Recent findings in understanding the biological function of vitamin K.

Vitamin K is required for the posttranslational modification of liver precursors of prothrombin, blood coagulation Factors VII, IX, and X and additional proteins of undetermined functions in plasma and other tissues. This modification involves the formation of gamma-carboxyglutamic acid, an acidic amino acid needed for the interaction of these proteins with calcium ions. The vitamin is a cofactor of a unique microsomal carboxylase which requires the reduced form of vitamin K, CO2, and molecular oxygen. The vitamin is apparently oxidized to its 2,3-epoxide during the carboxylation reaction. Liver microsomes also contain an enzyme which catalyzes the reduction of the epoxide to the vitamin, vitamin K epoxide reductase, and a number of vitamin K reductases. The epoxide reductase appears to be the site of the anticoagulant action of 4-hydroxycoumarins, commonly used as oral anticoagulants.