OPTIMIZATION OF CONDITIONS FOR THE CATALYTIC EFFECT OF THE FACTOR-IXA - FACTOR-VIII COMPLEX - PROBABLE ROLE OF THE COMPLEX IN THE AMPLIFICATION OF BLOOD-COAGULATION

Purified human factor IXa activates factor X in the plasma only to a moderate extent. The activity of factor IXa is increased about twentyfold, if factor IXa forms a complex with factor VIII, phospholipid and calcium. Already minute amounts of factor VIII enhance the activity of factor IXa. The amount of activated factor X is directly related to the IXa concentration of the complex, however below 0. 3 U/ml factor IXa concentration the relative excess of calcium, e. g. 30 mM CaCl2 inhibits the functioning of the complex. By decreasing the concentration of calcium an about 20-fold maximal activity increment can be attained even at low factor IXa concentration. The complex displays maximal activity at the following composition: 1 U/ml factor IXa, 1 U/ml factor VIII, 40 μg/ml phospholipid and 30 mM CaCl2. Under such circumstances 1 mole of factor IXa activates 23 moles of factor X in 30 sec. In the case of a maximally active complex an approximately 108 amplification value is calculated from the activation of factor XI to the formation of fibrin. In light of the experimental results and of the kinetics of activation of intrinsic clotting factors, the role of the complex of factors IXa and VIII in the physiological and pathological processes of blood coagulation is discussed. © 1979.