Functional difference between intrinsic and extrinsic coagulation pathways. Kinetics of factor X activation on human monocytes and alveolar macrophages.

Activation of coagulation factor X via the intrinsic pathway requires the assembly of factors IXa and VIII on lipid membranes. It is known that the platelet expresses membrane sites for assembly of factors IXa/VIII and promotes efficient factor X activation. We now show that human blood monocytes, but not lymphocytes or polymorphonuclear leukocytes, also express appropriate sites for factors IXa/VIII assembly. The maximal rate of factor X activation by factors IXa (0.75 nM) and VIII (1 unit/ml) assembled on monocytes is similar to the maximal rate on platelets. This rate, adjusted per micromole of lipid phosphorus, is 1636 +/- 358 nM factor Xa/min on monocyte, and 1569 +/- 54 nM factor Xa/min on platelets. At physiologic concentrations of factors X and VIII, the activation rate increases with factor IXa concentration asymptotically approaching a maximum. Half-maximal rate is achieved with 1.0 +/- 0.16 nM factor IXa. Monocytes and macrophages, but not platelets, can express membrane tissue factor and thus promote simultaneous assembly of two distinct factor X-activating protease complexes. In these studies, blood monocytes and alveolar macrophages are used as membrane sources in kinetic experiments comparing factor X activation by intrinsic (factor IXa/VIII) versus extrinsic (factor VII/tissue factor) protease complexes. At plasma concentration of factors VIII and VII, apparent Km on the monocyte is 14.6 +/- 1.4 nM for intrinsic and 117.0 +/- 10.1 nM for extrinsic activation. The apparent Km on alveolar macrophages is 12.1 +/- 1.9 and 90.6 +/- 10.2 nM for intrinsic and extrinsic activation, respectively. Maximal rates on monocytes at saturating concentration of factors IXa, VIII, and VII are 48.0 +/- 11.2 nM factor Xa/min, for intrinsic activation, and 16.5 +/- 5.5 nM factor Xa/min, for extrinsic activation. These data show that the monocyte/macrophage is the only blood-derived cell type with membrane sites for both intrinsic and extrinsic pathway assembly. We have exploited this characteristic of the monocyte/macrophage membrane to demonstrate that factor X activation by the intrinsic pathway protease is more efficient than activation via the extrinsic pathway protease complex.