[Modulation of the platelet aggregation capacity by modified forms of thrombin]

It was found that human platelets possess a high sensitivity towards alpha-thrombin (Km = 2 nM). Modified thrombin forms (beta/gamma-thrombin) with an impaired recognition site of high molecular weight substrates and DIP-alpha-thrombin and trypsin are incapable of inducing platelet aggregation when taken at concentrations corresponding to effective concentrations of alpha-thrombin. Beta/gamma-Thrombin and trypsin, unlike DIP-alpha-thrombin, cause platelet aggregation at concentrations of 100-200 nM. Studies on the modulating effects of modified thrombin forms, alpha-thrombin and trypsin, on platelet aggregation induced by alpha-thrombin revealed that beta/gamma-thrombin, alpha-thrombin and trypsin at concentrations causing no cell aggregation potentiate the platelet response after 2 min incubation and inhibit platelet aggregation upon prolonged (15 min) incubation. However, DIP-alpha-thrombin, irrespective of the incubation time (up to 30 min) increased the sensitivity of platelets to alpha-thrombin-induced aggregation. The activating effect of DIP-alpha-thrombin is characterized by an equilibrium constant (KA) of 17 nM. The experimental data confirm the hypothesis that the necessary prerequisite for an adequate physiological response of platelets to alpha-thrombin is the maintenance in the thrombin molecule of an intact active center and a recognition site for high molecular weight substrates. The specificity of thrombin as a potent platelet aggregation inducer is determined by the recognition site for high molecular weight substrates.