THE POTENT ANTIOXIDANT ACTIVITY OF THE VITAMIN-K CYCLE IN MICROSOMAL LIPID-PEROXIDATION

In the vitamin K cycle, vitamin K-hydroquinone, the active cofactor fo r gamma-glutamylcarboxylase, is continuously regenerated. The successi ve pathways contain oxidation of the hydroquinone to the epoxide, foll owed by reduction to the quinone and reduction to the hydroquinone. Vi tamin K-hydroquinone is a potent radical scavenging species (Mukai et ed., I Biol Chem 267: 22277-22281, 1992). We tested the potential anti oxidant activity of the vitamin K cycle in lipid peroxidation reaction s (thiobarbituric acid reactive substances, TEARS) in rat liver micros omes. As prooxidant we used Fe2+/ascorbate, NADPH-Fe3+/ATP, and NADPH/ CCl4. Vitamin K (less than or equal to 50 mu M) on its own did not inf luence the formation of TBARS. In combination with 1 mM dithiothreitol (DTT), the reductive cofactor for the microsomal enzyme vitamin K epo xide reductase, vitamin K suppressed lipid peroxidation with a concent ration that blocked the maximal response by 50% (IC50) of ca. 0.2 mu M . Vitamin K-1 (phylloquinone) and vitamin K-2 (menaquinone-4) were equ ally active. Warfarin (5 mu M) and chloro-vitamin K (50 mu M), inhibit ors of vitamin K epoxide reductase and gamma-glutamylcarboxylase, resp ectively, were able to completely abolish the antioxidant effect. Lipi d peroxidation was inversely related to the amount of vitamin K hydroq uinone in the reaction. Vitamin K epoxide reductase seemed sensitive t o lipid peroxidation, with half of the activity being lost within 10 m in during oxidation with NADPH/CCl4. The inactivation could be attenua ted by antioxidants such as vitamin E, reduced glutathione, and menadi one and also by a K vitamin in combination with DTT, but not by supero xide dismutase and catalase. The results show that the vitamin K cycle could act as a potent antioxidant, that the active species in all pro bability is vitamin K-hydroquinone, and that. the primary reaction pro duct is the semiquinone. The results also show that the reaction produ ct is processed in the vitamin K cycle to regenerate vitamin K-hydroqu inone. (C) 1997 Elsevier Science Inc.