Neutrality of amiodarone on the initiation and propagation of membrane lipid peroxidation

Amiodarone is an iodinated benzofuran derivative largely used as an antiarrhythmic. Owing to the sensitivity of heart tissue to radicals, amiodarone was assayed for putative effects on lipid peroxidation studied in liposomes of soybean phosphatidylcholine and of bovine heart mitochondrial lipids used as model systems. Lipid peroxidations were initiated with Fe2+/ascorbic acid, and with peroxyl radicals generated from the azocompounds, AAPH and AMVN. These assays were carried out by following the quenching of the fluorescent probe cis-parinaric acid and by monitoring oxygen consumption. It has been ascertained that amiodarone does not protect or potentiate significantly the lipid peroxidation both lipidic systems. To fully ascertain the neutral behaviour of amiodarone in the lipid peroxidation process, the degradation of phospholipid acyl chains has been checked by GLC. These data confirm that amiodarone does not protect or potentiate lipid peroxidation to a significant extent. It is concluded that the limited effects of amiodarone might be related only indirectly with the lipid peroxidation. It is possible that the drug causes limited conformational and biophysical alterations in membrane phospholipid bilayers that can affect the process of peroxidation. Therefore, it is concluded that the therapeutic effects and benefits as a heart antiarrhythmic agent are independent of lipid peroxidation processes. Furthermore, the interaction of the drug with lipid bilayers does not induce significant conformational perturbations that could significantly favour or depress the peroxidation process.

Antioxidant activity of 5-aminosalicylic acid against peroxidation of phosphatidylcholine liposomes in the presence of alpha-tocopherol: a synergistic interaction?

Oxidative damage has been implicated in the pathogenesis of inflammatory bowel diseases. 5-Aminosalicylic acid (5-ASA), the anti-inflammatory drug commonly used in the treatment of this condition, has been shown to possess antioxidant properties considered to be of particular importance in the pathologic context of these diseases. However, its action mechanisms are far from being completely elucidated, especially regarding its antioxidant properties in the presence of endogenous antioxidants such as alpha-tocopherol (alpha-T), the major defence system of biomembranes against lipid peroxidation. In this study we investigated the scavenging activity of 5-ASA toward peroxyl radicals generated at different sites of soybean PC liposomes, used as model membranes, either alone or in combination with alpha-T. 5-ASA, separately, shows strong scavenging activity toward peroxyl radicals generated in the aqueous phase by thermal decomposition of 2,2'-azobis(2-amidinopropane hydrochloride) (AAPH), inducing a clear concentration-dependent inhibition period, either of oxygen consumption or of conjugated diene hydroperoxides production. HPLC analysis indicates that 5-ASA is consumed, at a constant rate, throughout the reaction, and when the inhibition period is over, the oxidation rate is resumed. On the other hand, apart from a slight decrease in the rate of oxidation, 5-ASA is unable to suppress efficiently lipid peroxidation, when the reaction starts inside the lipid membranes, by thermal decomposition of 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN). When 5-ASA is combined with alpha-T, and the oxidation starts in the aqueous phase, an additive inhibitory effect occurs between both compounds. 5-ASA protects efficiently alpha-T against initial attack from AAPH-peroxyl radicals, delaying its consumption. On the other hand, if the reaction starts inside the lipid bilayer, 5-ASA prolongs significantly the inhibitory period produced by alpha-T on the initial rate of oxidation, as measured by oxygen consumption and conjugated diene hydroperoxides. This inhibitory effect points to a synergistic interaction between 5-ASA and alpha-T, since 5-ASA, by itself, is unable to suppress the oxidation reaction. Therefore, 5-ASA reveals an important cooperative effect with alpha-T, either affording an efficient protection to this antioxidant compound, when free radicals are generated in the aqueous site, or potentiating its activity when oxidation is initiated inside the lipid bilayer. Taking into account that the ascorbic acid content decreases significantly in the inflamed mucosa of patients with inflammatory bowel diseases, our data are, certainly, a very important contribution to the knowledge of the anti-inflammatory action of 5-ASA.

Antioxidant activities of some extracts of Thymus zygis

The antioxidant activities of methanol and ethyl ether extracts obtained from Thymus zygis, collected during the flowering or non-flowering period, were evaluated and compared. To investigate this potential, extracts were tested on their capacity to react with diphenylpicrylhydrazyl (DPPH) in a homogeneous medium, and to inhibit Fe2+/ascorbate-induced membrane lipid peroxidation, as estimated by the formation of thiobarbituric acid-reactive substances (TBARS). Although methanol extracts reduce DPPH radicals more efficiently than ethyl ether extracts, suggesting a potent radical scavenger activity, the ethyl ether extracts were found to be most active in inhibiting lipid peroxidation in sarcoplasmic reticulum (SR) membranes. In addition, both extracts present peroxyl and superoxide radical scavenging activities. Peroxyl radicals were generated by the water soluble 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) azoinitiator, and the scavenging activities of the extracts were measured by the inhibition of cis-parinaric acid (PnA) fluorescence decay in SR. Superoxide radicals were generated either by an enzymatic or a non-enzymatic system, and the scavenger ability was evaluated by the inhibition of nitroblue tetrazolium reduction. Methanolic extracts are more potent as scavengers of peroxyl and superoxide radicals than the ethyl ether extracts. Apparently, there is a relationship between antioxidant potency and the total phenolic groups content in each extract.

Action of phenolic derivatives (acetaminophen, salicylate, and 5-aminosalicylate) as inhibitors of membrane lipid peroxidation and as peroxyl radical scavengers

The action of the phenolic compounds acetaminophen, salicylate, and 5-aminosalicylate (5-ASA) as inhibitors of lipid peroxidation was studied under conditions suitable for establishing their antioxidant potencies. These phenolic compounds react differently with diphenylpicrylhydrazyl (DPPH) and protect differently sarcoplasmic reticulum membranes against lipid peroxidation induced by Fe2+/ascorbate, as evaluated by the formation of thiobarbituric acid-reactive substances (TBARS) and by the loss of the polyunsaturated fatty acyl chains. 5-Aminosalicylate reacts promptly with DPPH, suggesting a potent radical scavenger activity and was found to be the most active in inhibiting Fe2+/ascorbate-induced lipid peroxidation. These compounds also exhibit peroxyl radical scavenging activity generated by the water-soluble 2,2'-azobis-(2-amidinopropane hydrochloride) azoinitiator of peroxyl radicals, as evidenced by the inhibition of cis-parinaric acid fluorescence decay or oxygen consumption. 5-ASA rapidly scavenges peroxyl radicals in the aqueous phase, producing a concentration-dependent inhibition period similar to Trolox or cysteine, suggesting an antioxidant activity of chain-breaking type. By comparison, the reactivities of acetaminophen and salicylate are significantly weaker, acting essentially as oxidation retardants. Although closely related in structure, the antioxidant efficiencies of the three phenolic compounds are significantly different. The higher antioxidant activity of 5-ASA is putatively related with the p-amine relative to the hydroxyl group, potentially increasing the stability of the phenoxyl radical. Such a stabilization is not possible with salicylate and is decreased in acetaminophen by an electron withdrawing effect of the p-acetyl.

Tamoxifen and hydroxytamoxifen as intramembraneous inhibitors of lipid peroxidation. Evidence for peroxyl radical scavenging activity

Tamoxifen (TAM) is the antiestrogen most widely used in the chemotherapy and chemoprevention of breast cancer. It has been reported that TAM and its more active metabolite 4-hydroxytamoxifen (OHTAM) induce multiple cellular effects, including antioxidant actions. Here sarcoplasmic reticulum membranes (SR) were used as a simple model of oxidation to clarify the antioxidant action type and mechanisms of these anticancer drugs on lipid peroxidation induced by Fe2+/ascorbate and peroxyl radicals generated by the water-soluble 2,2'-azobis(2-amidinopropane)dihydrochloride (AAPH) and by the lipid-soluble 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN). Peroxidation was monitored by different assay systems, namely cis-parinaric acid (PnA) fluorescence quenching, production of thiobarbituric acid-reactive substances, polyunsaturated fatty acids (PUFA) degradation and oxygen consumption. TAM and OHTAM are efficient inhibitors of lipid peroxidation induced by Fe2+/ascorbate and strong intramembraneous scavengers of peroxyl radicals generated either in the water or lipid phases by AAPH and AMVN, respectively. However, these drugs are not typical chain-breaking antioxidant compounds as compared with vitamin E. Additionally, their antioxidant effectiveness enhances the protective capacity of vitamin E against lipid peroxidation induced by AMVN. OHTAM is a more powerful intramembraneous inhibitor of lipid peroxidation as compared with TAM; this effectiveness not correlating with alterations on membrane fluidity may be due to the presence of a hydrogen-donating HO-group in the OHTAM molecule and its preferential location in the outer bilayer regions where it can donate the hydrogen atom to quench free radicals capable of initiating the membrane oxidative degradation. The stronger OHTAM intramembraneous scavenger capacity over TAM also correlates with its higher partition in biomembranes. Therefore, the strong peroxyl radical scavenger activity of OHTAM in the hydrophobic membrane phase may putatively contribute to the mechanisms of cytostatic and chemopreventive action of its promoter TAM on development of breast cancer.

Lipid peroxidation in sarcoplasmic reticulum membranes: effect on functional and biophysical properties

The fluorescent polyunsaturated parinaric acid (PnA) incorporated in sarcoplasmic reticulum membranes (SR) was used to probe the initial stages of membrane lipid peroxidation. The experimental set up of the PnA assay was investigated by means of several peroxidation initiators to ascertain peroxidation conditions. This assay in SR is particularly useful to evaluate the membrane susceptibility to peroxidation and to ascertain suitable conditions (concentration of initiators and cofactors) to challenge peroxidation in each preparation under study. On the basis of the PnA assay, Fe2+/ascorbate was selected among the different initiator systems to assess the effect of lipid peroxidation upon biochemical and biophysical parameters of SR membranes. Under mildly controlled conditions at 25 degrees C, the lipid degradative process, as detected by fatty acid analysis, decreases the Ca2+ uptake (up to about 50% of control) and reduces the Ca2+ pump efficiency (Ca2+/ATP ratio) up to about 58% of control, without inactivation the ATPase enzyme turnover. The effect of lipid peroxidation on the SR bilayer organization is dependent either on the extent of lipid peroxidation or on the depth of the bilayer as probed by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene and by intramolecular excimerization of 1,3-di(1-pyrenyl)propane. It is concluded that the effect of mild lipid peroxidation on Ca2+ pump activity is partially exerted through the alteration of physical properties in the lipid phase or lipid-protein interfaces.