Involved in vitamin K metabolism. Can reduce inactive vitamin K 2,3-epoxide to active vitamin K (in vitro), and may contribute to vitamin K-mediated protection against oxidative stress. Plays a role in vitamin K-dependent gamma-carboxylation of Glu residues in target proteins. [protein]-disulfide + H2O + phylloquinone = 2,3-epoxyphylloquinone + [protein]-dithiol [protein]-disulfide + phylloquinol = [protein]-dithiol + phylloquinone Inhibited by warfarin (coumadin). Up-regulated in response to oxidative stress induced by hydrogen peroxide treatment. The location of two cysteine active-site residues within a proposed transmembrane is consistent both with the known hydrophobic environment of the thiol redox site of the enzyme and with the lipophilicity of vitamin K and warfarin (coumadin). Belongs to the VKOR family.