Establishing the inhibitory effects of bradykinin on thrombin

Bradykinin, RPPGFSPFG, has been reported to be an inhibitor of thrombin’s roles in blood clotting, platelet activation, and cellular permeability. The exact target, magnitude, and type of inhibition occurring are not well characterized. Based on the individual kinetic parameters calculated here, bradykinin is classified as a weak competitive inhibitor against hydrolysis of S-2238 and of a PAR4-like peptide. The Km values increased twofold in the presence of bradykinin, whereas the kcat values remained constant. The Ki values ranged from 170 to 326 μM. Other biochemical studies indicated that bradykinin inhibits release of fibrinopeptide A from fibrinogen. Furthermore, bradykinin hindered the time required for fibrin clot formation. The weak inhibitions observed in vitro suggest that the direct effects of bradykinin on the thrombin active site become significant only at high concentrations, levels that may be difficult to achieve physiologically. Clearly, bradykinin can target thrombin but whether this direct interaction can be achieved in vivo and is sufficient to elicit a response without contributions from other cofactors requires further investigation.