A B S T R A C T We have investigated the pathway of prothrombin activation in blood and plasma. By means of a rapid purification procedure involving chromatography on DEAE-cellulose and hydroxyapatite, we demonstrated that the major prothrombin fragment in serum is that representing the amino-terminal half of prothrombin (i.e. FL 2). The F1 2 isolated was characterized by its size, amino acid and antigenic compositions, amino-terminal residue, and the peptides (designated Fl and F2, respectively) it yielded upon hydrolysis by thrombin. Measurements by the isotope dilution technique showed that Fl 2 could account for the fate of at least 90% of the prothrombin originally present in plasma. By contrast, the serum concentration of the fragment representing the amino-terminal third of prothrombin (viz. F1) was <10% that of F1 -2. These results demonstrated that the major route of prothrombin conversion in blood or plasma involves the removal ofthe combined activation fragment (F1 *2) as a single peptide.
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