THE INHIBITION OF INTRINSIC PROTHROMBINASE AND ITS GENERATION BY HEPARIN AND 4 DERIVATIVES IN PROTHROMBIN POOR PLASMA

The effect of different heparins and a synthetic pentasaccharide on the inhibition of intrinsic prothrombinase and of its generation was studied by a new technique, using a defibrinated prothrombin poor human plasma, supplemented with phospholipids and calcium. Prothrombinase activity was evaluated on purified prothrombin with a chromogenic substrate. This technique is designed to bypass the interference of massive endogenous thrombin generation on the measurement of prothrombinase activity. We first validated the specificity of the technique by using specific Xa and Ila inhibitors. Then, the inhibition of prothrombinase generation and the inhibition of generated prothrombinase were oth studied. The results showed that anti-Xa activity measured on exogenous bovine factor Xa added to plasma was not correlated with the inhibition of prothrombinase generation or prothrombinase activity. The concentrations required for unfractionated heparin (the 4th International Standard : 4th IS UH), 1st International Standard Low Molecular Weight Heparins (1st IS LMWH), enoxaparin, Fraxiparine®, and pentasaccharide in order to inhibit preformed prothrombinase were significantly higher than those necessary to inhibit prothrombinase generation. These data suggest that anti-Xa activity of unfractionated heparin and its derivatives does not completely reflect the extent of the inhibition of intrinsic prothrombinase generation by UH, LMWH, and pentasaccharide. On the other hand, anti-Ila activity of heparins could be responsible for the inhibition of prothrombinase generation. The action of pentasaccharide devoid of anti-Ila activity on prothrombinase generation appears related to its indirect effect on the formation of initial thrombin traces. This new technique provides a tool to study the essential role played by thrombin during the early steps of coagulation. © 1990.