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The effects of vitamin K homologues (K1, K2 and K3) on lipid peroxidation of lecithin liposomes induced by ascorbic acid and ferrous ion were examined. Ubiquinone-10 (UQ-10) was used as a reference in evaluation of the effectiveness of these vitamins. The lipid peroxidation was assessed by measurements of thiobarbituric acid-reactive substance (TBARS) and conjugated diene formation during the reaction. Among them, vitamins K1 and K2 inhibited the lipid peroxidation, as did UQ-10, with the order of effectiveness: UQ-10 greater than K2 greater than K1. By contrast, vitamin K3 had no inhibitory effect on ascorbic acid/Fe(2+)-induced lipid peroxidation of the liposomes. The inhibitory effect of vitamins K1 and K2 appeared only when these vitamins were incorporated into the liposomes by sonication. Simple mixing of the liposomes with these vitamins or with UQ-10 did not inhibit peroxidation of the liposomes even at high concentrations. From measurements of nitroblue tetrazolium reduction and p-nitrosodimethylaniline bleaching of vitamin K1- or K2-incorporated liposomes in the presence of ascorbic acid/Fe2+, it was found that these vitamins prevent the formation of hydroxyl radicals, not superoxide anions, during the peroxidation reaction. However, the degree of ascorbic acid/Fe(2+)-induced TBARS formation of the liposomes was not inhibited by the addition of mannitol to the reaction mixture. From these results, it is suggested that the inhibitory effect of these vitamins is mainly involved in termination of radical-chain reaction. Experimental results using several radical scavengers and/or antioxidants supported this interpretation.(ABSTRACT TRUNCATED AT 250 WORDS)

Antioxidant effect of vitamin K homologues on ascorbic acid/Fe2+-induced lipid peroxidation of lecithin liposomes.