Erika Komuro

Membrane damage due to lipid oxidation.

The influence of free radical-mediated oxidations is amplified because it proceeds by a chain mechanism, ie, only one radical can initiate chain reaction which may propagate over and over again. It was found that the in vitro oxidations of erythrocyte membranes proceed by a chain mechanism with a long kinetic chain length. Thus, the role of chain-breaking antioxidants is quite important, since they scavenge chain-carrying radicals to break a chain reaction. In fact, it has been found experimentally that vitamin E, a lipophilic chain-breaking antioxidant present within the membranes, suppresses the oxidative damage of the membranes more efficiently than water-soluble chain-breaking antioxidants such as vitamin C, which scavenges aqueous radicals but can not scavenge chain-carrying radicals within the membranes.

Antioxidant activities of probucol against lipid peroxidations.

The antioxidant activities of probucol were measured in the oxidations of methyl linoleate in homogeneous solution and soybean phosphatidylcholine liposomal membranes and also of low-density lipoproteins. When an excess amount of probucol was reacted with galvinoxyl, the EPR spectrum of galvinoxyl disappeared and a new triplet EPR signal was found: g = 2.0058 and aH(2H) = 0.14 mT. The identical EPR spectrum was observed when probucol was reacted with tert-butoxyl radical generated from di-tert-butylperoxy oxalate. This EPR signal disappeared rapidly when reacted with either alpha-tocopherol or 6-O-palmitoyl-ascorbic acid. Probucol suppressed the free-radical-mediated oxidations of methyl linoleate in hexane and in acetonitrile, in a dose-dependent manner. Its antioxidant activity was 17.5-fold less than that of alpha-tocopherol in hexane. Probucol incorporated into soybean phosphatidylcholine liposomes suppressed its oxidation. The antioxidant activity of probucol was less than that of alpha-tocopherol, but the difference between the two antioxidant activities was smaller in the membranes than in homogeneous solution. Probucol also suppressed the oxidation of low-density lipoprotein. Interestingly, probucol suppressed the oxidation of LDL as efficiently as alpha-tocopherol, implying that physical factors as well as chemical reactivity are important in determining the overall activity of antioxidant in low-density lipoprotein.

Effects of solvents and media on the antioxidant activity of α-tocopherol

Abstract The effects of solvents and media on the antioxidant activity of α-tocopherol were studied. The antioxidant activities of α-tocopherol in different solvents decreased in the order of acetonitrile = hexane > ethanol = methanol, which indicates that the antioxidant activity of α-tocopherol is smaller in protic solvent than in aprotic solvent. The antioxidant activity of 2-(4,6,12-trimethyltridecyl)-5-hydroxy-2,4,6,7-tetramethylindan, which has similar structure to α-tocopherol but does not have ether oxygen, was also measured in protic and aprotic solvents. Its antioxidant activity was smaller than that of α-tocopherol in every solvent, but interestingly, substantially the same effects were observed. These results show that the hydrogen bonding between the protic solvent and ether oxygen is not important but that the hydrogen bonding between protic solvent and phenolic group reduces the activity of α-tocopherol. Antioxidant activities of α-tocopherol in micelle system and liposomal membrane were markedly reduced compared with that in homogeneous solution. Solvent effect on the α-tocopherolxyl radical was also studied by using electron spin resonance. The hyperfine splitting constants of a H 5CH3 and a H 7CH3 were smaller in protic solvent than in aprotic solvent, which shows that lone-pair orbital energy on 5-CH3 and 7-CH3 is smaller in protic solvent. The ESR spectra of α-tocopheroxyl radical in liposomal membrane and micelle were similar to those observed in aprotic solvent and in protic solvent, respectively, suggesting that α-tocopheroxyl radical is located predominantly in the lipophilic domain of the liposomal membrane but in or closer to water phase of micelle aqueous suspensions.

Inhibition of Peroxidation of Membranes

There is an increasing number of observations that suggest the involvement of oxygen radicals in the deterioration of food and in the development of a variety of forms of tissue damage and pathological events such as heart disease, cancer, and aging. It is accepted that the non-enzymatic, free radical-mediated chain oxidation of the membranes plays a crucial role. Aerobic organisms are protected from such oxygen toxicity by an array of defense systems,6–8 which are divided into two categories: the preventive antioxidants and chain-breaking antioxidants. The preventive antioxidants deactivate the active species and possible precursors of free radicals, thereby suppressing the generation of free radicals and reducing the rate of chain initiation.

Action of 21-aminosteroid U74006F as an antioxidant against lipid peroxidation.

The dynamics of the action of the 21-aminosteroid U74006F as an antioxidant against lipid peroxidation were studied in organic solution and membranes. It was confirmed that the reactivities of this compound toward stable phenoxyl radical and peroxyl radical were quite low. In fact, U74006F did not exert appreciable antioxidant effect against the free radical-driven oxidation of methyl linoleate in acetonitrile solution. However, it suppressed the oxidation of phosphatidylcholine liposomal membranes into which it was incorporated in a concentration-dependent manner. The 21-aminosteroid U74006F did not exert any sparing effect on the rate of alpha-tocopherol consumption in the oxidation of methyl linoleate in solution, but when they were simultaneously incorporated into the membrane, U74006F spared alpha-tocopherol and exerted a synergistic effect against the oxidation of liposomal membranes. This suggests that lipophilic U74006F acts as an antioxidant against lipid peroxidation through a physicochemical and not a pure chemical mechanism, and that a physical interaction with the liposomal membrane may facilitate the inhibition of lipid peroxidation with U74006F.

[3] Sulfhydryl free radical formation enzymatically by sonolysis, by radiolysis, and thermally: Vitamin A, curcumin, muconic acid, and related conjugated olefins as references

Publisher Summary Sulfhydryl (thiyl) free radicals can be generated in a variety of biochemical systems, particularly those containing a peroxidase or myoglobin and hydrogen peroxide. They have long been known to be formed thermally, by photolysis, and by radiolysis. They can also be readily formed by sonolysis in systems, such as thiyl free radicals can rapidly undergo electron transfer and hydrogen transfer reactions with a variety of biological molecules. They can also undergo addition reactions with oxygen and with several conjugated olefinic and aromatic compounds including vitamins A and D, styrene, and several polyaromatic dihydrodiols. This chapter discusses the use of vitamin A, the food additive curcumin, and the benzene metabolite muconic acid as reference models in studies of thiyl free radical reactivity together with some of the methods by which thiyl free radicals are generated in the laboratory. Sonolysis and other studies, with curcumin or β -carotene in particular, provide stimulating, relatively inexpensive, and practical teaching excercises.

Hemolysis of rabbit erythrocytes induced by cigarette smoke

Cigarette smoke has been found to induce the hemolysis of rabbit erythrocytes. The particulate phase had more profound effect than the gas phase. Neither free radical scavengers such as ascorbic acid, uric acid and water-soluble vitamin E analogue nor antioxidant enzymes such as catalase and superoxide dismutase suppressed the cigarette smoke-induced hemolysis, suggesting that free radicals, hydrogen peroxide, and superoxide were not the active species.