A cellular system for quantitation of vitamin K cycle activity: structure-activity effects on vitamin K antagonism by warfarin metabolites

Warfarin and other 4-hydroxycoumarins inhibit vitamin K epoxide reductase (VKOR) by depleting reduced vitamin K that is required for posttranslational modification of vitamin K–dependent clotting factors. In vitro prediction of the in vivo potency of vitamin K antagonists is complicated by the complex multicomponent nature of the vitamin K cycle. Here we describe a sensitive assay that enables quantitative analysis of γ-glutamyl carboxylation and its antagonism in live cells. We engineered a human embryonic kidney (HEK) 293–derived cell line (HEK 293-C3) to express a chimeric protein (F9CH) comprising the Gla domain of factor IX fused to the transmembrane and cytoplasmic regions of proline-rich Gla protein 2. Maximal γ-glutamyl carboxylation of F9CH required vitamin K supplementation, and was dose-dependently inhibited by racemic warfarin at a physiologically relevant concentration. Cellular γ-glutamyl carboxylation also exhibited differential VKOR inhibition by warfarin enantiomers (S > R) consistent with their in vivo potencies. We further analyzed the structure-activity relationship for inhibition of γ-glutamyl carboxylation by warfarin metabolites, observing tolerance to phenolic substitution at the C-5 and especially C-6, but not C-7 or C-8, positions on the 4-hydroxycoumarin nucleus. After correction for in vivo concentration and protein binding, 10-hydroxywarfarin and warfarin alcohols were predicted to be the most potent inhibitory metabolites in vivo.