Antithrombin BM from human plasma: an antithrombin binding moderately to heparin.

A human antithrombin was purified app. 60 fold from Cohn fraction IV, to give a single band of about 70.000 molecular weight in polyacrylamide gel electrophoresis. Compared to the similar antithrombin III, this glycoprotein binds only moderately to porcine heparin (hence its name Antithrombin BM), thus requiring higher heparin concentration for full thrombin inhibitor function, and lower ionic strength for elution from a heparin sepharose column. In these respects it resembles "heparin cofactor A", which is, however, characterized by a substantially larger molecular weight. From AT III, AT BM further differs in its absolute dependency on the presence of heparin(oids) for antithrombin activity, in its more pronounced inhibitory specificity largely restricted to thrombin, and in the absence of substantial immunological crossreaction with antibody to AT III. Based on comparative measurements of antithrombin activity in the presence of different amounts of heparin, up to 40% of the antithrombin activity present in human blood may be attributed to AT BM. The in vivo role of this new inhibitor remains to be elucidated.