Measurement of factor Xa-antithrombin III in plasma: relationship to prothrombin activation in vivo. 
The M(r) of the complexes formed when factor Xa reacts with antithrombin III (ATIII) in plasma were estimated by gel filtration and SDS-polyacrylamide electrophoresis.  The predominant species of factor Xa-ATIII detected after plasma and plasma to which factor Xa had been added were gel filtered on Sephadex G-200 and Sepharose 4B had apparent M(r) > 200,000, in which factor Xa-ATIII was associated with vitronectin.  Addition of factor Xa-ATIII to ATIII-depleted plasma also resulted in the formation of factor Xa-ATIII-vitronectin complexes with M(r) > 200,000.  Using polyclonal antibodies to human factor Xa-ATIII and ATIII as the capture and detector antibodies, respectively, a sensitive and specific enzyme-linked immunosorbent assay was developed to quantify factor Xa-ATIII in plasma.  The relationship between factor Xa-ATIII production and prothrombinase activity in vivo was investigated by quantifying factor Xa-ATIII and prothrombin fragment 1 + 2 endogenous to the plasmas of blood donors and patients with Hodgkin's and non-Hodgkin's lymphoma.  Whereas the concentrations of prothrombin fragment 1 + 2 in the 84 normal plasmas increased with age, those of factor Xa-ATIII (mean +/- SD of 34.7 +/- 13.8 pM) did not, and no correlation existed between the concentrations of the two parameters in normal plasmas.  In contrast, a highly significant correlation between the concentrations of these two parameters was found in the plasmas of the cancer patients which coincidentally also had higher concentrations of both factor Xa-ATIII and prothrombin fragment 1 + 2 than the normal plasmas.  Thus, ATIII may differentially influence prothrombinase formation and activity in normal individuals and cancer patients.