Inhibition of thrombin catalytic activity by released platelet PN-1. Thrombin catalytic activity was measured after incubation with the supernatants from resting platelets (■) or activated platelets () as described in “Preparation of washed platelets.” Products of human platelet secretion from (A) control donors or (B) GPS patients were incubated with thrombin in the presence or absence of an anti-PN-1 IgG (150 μg/mL) and/or an anti-PAI-1 IgG (150 μg/mL). (C) Secretion products of platelets from mice deficient for PN-1 or PAI-1 and their WT littermates. (D) Secretion products of human platelets were pretreated with polybrene (50 μg/mL) before the incubation with thrombin. Data are presented as mean ± SD of 3 independent experiments, each performed in triplicate. *P < .05, significantly different from products of resting platelet supernatant. †P < .05, significantly different from products of activated platelet supernatant.