A new familial variant of antithrombin III: 'antithrombin III Paris'.
A 59-year-old woman presented a recurrent history of thromboembolism. A qualitative defect of antithrombin III (AT III) was suggested by the discrepancy between a normal amount of AT III antigen and a decreased heparin cofactor activity. Six members of the same family showed a similar defect although clinically asymptomatic. The qualitative abnormality of AT III was confirmed by two-dimensional immunoelectrophoresis. In the absence of heparin, a single peak was obtained with both control and patients' plasmas. In the presence of heparin, two peaks of AT III were observed in the patients' plasmas: the mobility of one peak was similar to that of the control, whereas the other showed a decreased mobility, suggesting a lack of binding to heparin. The two populations of AT III were separated by affinity chromatography on heparin-agarose. 50% of the patients' AT III bound to the agarose beads. The remainder, recovered in the supernatant, migrated in two-dimensional immunoelectrophoresis as a single peak with the same mobility in the presence or absence of heparin, and was devoid of heparin cofactor activity. This familial AT III variant characterized a reduced affinity for heparin is tentatively named 'Antithrombin III Paris'.