The Metabolic Relationships between the Different K Vitamins and the Synthesis of the Ubiquinones

Abstract

Methylnaphthoquinone is converted in the animal organism into vitamin K₂(₂₀), which is the actual active form of the K vitamins. Vitamin K₁ (phylloquinone) as well as vitamin K₂(₃₀) (the K vitamin of bacteria) first lose the side chain and are thus converted into methylnaphthoquinone, which then yields vitamin K₂(₂₀). The side chain can be introduced in vitro in the presence of an enzyme system which occurs in liver mitochondria. The action of the enzyme system consists of condensing the pyrophosphates of isoprenalkohols with the quinone with elimination of pyrophosphoric acid and formation of the corresponding 3-substituted naphthoquinones. It has been shown that the synthesis of the ubiquinones (coenzyme A) takes place along the same lines, starting from 2,3-dimethoxy-5-methyl-benzoquinone. The properties of a new flavoprotein (vitamin K reductase) are described. This reductase transfers hydrogen from DPNH⁺ and TPNH⁺ to various quinones and is strongly inhibited by vitamin K antagonists (e.g., dicumarol). The importance of this enzyme for the interpretation of the mechanism of action of vitamin K is discussed.