Characterization of a proteolytically modified form of human prothorombin

Treatment of human prothrombin by α chymotrypsin results in the production of a single homogeneous product with an apparent M.W. of 69 000. This product fails to adsorb on barium citrate. NH2-terminal sequence analysis shows that a single cleavage has occurred after tyrosine 44. The proteolytically modified prothrombin which can therefore be referred to as prothrombin (des 1–44), lacks the whole vitamin K dependent part of the molecule. The region of the peptide chain around tyrosine 44 must be particularly exposed to proteolytic attack and serve as a junction between the vitamin K dependent domain and the other structural domains of prothrombin. In the presence of factor Xa alone, prothrombin (des 1–44) is indistinguishable from normal prothrombin when activation is monitored by the appearance of amidolytic activity on S2238. However, activation of prothrombin (des 1–44) is no longer enhanced by the presence of Ca++ and phospholipid in the activation mixture.