Identification of glycyrrhizin as a thrombin inhibitor

Glycyrrhizin (GL), an anti-inflammatory compound isolated from Glycyrrhiza glabra, was identified as a new thrombin inhibitor: (a) It prolonged plasma recalcification and thrombin and fibrinogen clotting times, and (b) it inhibited thrombin-induced, but not collagen-, PAF- or convulxin-induced platelet aggregation. On the other hand, GL did not block thrombin's amidolytic activity upon S-2238. Furthermore, the fluorescence emission intensity of dansyl-thrombin was increased upon GL binding. Moreover, GL displaced hirudin as an inhibitor of thrombin-catalyzed hydrolysis of S-2238. Our data provide evidence that GL is a selective inhibitor of thrombin (the first one isolated from plants) that is able to exert its anti-thrombin action by interacting with the enzyme's anion binding exosite 1. A pharmacophoric search identified GL as a sialyl Lewis X (SLe[X]) mimetic compound able to inhibit selectin binding to SLe(X). However, SLe(X) did not affect thrombin clotting activities, which indicates a lack of its interaction with thrombin and distinguishes both molecules. It is suggested that the anti-inflammatory effect of GL may be due to its effective anti-thrombin action.