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Blood. 2011 Mar 10;117(10):2967-74. doi: 10.1182/blood-2010-08-304303. Epub 2011 Jan 14.

Functional study of the vitamin K cycle in mammalian cells.

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Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA.


We describe a cell-based assay for studying vitamin K-cycle enzymes. A reporter protein consisting of the gla domain of factor IX (amino acids 1-46) and residues 47-420 of protein C was stably expressed in HEK293 and AV12 cells. Both cell lines secrete carboxylated reporter when fed vitamin K or vitamin K epoxide (KO). However, neither cell line carboxylated the reporter when fed KO in the presence of warfarin. In the presence of warfarin, vitamin K rescued carboxylation in HEK293 cells but not in AV12 cells. Dicoumarol, an NAD(P)H-dependent quinone oxidoreductase 1 (NQO1) inhibitor, behaved similarly to warfarin in both cell lines. Warfarin-resistant vitamin K epoxide reductase (VKOR-Y139F) supported carboxylation in HEK293 cells when fed KO in the presence of warfarin, but it did not in AV12 cells. These results suggest the following: (1) our cell system is a good model for studying the vitamin K cycle, (2) the warfarin-resistant enzyme reducing vitamin K to hydroquinone (KH₂) is probably not NQO1, (3) there appears to be a warfarin-sensitive enzyme other than VKOR that reduces vitamin K to KH₂, and (4) the primary function of VKOR is the reduction of KO to vitamin K.

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