Yasukazu Yoshida

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.

Action of ebselen as an antioxidant against lipid peroxidation

The action of ebselen (2-phenyl-1,2-benzoisoselenazol-3(2H)-one) as an antioxidant was studied under various conditions to clarify how it prevents oxidative damage. It did not react with diphenylpicrylhydrazyl nor did it suppress the oxidation of methyl linoleate in acetonitrile solution or in aqueous dispersions induced by free radical initiator, suggesting that ebselen does not act as a potent radical scavenging antioxidant. On the other hand, it suppressed the oxidation of methyl linoleate emulsions in aqueous dispersions induced by iron. It also suppressed the spontaneous oxidation of rat brain and liver homogenates, but it did not suppress the oxidation of these homogenates induced by a free radical initiator. It was also found that ebselen reduced the fatty acid hydroperoxides to their corresponding alcohols and this reaction was enhanced by the presence of glutathione. These results suggest that ebselen acts as an antioxidant by reducing hydroperoxides, but that it does not act as a radical-scavenging antioxidant.

Oxidation of methyl linoleate in aqueous dispersions induced by copper and iron

The oxidations of methyl linoleate micelles in aqueous dispersions induced by copper and iron have been studied, aiming specifically at elucidating the action of the copper ion in the chain initiation. Sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB) were used as anionic and cationic surfactants, respectively, in order to see the effect of the electric charge of the micelle surface. Both copper and iron induced the oxidations of methyl linoleate micelles by decomposing lipid hydroperoxide contained initially in methyl linoleate, tert-butyl hydroperoxide, or hydrogen peroxide added to the aqueous phase. The rate of oxidation induced by cupric ions was proportional to the first power of methyl linoleate concentration and to the half power of both cupric ion and hydroperoxide concentrations, suggesting that the oxidation was initiated by the peroxyl and alkoxyl radicals formed in the decomposition of hydroperoxide by copper. The formation of alkoxyl radicals was confirmed by its trapping with a spin trap. The rate of oxidation was dependent on the type of surfactant. Methyl linoleate containing a very small amount of hydroperoxide was oxidized by copper in the SDS system, but the rate of its oxidation was negligible when TTAB was used. However, the addition of tert-butyl hydroperoxide induced the oxidation even in the TTAB system. Hydroperoxyl and hydroxyl radicals formed in the SDS system induced the oxidation, but those formed in the TTAB system did not. It was shown that the effect of radicals on the initiation of lipid peroxidation depends on the type of radicals and site of radical formation.

Free radical-mediated oxidation of lipids induced by hemoglobin in aqueous dispersions.

In order to elucidate the effects of hemoglobin on the oxidative damage, the oxidations of soybean phosphatidylcholine (PC) liposomes and low density lipoprotein (LDL) in aqueous dispersions induced by ruptured erythrocytes (hemolysate) and methemoglobin were studied. The rate of oxidation of soybean PC liposomes induced by hemolysate was much faster than that induced by the same concentration of ferric sulfate or cupric chloride and increased with the increase in the amount of hemolysate. When soybean PC was treated beforehand with ebselen, which decomposes hydroperoxides but can not scavenge free radicals, the oxidation did not proceed. Furthermore, the oxidation was suppressed by the addition of pentamethylchromanol in liposomal membrane. The reaction of hemoglobin with methyl linoleate hydroperoxide in aqueous dispersions was also studied. It was found that hemolysate and methemoglobin decomposed the hydroperoxides much faster than cupric chloride and ferric sulfate. The electron spin resonance study strongly suggested that both hemolysate and methemoglobin decomposed hydroperoxides rapidly to form alkoxyl and peroxyl radicals. It was also found out by spectrophotometrical study and by using carbon monoxide and potassium cyanide that hemolysate was oxidized by organic hydroperoxide to form methemoglobin and ferrylhemoglobin. The rate of oxidation of LDL induced by hemolysate was much slower than that of soybean PC liposomes, while LDL was oxidized faster than soybean PC liposomes by cupric chloride under the same conditions. However, in the presence of tert-butyl hydroperoxide, the oxidation of LDL induced by hemolysate proceeded rapidly. These results were interpreted by both the chemical reactivity and the accessibility of metal catalyst to the lipid hydroperoxides in LDL and liposomes. It was suggested that hemoglobin released by hemorrhage might play a crucial role in the initiation of oxidative damage in vivo.

Unregulated Lipid Peroxidation in Neurological Dysfunction

Lipid peroxidation has been the subject of extensive studies focusing on its biochemical mechanisms, dynamics, product analysis, involvement in diseases, inhibition, and biological signaling. Lipid hydroperoxides are formed as major primary products, but they are substrates for various enzymes and they also undergo various secondary reactions. Recently, the biological roles of lipid peroxidation products have received much attention, not only for their pathological mechanisms, but also for their practical clinical applications as biomarkers. Hydroxyoctadecadienoic acid from linoleates, F 2 -isoprostanes from arachidonates, and neuroprostanes from docosahexanoates have been proposed as biomarkers for evaluating oxidative stress in vivo and in oxidative stress-related diseases. Although neurodegenerative disorders have various causes, there is much accumulated evidence to show that oxidative stress is involved in their pathologies. Moreover, the mechanism of reactive oxygen species production differs in each disease. Here, the mechanisms underlying Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis, stroke, and Down syndrome are described.