Is the Noxious Hemolytic Effect of Copper a Result of Lipoperoxidation?

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

The interaction between alpha-tocopherol and copper ion and its effect on the oxidations of methyl linoleate micelles and soybean phosphatidylcholine liposomes in aqueous dispersions have been studied. alpha-Tocopherol reacted with copper in methanol with a rate constant estimated as 0.56 M-1s-1 at 37 degrees C. Similarly, alpha-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 alpha-tocopheroxyl radical and reduction of cupric ion to cuprous ion were observed. Under these circumstances, alpha-tocopherol acted as a prooxidant rather than antioxidant. This interaction was also observed between endogenous alpha-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 alpha-tocopherol in LDL. It was concluded that alpha-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 alpha-tocopherol may not be important in vivo, where substantially all the copper ion must be sequestered.

Effects of metal chelating agents on the oxidation of lipids induced by copper and iron

The non-enzymatic oxidations of soybean phosphatidylcholine liposomes, methyl linoleate micelles and low-density lipoprotein in aqueous dispersions induced by copper and iron have been studied aiming specifically at elucidating the action of the metal chelating agents such as ethylenediaminetetraacetic acid disodium salt (EDTA), nitrilotriacetic acid (NTA), adenosine-5'-diphosphate disodium salt (ADP), desferrioxamine (DFO), penicillamine (PCM), and triethylene tetramine (TTM). The effects of chelators on chemiluminescence emitted from its probe luminol in the decomposition of tert-butyl hydroperoxide by metal ion were also studied. The effects of chelators on the oxidations depended both on the metal ion and the substrate. Namely, in the oxidations of both liposomes and micelles, EDTA and NTA suppressed the copper-induced oxidations, whereas they enhanced the oxidations induced by iron. ADP had little effect, while PCM and TTM had accelerating effect for both metal ions. On the other hand, in the oxidation of LDL, none of these chelators enhanced the oxidation. Especially, TTM and PCM suppressed the copper-induced oxidation of LDL, suggesting that the chelating agents blocked the access of the metal ion to the hydroperoxide within LDL. The effects of chelators on chemiluminescence emission were similar to those on the oxidations of liposomes and micelles. The cyclic voltammograms of metal complexes were also measured. The multiple effects of chelators on the rate of non-enzymatic, metal-catalyzed oxidations of lipids were interpreted by their influence on redox-potential and accessibility to hydroperoxide of the metal-chelator complex.

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.

Polyunsaturated fatty acid profiles and alpha-tocopherol levels in plasma and whole blood incubated with copper. Evidence of inhibition of lipoperoxidation in plasma by hemolysate

Polyunsaturated fatty acid (PUFA) profiles and alpha-tocopherol levels were studied in human plasma and whole blood incubated with copper under air or nitrogen. In plasma, both PUFAs and alpha-tocopherol disappeared. The results were completely different in whole blood: (i) in plasma, while alpha-tocopherol decreased in the same manner as in plasma incubated alone, profiles of PUFA were only slightly modified. So, in spite of the absence of alpha-tocopherol, lipoperoxidation was not very marked. That is why the release of a protective factor from erythrocytes during hemolysis was under consideration. This was confirmed by the complete inhibition of degradation of PUFAs in plasma when hemolysate was added; (ii) In erythrocytes, no modification in PUFA profiles could be detected while alpha-tocopherol decreased slightly. Thus, not only do erythrocytes resist the copper-dependent oxidative stress in an incredible manner, but they also seem to protect plasma at the time of hemolysis.