Vitamin K1 (Phytomenadione) is a procoagulant factor. As a component of a hepatic carboxylase system, Vitamin K1 is involved in the post-translational carboxylation of clotting factors II (Prothrombin), VII, IX and X and the clotting inhibitors protein C and protein S. Coumarins inhibit the reduction of Vitamin K1 (quinone form) to Vitamin K1 (hydroquinone) and also prevent the Vitamin K1 epoxide arising after carboxylation from being reduced to the quinone form.
Vitamin K1 is an antagonist of coumarin-type anticoagulants, e.g. Phenprocoumon. It does not, however, neutralise the activity of Heparin; Protamine is the antagonist of Heparin.
Vitamin K1 is ineffective in hereditary hypoprothrombinemia or hypoprothrombinemia induced by severe hepatic failure.
Lack of Vitamin K1 leads to an increased tendency to haemorrhagic disease in the newborn. Vitamin K1 administration, which promotes synthesis of the above-mentioned coagulation factors by the liver, can reverse an abnormal coagulation status and bleeding due to vitamin K1 deficiency.