Purification of an Apparent Rat Liver Prothrombin Precursor: Characterization and Comparison to Normal Rat Prothrombin

Current evidence would suggest that prothrombin is synthesized from a liver precursor molecule in a vitamin K dependent step which involves the attachment of calcium binding groups to the precursor. A protein has now been isolated from the liver of warfarin-treated rats which has the properties predicted for this precursor. The purified precursor is a glycoprotein with a molecular weight indistinguishable from rat prothrombin. Both electrophoretic and isofocusing analyses indicate that the precursor is less negatively charged than prothrombin. Specific proteolysis of the precursor by thrombin, taipan snake venom, or clotting factor Xa yielded fragments indistinguishable from those formed by similar proteolysis of prothrombin. The rate of activation of the precursor to thrombin by factor Xa and Ca²⁺was not stimulated by the addition of phospholipid, while prothrombin activation is greatly stimulated under these conditions. All of the data obtained are consistent with the hypothesis that the protein isolated is a precursor to prothrombin, and that under the influence of vitamin K, this precursor is converted to prothrombin by the addition of a number of acidic calcium binding groups.