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.