What enzyme is responsible for activating Vitamin K in the liver?

The activation of Vitamin K in the liver is primarily facilitated by the enzyme epoxide reductase. This enzyme plays a crucial role in the vitamin K cycle by converting vitamin K epoxide back into its active form, which is necessary for the synthesis of several clotting factors that depend on vitamin K for their activity. The conversion is critical because it allows the vitamin to function as a cofactor for gamma-carboxylase, an enzyme that modifies specific glutamic acid residues on these clotting factors, enabling them to bind calcium ions—a vital step for proper coagulation.

In terms of functionality, epoxide reductase is essential for maintaining adequate levels of reduced vitamin K. If vitamin K remains in its oxidized form, it cannot participate in the gamma-carboxylation process, which is needed both for the activation of clotting factors II (prothrombin), VII, IX, and X, as well as for proteins C and S that are involved in anticoagulation processes. Therefore, epoxide reductase is key to ensuring a continuous supply of active vitamin K in the liver, necessary for maintaining hemostasis.