“…The activity of antithrombin is modulated by the glycosaminoglycan heparin that binds to it and activates it: in the absence of heparin, antithrombin is a rather poor inhibitor of thrombin. The mechanism of this activation is unknown, but it seems likely from observations of susceptibility of the reactive centre loop of antithrombin to cleavagelo, 11 that it involves release of the loop from steric hindrance. Antitrypsin does not bind heparin and antitrypsin Pittsburgh has full inhibitory activity irrespective of heparin concentration.…”