affected the physical properties of both the phosphatidylcholine (PC) and phosphatidylethanolamine (PE) bilayers dramatically. Galloylated catechins showed higher phospholipid/water partition coefficients than their homologues and were immersed in the phospholipid palisade intercalating within the hydrocarbon chains, ECG being at the deepest position. In contrast, nongalloylated catechins presented a shallow location close to the phospholipid/water interface. ECG also exhibited the highest antioxidant capacity against lipid peroxidation, which correlated with its strong effect on DPH fluorescence anisotropy (as observed by the increase of the lipid order of fluid PC bilayers) and with the presence of highly cooperative transitions as seen by DSC. We propose that the high antioxidant capacity of some galloylated catechins such as ECG could be partially due to the formation of membrane structures showing resistance to detergent solubilization and in which the phospholipids have tightly packed acyl chains and highly hydrated phosphate groups. Significantly, PE was found to be essential to the promotion of carboxyfluorescein leakage from bacterial model membranes by galloylated catechins, indicating that their bactericidal activity, at least at the membrane level, could be due to the specific effect of these catechins on PE.

Antioxidative effect of citrus essential oil components on human low-density lipoprotein in vitro.

We studied the antioxidative action to evaluate the effect of citrus essential oil components on human LDL in vitro. Among the authentic volatile compounds tested, gamma-terpinene showed the strongest antioxidative effect, and inhibited both the Cu(2+)-induced and AAPH-induced oxidation of LDL. gamma-Terpinene added after 30 min (mid-lag phase) and 60 min (propagation phase) of incubation of LDL with Cu(2+) inhibited LDL oxidation.

Protein Protection by Antioxidants: Development of a Convenient Assay and Structure-Activity Relationships of Natural Polyphenols

In this work, a convenient test of antioxidant activity was developed, with BChE-contaminated HSA as the target of AAPH-induced oxidation and its esterase activity as the marker of protein integrity or degradation. The method is relatively simple, of low cost, and convenient to use. Its application to natural polyphenols showed that quercetin (1), verbascoside (2), chlorogenic acid (3), caffeic acid (4), 1,3,6,7-tetrahydroxyxanthone (5), and mangiferin (6), are good antioxidants (IC50<9 μM). 1,5-Dihydroxy-3-methoxyxanthone (7), flemichin D (8), and cordigone (9) showed modest activities (ca. 50 μM<IC50<350 μM), whereas danthrone (10) was inactive. Complementary experiments with two of the more active antioxidants, namely quercetin (1) and chlorogenic acid (3) showed that both antioxidants were better radical scavengers than chain-breaking antioxidants. The relative adiabatic oxidation potential (ΔHox), the relative H-bond dissociation energy (ΔHabs), and the first oxidation potential measured by cyclic voltammetry were found to be related to the radical-scavenging activity of these antioxidants.

Red wine antioxidants bind to human lipoproteins and protect them from metal ion-dependent and -independent oxidation.

Plant-derived polyphenols may exert beneficial effects on atherosclerosis and cardiovascular diseases, in part, because of their antioxidant properties. In this study we compared the effects of unbound (free) and lipoprotein-associated red wine components on in vitro antioxidant protection of human low-density lipoprotein (LDL). Preincubation of LDL (1 mg protein/mL) with 0-2.5% (v/v) red wine for 3 h at 37 degrees C followed by gel filtration to remove unbound red wine components resulted in a dose-dependent, up to 4-fold increase in LDL-associated antioxidant capacity (measured as Trolox equivalents). Similar results were obtained with high-density lipoprotein (HDL) and bovine serum albumin (BSA). Furthermore, LDL was subjected to oxidation by copper and aqueous peroxyl radicals (2,2'-azobis[2-amidinopropane] dihydrochloride, AAPH). Under both types of oxidative stress, LDL-associated and free red wine components significantly decreased oxidation of the lipoprotein's protein moiety (assessed by tryptophan fluorescence) and lipid moiety (assessed by thiobarbituric acid-reactive substances and conjugated dienes). Similar protective effects of red wine components were observed against HDL oxidation. In contrast, red wine exerted a pro-oxidant effect on copper-induced oxidation of BSA tryptophan residues, while protecting them from AAPH-induced oxidation. Ascorbate strongly enhanced the protective effect of red wine against copper-induced LDL oxidation, and had an additive effect against AAPH-induced oxidation. Our data indicate that red wine components bind to LDL and HDL and protect these lipoproteins from metal ion-dependent and -independent protein and lipid oxidation.

The Kinetics of Copper-Induced LDL Oxidation Depend upon Its Lipid Composition and Antioxidant Content

Copper promotes oxidation of human low-density lipoprotein (LDL) through molecular mechanisms that are still under investigation. We employed native human LDL, phospholipid-containing delipidated LDL ghosts, or trilinolein-reconstituted, phospholipid-containing LDL to investigate both LDL oxidation and the associated process of copper reduction. Both LDL ghosts and trilinolein-reconstituted LDL were devoid of antioxidants and were extremely susceptible to AAPH-induced oxidation but, paradoxically, were rather resistant to copper-mediated oxidation. The dynamic reduction of Cu(II) to Cu(I) was quantitatively decreased in LDL ghosts and in trilinolein-reconstituted LDL, also lacking the initial rapid reduction and the subsequent inhibition phases, due to the absence of endogenous antioxidants. Conversely, the rate of copper reduction was linear and likely due to lipid peroxides, either already present or formed during copper-induced oxidation. We suggest that copper undergoes redox transitions in LDL by utilizing reducing equivalents originating from endogenous antioxidants and/or from lipid peroxides in the LDL lipid core.

The protective effect on Cu 2+- and AAPH-mediated oxidation of human low-density lipoproteins depends on glycosaminoglycan structure

The effect of various glycosaminoglycans on Cu 2+- and AAPH-induced oxidation of human low-density lipoprotein (LDL) was studied by monitoring conjugated diene formation. Heparin (Hep) increased the lag phase (t lag) of LDL oxidation, and fast moving and slow moving Hep species modified the kinetics of LDL oxidation to the same extent. Beef spleen heparan sulfate (HS) sample produced a significant increase of the t lag and a decrease of the conjugated diene formation of LDL whilst beef kidney HS species modified LDL oxidation kinetics to a lower extent. Dermatan sulfate (DS) from different sources caused a significant increase of the t lag and a decrease of the conjugated diene formation of LDL. Hyaluronic acid had no effect. Chondroitin sulfate (CS) from beef trachea produced a very strong protective antioxidant effect evaluated by increasing of the t lag and decreasing of the conjugated diene formation. Hep was completely ineffective in protecting LDL from 2,2'-azobis(2-amidinopropane) hydrochloride (AAPH)-mediated oxidation, whilst DS was moderately effective. Beef trachea CS showed a very strong ability to protect LDL oxidation induced by 1 mM AAPH. The different protective effect on Cu 2+- and AAPH-induced LDL oxidation by glycosaminoglycans is discussed considering their various structures and properties, and their capacity to interact to different extents with hydrophobic regions of LDL protein is confirmed by measuring the LDL-tryptophan fluorescence kinetics.

Enhancement of Antioxidative Ability of Rat Plasma by Oral Administration of (−)-Epicatechin

To check whether ingestion of (−)-epicatechin (EC) affects the antioxidative defense in blood plasma, we studied the oxidizability of plasma from Wistar rats after intragastrical EC administration at 10 or 50 mg/rat. The plasma pool obtained from control or EC-administered rats was oxidized with copper sulfate or 2,2′-azobis(2-amidinopropane)dihydrochloride (AAPH). EC metabolites in plasma 1 h after EC administration contained glucuronide and glucuronide-sulfate conjugates in both the free and O-methylated form. After 6 h, the plasma concentration of total EC metabolites decreased and the remaining conjugates were mostly present as the O-methylated form. Compared to the control group, the plasma obtained from rats 1 and 6 h after EC administration was more resistant against copper sulfate-induced oxidation on the basis of cholesteryl ester hydroperoxide (CE-OOH) accumulation. Also, the consumption of α-tocopherol during oxidation was suppressed in the plasma obtained from EC-treated rats. The content of CE-OOH and consumption of α-tocopherol in the plasma from EC-administered animals was much lower than those expected from the amount of nonmetabolized EC present in the plasma. Similar results were obtained from AAPH-induced oxidation of rat plasma after EC administration, except for the fact that CE-OOH accumulation was less suppressed in the plasma 6 h following administration. The O-methylated form was found to be more stable than the free form when EC-administered rat plasma was auto-oxidized at 37°. These results suggest that EC metabolites, particularly conjugates in the free form, possess an effective antioxidative activity in blood plasma.

Protection and Recycling of α-Tocopherol in Human Erythrocytes by Intracellular Ascorbic Acid

Ascorbic acid can recycle α-tocopherol from the tocopheroxyl free radical in lipid bilayers and in micelles, but such recycling has not been demonstrated to occur across cell membranes. In this work the ability of intracellular ascorbate to protect and to recycle α-tocopherol in intact human erythrocytes and erythrocyte ghosts was investigated. In erythrocytes that were 80% depleted of intracellular ascorbate by treatment with the nitroxide Tempol, both 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) and ferricyanide oxidized α-tocopherol to a greater extent than in cells not depleted of ascorbate. In contrast, in erythrocytes in which the intracellular ascorbate concentration had been increased by loading with dehydroascorbate, loss of α-tocopherol was less with both oxidants than in control cells. Protection against AAPH-induced oxidation of α-tocopherol was not prevented by extracellular ascorbate oxidase, indicating that the protection was due to intracellular and not to extracellular ascorbate. Incubation of erythrocytes with lecithin liposomes also generated an oxidant stress, which caused lipid peroxidation in the liposomes and depleted erythrocyte α-tocopherol, leading to hemolysis. Ascorbate loading of the erythrocytes delayed liposome oxidation and decreased loss of α-tocopherol from both cells and from α-tocopherol-loaded liposomes. When erythrocyte ghosts were resealed to contain ascorbate and challenged with free radicals generated by AAPH outside the ghosts, intravesicular ascorbate was totally depleted over 1 h of incubation, whereas α-tocopherol decreased only after ascorbate was substantially oxidized. These results suggest that ascorbate within the erythrocyte protects α-tocopherol in the cell membrane by a direct recycling mechanism.

Effects of methyl 9(or 10)-hydroxy-10(or 9)-mercaptostearate and hexadecanethioic S-acid on cupric ion- or 2,2-azo-bis(2-amidinopropane) dihydrochloride (AAPH)-induced oxidation of low density lipoprotein.

The preventive effects of two antioxidants, methyl 9(or 10)-hydroxy-10 (or 9)-mercaptostearate (SH-S) and hexadecanethioic S-acid (thiopalmitic acid, SH-Pal) against the oxidative modification of low density lipoproteins (LDL) induced by cupric ion or a water soluble initiator of peroxyl radicals, 2,2-azobis(2-amidinopropane) dihydrochloride (AAPH), were studied by measuring thiobarbituric acid-reactive substances (TBARS). SH-S acted as an effective antioxidant in the oxidative modification of LDL induced by either cupric ion or AAPH. Interestingly, SH-S completely inhibited the formation of fluorescence products and decreased both the fluorescence and alpha-tocopherol content in LDL induced by cupric ion, and reduced 1,1-diphenyl 2-picrylhydrazyl (DPPH) used as a stable free radical model. The antioxidative effect was effectively prevented by the addition of increasing amounts of N-ethylmalemide (NEM) to the system. SH-Pal also inhibited the cupric ion-induced LDL oxidation, but showed little inhibitory effect on the AAPH-induced LDL oxidation. Moreover, SH-Pal was reduced to palmitic acid during the AAPH-induced LDL oxidation. These findings indicate that SH-S protects against oxidative damage of LDL in vitro, and that it acts as a free radical in peroxidation. In addition, this study shows that SH-Pal doesn't act as an efficient antioxidant in AAPH-induced lipid peroxidation.

Antioxidant Paradoxes of Phenolic Compounds: Peroxyl Radical Scavenger and Lipid Antioxidant, Etoposide (VP-16), Inhibits Sarcoplasmic Reticulum Ca 2+-ATPase via Thiol Oxidation by Its Phenoxyl Radical

The effectiveness of a phenolic antioxidant as a radical scavenger is determined by its reactivity toward peroxyl radicals and also by the reactivity of the antioxidant phenoxyl radical toward oxidation substrate. If the phenoxyl radical efficiently interacts with vitally important biomolecules, this interaction may result in oxidative damage rather than antioxidant protection, In the present work, we studied effects of phenoxyl radicals generated from a phenolic antitumor drug, Etoposide (VP-16), on oxidation of thiols and activity of Ca 2+-ATPase in sarcoplasmic reticulum (SR) membranes from skeletal muscles. We found that VP-16 is an effective scavenger of peroxyl radicals as judged by its ability to inhibit a water-soluble ate-initiator, 2,2′-azobis(2-amidinopropane)dihydrochloride (AAPH)-induced (i) chemiluminescence (oxidation) of luminol, (ii) fluorescence decay (oxidation) of cis-parinaric acid incorporated in SR membranes, and (iii) peroxidation of SR membrane lipids, VP-16 did not prevent AAPH-induced oxidation of sulfhydryl groups and inhibition of Ca 2+-ATPase in SR membranes, Electron spin resonance measurements showed that AAPH-induced VP-16 phenoxyl radicals were reduced by interaction with SR thiols. By using tyrosinase to generate VP-16 phenoxyl radicals as the only source of free radicals in the model system, we found that inhibition of Ca 2+- ATPase was accompanied by oxidation of about 5 mol of Ca 2+-ATPase SH groups per 1 mol of oxidized VP-16, Secondary products of VP-16 oxidation (including VP-16 o-quinone) were not efficient in inhibiting SR Ca 2+-ATPase. Reduction of VP-16 phenoxyl radicals by ascorbate protected against AAPH- and tyrosinase-induced thiol oxidation and Ca 2+-ATPase inhibition. The results suggest that efficient phenolic scavengers of peroxyl radicals such as VP-16-which are commonly considered as potent antioxidants-may themselves produce oxidative stress due to secondary reactions of their phenoxyl radicals with thiols.

Phenoxyl Radicals of Etoposide (VP-16) Can Directly Oxidize Intracellular Thiols: Protective Versus Damaging Effects of Phenolic Antioxidants

Phenolic compounds can act as radical scavengers due to their ability to donate a mobile hydrogen to peroxyl radicals producing a phenoxyl radical if the phenoxyl radical formed in the radical scavenging reaction efficiently interacts with vitally important biomolecules, then this interaction may result in cytotoxic effects rather than in antioxidant protection. In the present work we have chosen two model compounds--a phenolic antitumor drug, VP-16, known to be highly cytotoxic, and a homolog of vitamin E, 2,2,5,7,8-pentamethyl-6-hydroxychromane (PMC)--as typical representatives of phenoxyl radicals to study interactions of their phenoxyl radicals with intracellular thiols. Using a water-soluble source of peroxyl radicals, the azo-initiator 2,2'-azobis(2-aminodinopropane) (AAPH), we found that both PMC and VP-16 are very efficient scavengers of peroxyl radicals as evidenced by their ability to inhibit AAPH-induced chemiluminescence of luminol and oxidation of PnA incorporated into DOPC liposomes. Both PMC and VP-16 were also able to protect against AAPH-induced oxidative degradation of DNA in nuclei from human leukemic K562 cells. In contrast, there was a dramatic difference in the ability of VP-16 and PMC to protect GSH against AAPH-induced oxidation: while PMC inhibited AAPH-induced oxidation of GSH in a concentration-dependent manner, VP-16 did not protect GSH against oxidation. We hypothesized that this was due to different reactivities of the phenoxyl radicals formed by AAPH-derived peroxyl radicals from VP-16 and PMC toward GSH. To substantiate this hypothesis, we compared interactions of the phenoxyl radicals generated from VP-16 and PMC with intracellular thiols in K562 cell homogenates. While the PMC phenoxyl radicals were only slightly affected by thiols, the VP-16 phenoxyl radicals were reduced by thiols. This is evidenced by (i) a significant inhibition of the tyrosinase-induced VP-16 consumption upon addition of K562 cell homogenates, (ii) a depletion of endogenous thiols in K562 cell homogenates induced by VP-16+tyrosinase, (iii) a transient disappearance of the VP-16 phenoxyl radical signal from the ESR spectra and its reappearance after depletion of endogenous thiols, and (iv) elimination of the lag period for the appearance of the VP-16 phenoxyl radical ESR signal subsequent to depletion of thiols by mersalyl acid. To evaluate the contribution of GSH and protein thiols to reduction of the VP-GSH-peroxidase + cumeme hydroperoxide to specifically deplete endogenous GSH.(ABSTRACT TRUNCATED AT 400 WORDS)

Supplementation with β-carotene in vivo and in vitro does not inhibit low density lipoprotein oxidation

The inhibition of low density lipoprotein (LDL) oxidation has been postulated as one mechanism by which antioxidants may prevent the development of atherosclerosis. Available data on the ability of β-carotene to inhibit LDL oxidation are conflicting. We examined the role of in vivo and in vitro supplementation with β-carotene on metal ion-dependent (cupric ions, Cu 2+) and metal ion-independent (2,2′-azobis[2-amidinopropane]dihydrochloride, AAPH) oxidation of LDL as measured by the formation of conjugated dienes (absorbance at 234 nm). Sixteen subjects were supplemented with 50–100 mg of β-carotene on alternate days for 3 weeks following a week-long loading dose of 100 mg/day. Plasma β-carotene levels rose 5.5-fold, while LDL β-carotene levels rose 8.5-fold. Oxidation of LDL by Cu 2+ or AAPH was not significantly delayed after in vivo supplementation with β-carotene compared with baseline. For AAPH, the lag phase (in minutes) was 75 ± 8 at baseline and 83 ± 14 after supplementation ( P = 0.07). For Cu 2+, the lag phase was 172 ± 41 at baseline and decreased to 130 ± 24 after supplementation ( P < 0.01). Similarly, no protective effect against Cu 2+-induced oxidation was observed when β-carotene was added to LDL in vitro. Supplementation of plasma with β-carotene in vitro prior to LDL isolation also did not enhance LDL's resistance to Cu 2+- or AAPH-induced oxidation, despite a 5-fold increase in LDL β-carotene levels over vehicle control. These data indicate that supplementation with β-carotene in vivo or in vitro does not enhance the protection of LDL against metal ion-dependent and -independent oxidation; rather, in vivo β-carotene supplementation may lead to a shortening of the lag phase of Cu 2+-induced lipid peroxidation in LDL.