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Thromb Haemost 1985; 53(03): 396-400
DOI: 10.1055/s-0038-1661322
Original Article
Schattauer GmbH Stuttgart

Assembly of the Intrinsic Factor X Activating Complex – Interactions between Factor IXa, Factor VIIIa and Phospholipid

Gerbrand van Dieijen
The Department of Biochemistry, University of Limburg, Maastricht, The Netherlands
,
Jan L M L van Rijn
The Department of Biochemistry, University of Limburg, Maastricht, The Netherlands
,
José W P Govers-Riemslag
The Department of Biochemistry, University of Limburg, Maastricht, The Netherlands
,
H Coenraad Hemker
The Department of Biochemistry, University of Limburg, Maastricht, The Netherlands
,
Jan Rosing
The Department of Biochemistry, University of Limburg, Maastricht, The Netherlands
› Author Affiliations
Further Information

Publication History

Received 05 November 1984

Accepted 22 March 1985

Publication Date:
18 July 2018 (online)

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Summary

The activation of blood coagulation factor X by factor IXa is strongly stimulated by the non-enzymatic cofactors phospholipid, Ca2+ and activated factor VIII. In this paper we present a method by which we were able to determine binding affinities of factor IXa for phospholipids (either in the absence or presence of factor VIIIa) from kinetic measurements of factor X activation. It is shown that rates of factor X activation in the presence of phospholipids can be saturated with an excess factor VIIIa at limiting amounts of factor IXa and vice versa. Our data indicate that the enzymatic unit in the intrinsic factor X activator is a 1:1 stoichiometrical complex of factor IXa and factor VIIIa bound to phospholipid. Titrations with factor IXa at fixed concentrations of phospholipid and factor X show that the apparent dissociation constant of factor IXa for phospholipid is lowered from 10-6 M to 10-8 M by the presence of factor VIIIa. We conclude, that in analogy with the role of factor Va in prothrombin activation, phospholipid-bound factor VIIIa functions as a high-affinity binding site (»receptor«) for factor IXa in the intrinsic factor X activating complex. Therefore, factor VIIIa increases the observed Vmax of factor X activation by 1) enhancing the kcat of the reaction and 2) increasing the amount of phospholipid-bound factor IXa that participates in factor X activation.

Keywords

Intrinsic factor X activation - Factor VIII - Kinetics and mechanism - Binding parameters
 

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