Evidence that warfarin anticoagulant action involves two distinct reductase activities.

Area of Science:

• Biochemistry
• Pharmacology
• Molecular Biology

Background:

• Vitamin K is essential for blood coagulation.
• Warfarin is an anticoagulant that inhibits vitamin K metabolism.
• Understanding vitamin K reductase activity is crucial for explaining warfarin resistance.

Purpose of the Study:

• To compare vitamin K and vitamin K 2,3-epoxide reductase activities in warfarin-susceptible and resistant rats.
• To elucidate the role of these reductases in vitamin K metabolism and warfarin's anticoagulant mechanism.

Main Methods:

• Microsomal preparations from rat livers were used.
• Enzyme kinetics were measured using dithiothreitol-dependent reductase assays.
• Inhibition of reductase activity by warfarin was assessed.
• Gamma-carboxylation of precursor protein was measured.

Main Results:

• Warfarin-resistant rats showed 3- to 6-fold higher warfarin concentrations (I50) for inhibiting vitamin K and vitamin K 2,3-epoxide reduction.
• Higher warfarin concentrations were required to inhibit gamma-carboxylation in resistant rat microsomes.
• These findings indicate reduced sensitivity of vitamin K reductases to warfarin in resistant rats.

Conclusions:

• Vitamin K reductase activity plays a significant role in vitamin K metabolism in rats.
• Warfarin's anticoagulant effect is mediated by the inhibition of both vitamin K and vitamin K 2,3-epoxide reductases.
• Differences in reductase activity contribute to warfarin resistance.