Antioxidant activity of reduced menadione in solvent solution and in model membranes

The antioxidant activity of reduced menadione was investigated and compared with that of alpha-tocopherol both in solvent solution and in large unilamellar vesicles by using azocompounds as free radical generators. The results show that: i) reduced menadione behaves as a chain-breaking antioxidant; ii) its inhibition rate constant is similar to that of alpha-tocopherol in homogeneous solution, whereas it is 4 times larger in egg yolk lecithin vesicles; iii) the stoichiometric factor is found lower than 1 in both systems, since a substantial portion of menadiol is consumed by autoxidation and does not contribute to radical trapping; iv) when both alpha-tocopherol and menadiol are present in vesicles, reduced menadione can spare alpha-tocopherol. Data presented here suggest that the reduced form of vitamin K may protect, when present, cellular membranes from free radical damage.