The vitamins phylloquinone (K1), menadione (K3) and various menaquinones (K2) were compared for their ability to serve as a cofactor for the hepatic vitamin K-dependent carboxylase. It was found that the cofactor activity of the menaquinones varied with the length of the aliphatic side-chain and showed an optimum at MK-3. Menadione was not active at all. The concentration required for half-maximal reaction velocity (K 1/2) was determined for the various menaquinones and decreased at increasing chain length. The K 1/2 value for MK-4 was 3-times lower than that for vitamin K1. Under our in vitro conditions both vitamin K1 and the K2 vitamins were rapidly metabolized into a mixture of the quinone, the hydroquinone and the epoxide form. The fact that at equilibrium the level of these three metabolites was independent of the starting material shows that the vitamin K cycle is operational for vitamin K1 as well as for K2.
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