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.
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 degrees C in order to study the antioxidant behaviour of ascorbate itself and ascorbate in combination with either alpha-tocopherol or a water-soluble alpha-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 alpha-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 alpha-tocopherol is complicated by the fact that under the experimental conditions employed alpha-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.
D000975:Antioxidants; D001205:Ascorbic Acid; D001578:Benzopyrans; D002839:Chromans; D007700:Kinetics; D008081:Liposomes; D008954:Models, Biological; D010084:Oxidation-Reduction; D010101:Oxygen Consumption; D010713:Phosphatidylcholines; D014810:Vitamin E
D000975:Antioxidants; D001578:Benzopyrans; D002839:Chromans; D008081:Liposomes; D010713:Phosphatidylcholines; D014810:Vitamin E; C002448:1,2-linoleoylphosphatidylcholine; D001205:Ascorbic Acid; C010643:6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid