Comparative uptake, metabolism, and utilization of menaquinone-4 and phylloquinone in human cultured cell lines

It is generally accepted that the availability of vitamin K in vivo depends on its homologues, the biological activities of which would differ among organs. To test this hypothesis, we examined the uptake, metabolism, and utilization of menaquinone-4 (MK-4) and phylloquinone (PK) using ¹⁸O-labeled compounds in two cultured human cell lines (HepG2 and MG-63). Lipid extracts were prepared from the cells and media after 1, 3, and 6 h of incubation. The detection of the vitamin K analogues (¹⁸O-, ¹⁶O-quinone, and epoxide forms) was carried out with LC-APCI-MS/MS as previously reported. The ¹⁸O of vitamin K was replaced with atmospheric ¹⁶O₂ during the formation of vitamin K epoxide with a carboxylative catalytic reaction. As a result, a significant difference was observed between MK-4 and PK in the amounts taken up into the cells. The ¹⁸O-labeled MK-4 was rapidly and remarkably well absorbed into the cells and metabolized to the epoxide form via a hydroquinone form as compared to the ¹⁸O-labeled PK. The difference in uptake of MK-4 and PK was not affected by treatment with warfarin although the metabolism of both compounds was markedly inhibited. This methodology should be utilized to clarify some of the actions of vitamin K in target cells and facilitate the development of new vitamin K drugs.