Activation of human prothrombin by human prothrombinase. Influence of factor Va on the reaction mechanism.

The kinetics of the activation of human prothrombin catalyzed by human prothrombinase was studied using the fluorescent alpha-thrombin inhibitor dansylarginine-N-(3-ethyl-1,5-pentanediyl)amide (DAPA). Prothrombinase proteolytically activates prothrombin to alpha-thrombin by cleavages at Arg273-Thr274 (bond A) and Arg322-Ile323 (bond B). The differential fluorescence properties of DAPA complexed with the intermediates and products of human prothrombin activation were exploited to study the kinetics of the individual bond cleavages in the zymogen. When the catalyst was composed of prothrombinase (human factor Xa, human factor Va, synthetic phospholipid vesicles, and calcium ion), initial velocity studies of alpha-thrombin formation indicated that the kinetic constants for the cleavage of bonds A or B were similar to the constants that were obtained for the overall reaction (bonds A + B). The progress of the reaction was also monitored by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The results indicated that the activation of human prothrombin catalyzed by prothrombinase proceeded exclusively via the formation of meizothrombin (bond B-cleaved) as an intermediate. Kinetic studies of the cofactor dependence of the rates of cleavage of the individual bonds indicated that, in the absence of the cofactor, cleavage at bond B would constitute the rate-limiting step in prothrombin activation. Progress curves for prothrombin activation catalyzed by prothrombinase and monitored using the fluorophore DAPA were typified by the appearance of a transient maximum, indicating the formation of meizothrombin as an intermediate. When factor Xa alone was the catalyst, progress curves were characterized by an initial burst phase, suggesting the rapid production of prethrombin 2 (bond A-cleaved) followed by its slow conversion to alpha-thrombin. Gel electrophoresis followed by autoradiography was used to confirm these results. Collectively, the results indicate that the activation of human prothrombin via the formation of meizothrombin as an intermediate is a consequence of the association of the cofactor, human factor Va, with the enzyme, human factor Xa, on the phospholipid surface.