The Biophysical and Functional Properties of Antithrombin III (AT III) in Vitro and in Vivo

Abstract

Human and rabbit AT III were purified by a two-step procedure of heparin-agarose affinity chromatography and Sephadex G150 gel filtration. The resulting preparation purified approximately 900 fold over plasma was homogeneous by SDS gel electrophoresis (SDSE), immunoelectrophoresis and gel exclusion chromatography (GEC). Both human and rabbit AT III possessed mol wt’s of approximately 68,000 daltons. As previously observed (Rosenberg and Damus: J. Biol. Chern., 248, 0490, 1973), we noted that human as well as rabbit AT III form firm stoichiometric complexes with thrombin demonstrable by SDSE and GEC. 125I labeled rabbit AT III possessing biophysical and biological properties identical to the unlabeled product was injected into rabbits. Radiolabeled AT III displayed complex multiphasic disappearance curves with a half-life of the major component of 36 hours. The apparent mol wt of AT III in vivo was reduced to approximately 30,000 daltons within minutes after its administration into rabbits as evidenced by GEC and SDSE. The 30,000 dalton AT III was as capable as the 68.000 dalton AT III to form firm stoichiometric complexes with thrombin. Thus, AT III may exist in different molecular forms possessing similar biological activities.