Inhibition of the Thrombin‐Fibrinogen Reaction by Heparin and Purified Cofactor

The inhibition of the thrombin‐fibrinogen reaction was studied by recording the appearance of N‐terminal glycine. The reaction was progressively inhibited by heparin and purified human cofactor. No immediate effect could be demonstrated Thrombin activity was not restored following neutralization of heparin with protamine. The inhibition was therefore apparently due to ‘progressive’ inactivation of thrombin. The thrombin half life was approximately inversely proportional to the cofactor concentration. As compared to inactivation by cofactor alone, heparin (0.1 I.U./ml.) increased the inactivation rate about 20 times. Consequently, an inhibition could be demonstrated at much higher thrombin concentrations than with cofactor alone. α1‐glucoprotein counteracted the inhibition, but calcium had no effect. The inhibition decreased as the pH and ionic strength was increased. The inhibition increased as the temperature was increased from 17° to 37° C.