Enzymatic Properties Of Staphylocoagulase-Human Prothrombin Complex (Staphylothrombin)

<jats:p>Staphylocoagulase(SC) is an extracellular protein produced by Staphylococcus aureus. This protein is known to coagulate several mammalian plasmas, by forming an active molecular complex with prothrombin. However, there is not enough knowledge of the molecular mechanism of prothrombin activation by SC. The present work was undertaken to elucidate the enzymatic properties of SC-human prothrombin (HP) complex. A highly purified SC was prepared from a culture filtrate of Staphylococcus aureus, strain st-213, by a bovine prothrom- bin-Sepharose affinity column developed in our laboratory. This SC could activate HP but not bovine prothrombin. A SC-HP complex formed with the molar ratio of 1 to 1 had a strong amidase activity towards α-thrombin fluorogenic substrate, Boc-Val-Pro-Arg-4-methylcoumary1-7-amide. The activity of the complex was stable at 0°C for at least 8 hrs, while the activity was rapidly decreased with incubation at 37°C. SDS-polyacrylamide gel electrophoretic analysis on the complex formed at 37°C indicated not only a proteolysis of SC portion to small derivatives, but also the fragmentation of HP portion to prethrombin 1 and profragment 1. This proteolytic fragmentation seemed to be due to a self-catalytic reaction of the SC-HP complex, since no endogeneous proteolytic activities were detected in each preparation of SC and HP. Bovine prothrombin, prethrombin 1 and prethrombin 2 inhibited the complex formation between SC and HP, but bovine profragment 1 and 2 had no effect on the complex formation. These results indicate that a high affinity site of HP to SC must be located in the prethrombin 2 region. Like α-thrombin, the amidase activity of SC-HP complex was competitively inhibited with benzamidine (Ki=1.2 x 10-1 M). However, both clotting and amidase activities of the complex could not be inhibited by 4 molar excess of hirudin or antithrombin III in the presence of heparin.</jats:p>