Isolation and partial characterization of two distinct types of antithrombin III from rabbit

Heparin-Sepharose chromatography of rabbit plasma or serum yields two fractions of free antithrombin III (AT). The first of these elutes at a lower salt concentration, represents about 90% of the AT in plasma, and has an approximately 2000 dalton higher molecular weight by SDS polyacrylamide electrophoresis. An antibody to the lower affinity species reacts with the second form. The higher affinity AT is not formed from the lower affinity type during blood coagulation as demonstrated by approximately equal levels in plasma and serum, and lack of conversion of 125I-labelled lower affinity AT to the higher affinity form during blood coagulation. Heparin cofactor and progressive AT activities of the two forms are essentially identical when assayed by chromogenic substrates. The forms are separable by crossed immunoelectrophoresis.