Antioxidant and co-antioxidant activity of vitamin C. The effect of vitamin C, either alone or in the presence of vitamin E or a water-soluble vitamin E analogue, upon the peroxidation of aqueous multilamellar phospholipid liposomes

Abstract

Thermally labile azo-initiators, dissolved in either the aqueous or lipid phase, have been used to generate peroxyl radicals at a known, steady rate in an aqueous dispersion of dilinoleoylphosphatidylcholine multilamellar liposomes at 37° C in order to study the antioxidant behaviour of ascorbate itself and ascorbate in combination with either α-tocopherol or a water-soluble α-tocopherol analogue (TROLOX(−)). It is found that ascorbate is an effective inhibitor of peroxidations initiated in the aqueous phase, with each ascorbate terminating 0.6 radical chains (i.e., n = 0.6), but it is a very poor inhibitor of peroxidations initiated in the lipid phase. Peroxidations initiated in the lipid-phase in the presence of either α-tocopherol or TROLOX(−) indicate that ascorbate is an excellent synergist with both phenolic antioxidants ( n = 0.4). In peroxidations initiated in the aqueous phase ascorbate acts as a co-antioxidant with TROLOX(−) ( n = 0.7), but the interpretation of the approximately additive effect obtained in the presence of α-tocopherol is complicated by the fact that under the experimental conditions employed α-tocopherol alone does not give a distinct, measurable inhibition period. The latter problem is shown to be due to a non-uniform distribution of the water-soluble initiator within the liposome. Other examples of the complicating effects of non-uniform distributions of reactants in kinetic studies of the autoxidation of organic substrates dispersed in water are described.