Further studies of the turnover of dog antithrombin III. Study of 131I-labelled antithrombin protease complexes

Fresh plasma containing 131I-antithrombin III (∗I-AT) was coagulated and incubated at 37°C for 2 hr. A “complex peak”, separated on heparin-agarose contained AT and ∗I-AT antigen but no heparin cofactor activity. Crossed immunoelectrophoresis showed only AT complexes. SDS PAGE showed 80% of the ∗I-AT in a major band (∼80,000 daltons), 15% in a minor band (∼100,000 daltons) and the rest in trace bands (∼60,000 and/or 115,000 daltons). Ammonia treatment of the complex peak released α-thrombin. After i.v. injection 80% of the complexed ∗I-AT, chiefly as the major band, left the plasma with t 1 2 ∼15 min and was almost immediately catabolized to low molecular weight breakdown products. A major catabolic site was the liver. A simple kinetic model describes the findings approximately.