The Conversion of Human Prothrombin to Thrombin by Sodium Citrate. Analysis by Sodium Citrate. Analysis of the Activation Mixture.

Since the discovery that bovine prothrombin when dissolved in 25% sodium citrate is autocatalytically converted to thrombin (2), several reviews have appeared which summarize the many chemical and biological changes occurring during this transformation (3-5). Recently the findings that poly-L-lysine, as well as a number of polyamino acids, initiates prothrombin activation (6) have suggested the importance of considering nonenzymic activators in the blood coagulation mechanism. Apart from some ultracentrifugal observations of bovine prothrombin during citrate activation mentioned briefly in review articles (4, 7), the protein changes which occur during this activation process, to our knowledge, have not been examined in detail, Also, the conversion of human prothrombin to thrombin by citrate has not been studied as extensively as the corresponding conversion in the bovine system, except for the work of Alexander (8). His studies have dealt mainly with the mechanism whereby citrate and other anions activate certain clotting factor impurities found in most prothrombin preparations; his point of view is not shared by Seegers and Marciniak (9) (see discussion following and Reference 8). In the present work with human preparations we have not attempted to study the mechanism of prothrombin activation, but have focused our attention on the changes in molecular size and thrombin activity which occur when prothrombin is exposed to concentrated citrate solutions. The data obtained indicate certain similarities and dissimilarities to the phenomena reported for the bovine species. In addition, smaller molecules obtained after activation of the zymogen have been identified and partially characterized. Similar experimental approaches might be helpful in studies of the activation mechanism itself and eventually for an understanding of the role of other clotting factors in this transformation.