Placental protein 5 (PP5) inhibits thrombin-induced coagulation of fibrinogen.

The effect of Placental Protein 5 (PP5) on thrombin-induced coagulation of diluted fibrinogen is described. In contrast to previous reports which failed to demonstrate antithrombin-like activity for PP5 on synthetic substrates, we show in this study that PP5 inhibits thrombin activity in a dose-dependent manner. PP5 acts thus in a very similar way to antithrombin III (ATIII). Although PP5 binds to heparin, it does not show any heparin-cofactor activity. Whereas catalytic amounts of heparin accelerate greatly the inhibitory effect of ATIII, no accelerating effect of heparin on PP5 could be observed under the same conditions. In spite of its anti-thrombin activity, PP5 cannot be considered as a pregnancy-analogue of ATIII, as it lacks the heparin-cofactor activity.