Formation of 3-hydroxy-2,3-dihydrovitamin K1 in vivo: relationship to vitamin K epoxide reductase and warfarin resistance.

Hydroxy vitamin K [3(2)-hydroxy-2,3- dihydrovitamin K1] has been identified as a quantitatively important metabolite of injected vitamin K epoxide in vivo. The metabolite has been isolated and identified by comparison of its UV, mass spectra and high-performance liquid chromatography (HPLC) retention times with those of synthetic standards, and by its characteristic conversion to vitamin K quinone on treatment with the base triethylamine. This metabolite is formed from the vitamin K epoxide, not from the vitamin K quinone and can represent up to 3.5% of dose and 13% of hexane-extractable metabolites present in liver 1 hour after injection of 330 micrograms vitamin K1 epoxide per kilogram body weight. It is formed in both normal and warfarin-resistant rat strains, but to a significantly greater extent in the latter. Unlike the hydroxy vitamin K formed by warfarin-resistant rat liver microsomes in vitro, the metabolite formed from racemic vitamin K epoxide in vivo was not optically active, nor was its formation inhibited by coumarin anticoagulants under conditions that completely blocked vitamin K epoxide reduction in vivo. On this basis, hydroxy vitamin K formation in vivo differs from its formation in vitro; it is not a product of vitamin K epoxide reductase in vivo, but of some other possibly non-enzymatic reaction.