R- and S-warfarin inhibition of vitamin K and vitamin K 2,3-epoxide reductase activities in the rat

Abstract

Reduction of vitamin K 2,3-epoxide and vitamine K catalyzed by hepatic microsomal enzymes is required for normal, postribosomal, γ-carboxyglutamate formation in the prothrombin complex Factors II, VII, IX, and X. The R- and S-warfarin enantiomers differentially inhibit (S-warfarin is 2 to 5 times more active) vitamin K function by mechanisms which have not been unambiguously determined. As a step toward determining the physiologically relevant site(s) of warfarin-antivitamin K activity we investigated in Wistar rats the effects of R- and S-warfarin on vitamin K 2,3-epoxide and vitamin K reductase activities and correlated them with effects on plasma concentrations of the Factors II, VII and X. Based on the results of these studies we conclude that: warfarin inhibition of the vitamin K 2,3-epoxide and vitamin K reductases is essentially irreversible; S-warfarin stereoselectively inhibits both reductases in vivo but not in vitro; the vitamin K reductase which utilizes dithiothreitol as cofactor in vitro is primarily responsible for vitamin K reduction to vitamin K hydroquinone under physiological conditions; warfarin initially inhibits γ-carboxyglutamate formation by inhibiting simultaneously the vitamin K 2,3-epoxide and vitamin K reductases; and following enantiomer administration there is an apparent lack of correlation between the restoration of the reductase activities and the reinitiation of coagulation factor synthesis.