The Effect of Platelets in the Activation of Human Blood Coagulation Factor IX by Factor XIa

• Published in: Blood 1986-07, Vol.68 (1), p.140-148
• Main Authors: Soons, Hans, Janssen-Claessen, Truus, Hemker, H. Coenraad, Tans, Guido
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; Blood coagulation. Blood cells; Blood Platelets - drug effects; Blood Platelets - physiology; Blood Proteins - analysis; Coagulation factors; Factor IX - analysis; Factor XI - physiology; Factor XIa; Fundamental and applied biological sciences. Psychology; Humans; In Vitro Techniques; Kinetics; Male; Molecular and cellular biology
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood.V68.1.140.140

We report here the effect of activated human platelets on the activation of human factor IX by human factor Xla. Factor IXa formed during activation was determined via its ability to activate bovine factor X. To increase sensitivity, phospholipids and bovine factor Villa were present in the assay. The kinetic parameters of the factor IX activation were determined in the presence of 10 mmol/L CaCI2. The Km for factor IX was 0.30 μmol/L and kcet was 2.4 s-1. Activated human platelets inhibited factor IX activation by factor Xla in a dose-dependent manner, whereas unstimulated platelets had no effect. Factor IX activation was inhibited for more than 90% at a platelet concentration of 4 × 108/mL, whereas concentrations of less than 106/mL had no influence. The inhibitory effect could be induced by thrombin, collagen, calcium ionophore A 23187, and adrenalin. The appearance of inhibitory activity could be blocked by the addition of the prostacyclin analogue ZK 36374 at any time during platelet activation. Stirring during platelet activation was not necessary. These results suggest that the inhibition is caused by a release reaction. This was confirmed by centrifugation experiments that showed that the inhibitory activity could be recovered from the supernatant of the activated platelets. The inhibitory activity was destroyed upon boiling and was susceptible to trypsin digestion. Passage of platelet supernatant over ACA 22 showed that the inhibitory activity eluted with an apparent molecular weight of less than 1,200,000 but greater than 669,000. The inhibition of factor Xla was reversible. These data suggest that platelets release an antiprotease of factor Xla that reversibly inhibits factor Xla. Lineweaver-Burk analysis showed that the inhibitor caused both an increase in Km for factor IX and a decrease in kcat of factor IXa formation by factor Xla. © 1986 by Grune & Stratton, Inc.