Antioxidant and prooxidant effects of alpha-tocopherol in a linoleic acid-copper(II)-ascorbate system

Abstract

The peroxidation of linoleic acid (LA) in the absence and presence of either Cu(II) ions alone or Cu(II)-ascorbate combination was investigated in aerated and incubated emulsions at 37 degrees C and pH 7. LA peroxidation induced by either copper(II) or copper(II)-ascorbic acid system followed pseudo-first order kinetics with respect to primary (hydroperoxides) and secondary (aldehydes- and ketones-like) oxidation products, detected by ferric-thiocyanate and TBARS tests, respectively. -Tocopherol showed both antioxidant and prooxidant effects depending on concentration and also on the simultaneous presence of Cu(II) and ascorbate. Copper(II)-ascorbate combinations generally led to distinct antioxidant behavior at low concentrations of -tocopherol and slight prooxidant behavior at high concentrations of -tocopherol, probably associated with the recycling of tocopherol by ascorbate through reaction with tocopheroxyl radical, while the scavenging effect of -tocopherol on lipid peroxidation was maintained as long as ascorbate was present. On the other hand, in Cu(II) solutions without ascorbate, the antioxidant behavior of tocopherol required higher concentrations of this compound because there was no ascorbate to regenerate it. Practical applications: Linoleic acid (LA) peroxidation induced by either copper(II) or copper(II)-ascorbic acid system followed pseudo-first order kinetics with respect to primary (hydroperoxides) and secondary (e.g., aldehydes and ketones) oxidation products. -Tocopherol showed both antioxidant and prooxidant effects depending on concentration and also on the simultaneous presence of Cu(II) and ascorbate. The findings of this study are believed to be useful to better understand the actual role of -tocopherol in the preservation of heterogenous food samples such as lipid emulsions. Since -tocopherol (vitamin E) is considered to be physiologically the most important lipid-soluble chain-breaking antioxidant of human cell membranes, the results can be extended to in vivo protection of lipid oxidation.