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This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Rights and Permissions Citing Articles Citing Articles via HighWire Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by Soons, H. Articles by Tans, G. Search for Related Content PubMed PubMed Citation Articles by Soons, H. Articles by Tans, G. Social Bookmarking                   What's this?  Previous Article  |  Table of Contents  |  Next Article  The effect of platelets in the activation of human blood coagulation factor IX by factor XIa H Soons, T Janssen-Claessen, HC Hemker and G Tans We report here the effect of activated human platelets on the activation of human factor IX by human factor XIa. Factor IXa formed during activation was determined via its ability to activate bovine factor X. To increase sensitivity, phospholipids and bovine factor VIIIa were present in the assay. The kinetic parameters of the factor IX activation were determined in the presence of 10 mmol/L CaCl2. The Km for factor IX was 0.30 mumol/L and kcat was 2.4 s-1. Activated human platelets inhibited factor IX activation by factor XIa 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 X 10(8)/mL, whereas concentrations of less than 10(6)/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 XIa was reversible. These data suggest that platelets release an antiprotease of factor XIa that reversibly inhibits factor XIa. 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 XIa. Volume 68, Issue 1, pp. 140-148, 07/01/1986 Copyright © 1986 by The American Society of Hematology CiteULike    Connotea    Del.icio.us    Digg    Reddit    Technorati    What's this? This article has been cited by other articles: J. A. Oliver, D. M. Monroe, H. R. Roberts, and M. Hoffman Thrombin Activates Factor XI on Activated Platelets in the Absence of Factor XII Arterioscler Thromb Vasc Biol, January 1, 1999; 19(1): 170 - 177. [Abstract] [Full Text] [PDF] R. Smith, D. Higuchi, and G. Broze Jr Platelet coagulation factor XIa-inhibitor, a form of Alzheimer amyloid precursor protein Science, June 1, 1990; 248(4959): 1126 - 1128. [Abstract] [PDF]

	Copyright © 1986 by American Society of Hematology         Online ISSN: 1528-0020