Jyunichi Tsuchiya

Interaction of α-tocopherol with copper and its effect on lipid peroxidation

Abstract The interaction between α-tocopherol and copper ion and its effect on the oxidations of methyl linoleate micelles and soybean phosphatidylcholine liposomes in aqueous dispersions have been studied. α-Tocopherol reacted with copper in methanol with a rate constant estimated as 0.56 M −1 s −1 at 35°C. Similarly, α-tocopherol incorporated into methyl linoleate and ethyl palmitate micelles and also phosphatidylcholine liposomal membranes interacted with copper at roughly the similar rate. In every case, the formation of α-tocopheroxyl radical and reduction of cupric ion to cuprous ion were observed. Under these circumstances, α-tocopherol acted as a prooxidant rather than antioxidant. This interaction was also observed between endogenous α-tocopherol in human low density lipoprotein and copper, and the rate was estimated to be higher than that in methanol, implying the facile interaction of the two at LDL surface. However, copper incorporated in ceruloplasmin or chelated with albumin did not interact with endogenous α-tocopherol in LDL. It was concluded that α-tocopherol reacts with free copper(II) ion to give more reactive copper(I) ion and may act as a prooxidant for lipid peroxidation in the presence of free copper ion. However, such a prooxidant effect of α-tocopherol may not be important in vivo, where substantially all the copper ion must be sequestered.

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.

Inhibition of oxidation of low density lipoprotein by troglitazone

The effect of a new oral hypoglycemic agent troglitazone, (+/-)-5-[4-(6-hydroxy-2,5,7,8-tetramethylchroman-2-yl-methoxy)benz yl]-2,4-thiazolidinedione as an antioxidant against the free radical-mediated oxidation of low density lipoprotein (LDL) was studied. The oxidation of LDL gives cholesteryl ester hydroperoxide and phosphatidylcholine hydroperoxide as major primary products. Troglitazone incorporated exogenously into LDL inhibited the oxidations of LDL induced by either aqueous or lipophilic peroxyl radicals and suppressed the formation of lipid hydroperoxides efficiently. Ascorbic acid added into the aqueous phase spared both endogenous alpha-tocopherol and troglitazone in LDL. It was also found by absorption spectroscopic and electron spin resonance (ESR) studies that troglitazone reacted rapidly with a galvinoxyl radical to give a chromanoxyl radical which gives the same ESR spectrum as alpha-tocopherol. This ESR spectrum disappeared rapidly when ascorbic acid was added into the system. These results show that troglitazone acts as a potent antioxidant and protects LDL from oxidative modification.

Inhibition of Oxidation of Low Density Lipoprotein by Vitamin E and Related Compounds

The oxidation of low density lipoprotein (LDL) was carried out aiming specifically at elucidating the anti-oxidant action of alpha-tocopherol. Lipophilic and hydrophilic azo compounds and copper induced the oxidation of LDL similarly to give cholesterol ester and phosphatidylcholine hydroperoxides as major products. The antioxidant potency of alpha-tocopherol in LDL was much poorer than in homogeneous solution. Doxyl stearic acids were used as spin probe and incorporated in LDL. The rate of reduction of doxyl nitroxide in LDL by ascorbate decreased with increasing distance from the LDL surface. From the competition between the spin probe and alpha-tocopherol in scavenging radical, it was found that the efficacy of radical scavenging by alpha-tocopherol became smaller as the radical went deeper into the interior of LDL. On the other hand, 2,2,5,7,8-pentamethyl-6-chromal spared the spin label regardless of the position of nitroxide. The antioxidant activity of chromanols against LDL oxidation increased with decreasing length of isoprenoid side chain at the 2-position. All these results were interpreted by location and low mobility of alpha-tocopherol in LDL. The tocopherol mediated propagation was observed notably at low rate of radical flux, but this was suppressed by reductant such as ascorbic acid and ubiquinol.

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.

Oxidation of lipids. XI. Spin trapping and identification of peroxy and alkoxy radicals of methyl linoleate

Abstract Spin trapping of peroxy and alkoxy radicals generated from the hydroperoxide of methyl linoleate was studied using methyl- N -duryl nitrone (MDN) and phenyl- N -tert-butyl nitrone (PBN) as spin traps. The conjugated dienyl carbon radical was also generated from methyl linoleate and spin-trapped. The spin adducts of peroxy, alkoxy, and dienyl carbon radicals were observed by ESR and their hyperfine splitting constants were determined. The spin adducts of peroxy and alkoxy radicals could be distinguished clearly with MDN.

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.