Three different synthetic chromogenic substrates (H glutamylglycyl-L-argnine p-nitroanilide (S-2227), pyro-glutamyl-glycyl-L-argininep-nitroanilide (S-2444), and H D-isoleucyl-L-prolyl-L-arginine p-nitroanilide (S-2288)) were investigated for use in the measurement of plasminogen activator activity with high molecular weight urokinase (H-UK), low molecular weight urokinase (L-UK), and tissue plasminogen activator (TPA). The three substrates were hydrolyzed by both TPAtype and UK-type plasminogen activator. As regards the amidolytic activity of S-2227, TPA exhibited a weaker amidolytic activity, and L-UK a stronger activity. In the case of the amidolytic activity of S-2444, no great difference between the three activators was observed in terms of Vmax. As regards the amidolytic activity of S-2288, L-UK exhibited a stronger activity, and TPA a weaker activity. It is suggested that the molecular size of the synthetic chromogenic substrate was too small when compared to natural substrate (fibrin), and therefore that fibrin-binding sites around the catalytic site in TPA are not recognized.
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