Warfarin resistance. Vitamin K epoxide reductase of Scottish resistance genes is not irreversibly blocked by warfarin.

The dithiothreitol-dependent vitamin K 2,3 epoxide (vitamin KO) reductase activity in liver microsomes of Scottish-derived warfarin-resistant Wistar rats (Tolworth Laboratory) was compared to that of susceptible Wistar rats. Under the test conditions, reductase activities in liver homogenates and in liver microsomes were comparable for both strains. The in vitro i50 of S-warfarin for microsomal reductase activity was 1 to 2 microM in both strains. The effect of in vivo S-warfarin was investigated after single doses, i.e. 0.2 and 1 mg/kg for the susceptible rats, and 1 and 5 mg/kg for the resistant rats. At 20 hr following the warfarin administrations in the susceptible strain, microsomal reductase was suppressed to about 30% of control. Microsomal reductase activity in the resistant strain was not reduced. Tissue and microsomal warfarin concentrations, however, were comparable in both strains. Wash experiments with microsomes which were treated in vitro with S-warfarin showed that vitamin KO reductase of the warfarin-resistant strain was not irreversibly inactivated by warfarin. The reactivation was mediated by DDT. The results suggest the following characteristic of the vitamin KO reductase of the Scottish resistance gene: contrary to the "normal" enzyme, the tight complex between the inhibitor and the resistant enzyme is liable to reactivation by reduction of the disulfide bridge in the active centre of the enzyme. This property explains the resistance for 4-hydroxycoumarin anticlotting activity.