The Different Forms of Antithrombin III in Serum

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Abstract
Antithrombin III (AT III) complexes were isolated from human serum by affinity chromatography and gel filtration. In the first step of the preparation, using heparin-agarose chromatography, we observed that the complexed form of AT III bound less strongly to the gel than the free form and that about half of the AT III was free. With further purification a 2.5 × 105 molecular weight complex was isolated. Using 125I labelled human thrombin, this complex was radioactive indicating the presence of thrombin. Only in a synthetic thrombin-AT III system was a 9 × 104 molecular weight complex detected, but not in serum. These facts suggest that in serum AT III complexes may exist in a polymeric form. Also, an AT III antigen derived from the original AT III molecule, but not complexed, was isolated which may be a degradation product. Abbreviations used: AT-III, antithrombin III. Hepes, N-2-Hydroxyethylpiperazine-N-2-Ethanesulphonic acid. * A preliminary account of this work was presented at the one day Scientific meeting of the British Society of Haematology, held at the Middlesex Hospital, London, in January, 1976 (Pepper and Banhegyi, 1976). ** Present address: 1st Department of Medicine, Postgraduate Medical School, Budapest, Hungary. Correspondence and reprint requests to D. S. Pepper.