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  • Title: Metabolism of vitamin K and prothrombin synthesis: anticoagulants and the vitamin K--epoxide cycle.
    Author: Bell RG.
    Journal: Fed Proc; 1978 Oct; 37(12):2599-604. PubMed ID: 359368.
    Abstract:
    Vitamin K is primarily located in hepatic microsomes, where the vitamin K-dependent carboxylation in prothrombin synthesis occurs. Recent evidence supports the idea that the carboxylation is linked to the metabolism of the vitamin--specifically the cyclic interconversion of vitamin K and vitamin K epoxide. The primary site of action of coumarin and indandione anticoagulants appears to be an inhibition of the epoxide-to-vitamin K conversion in this cycle. There is a correlation between the inhibition of prothrombin synthesis and the regeneration of vitamin K from the epoxide by anticoagulants. In hamsters and warfarin-resistant rats prothrombin synthesis and the epoxide-K conversion are less sensitive to warfarin than in the normal rat. The epoxide-K conversion is impaired in resistant rats, which may explain their high vitamin K requirement. There is also a correlation between vitamin K epoxidation and vitamin K-dependent carboxylation, but the apparent link may be because vitamin K hydroquinone is an intermediate in the formation of the epoxide and also the active form in carboxylation. The vitamin K-epoxide cycle is found in extrahepatic tissues such as kidney, spleen, and lung and is inhibited by warfarin.
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