The influence of Zn+' on fibrin clot formation was investigated by measuring its effect on the clotting times of fibrinogen exposed to thrombin. It was observed with either human or bovine thrombin that 0.01-0.1 mM Zn+2 induced significant reductions of clotting times in a concentration-dependent manner. The procoagulant effect of Zn+2 occurred in the presence of Ca+2 but was inhibited by metal chelating agents.
Higher levels of Zn+' (> 0.2 mM final concentration) were required to accelerate thrombin-induced clot formation in the presence of citrate or oxalate. Similarly with oxalated human plasma, > 0.2 mh4 Znf2 decreased the clotting time. Cations such as Mgf2 and Mn+' caused little change in clotting times. As an extension of these findings, we examined the effect of Zn+2 on the inhibition of thrombin by antithrombin-I11 (AT-111). The presence of as little as 0.006 mM Znt2 in an incubating mixture of thrombin and AT-111 severely reduced the inhibitory activity of AT-I11 towards thrombin. It was observed that the relative intrinsic fluorescence emission of human thrombin decreased upon exposure to Zn+2 but was unaffected by Mgf2 or Mn+2. It is suggested that Zn+' can form a complex with thrombin, which results in altered reactivity towards fibrinogen and decreased inhibition by