HEPARIN INDEPENDENT PURIFICATION OF ANTITHROMBIN III (AT III) BY IMMUNO-AFFINITY CHROMATOGRAPHY RESULTING IN A FUNCTIONALLY INTACT MOLECULE

Abstract

Previous methods for purification of AT III are based on its heparin-binding capacity. However, in congenital AT III deficiency abnormal inhibitor molecules with impaired binding of heparin and/or thrombin has been reported. The aim of the present study was to develop a purification method based on immuno-affinity chromatography, and thus independent of the heparin binding capacity.Rabbits were immunized with human AT III purified by a three-step procedure involving dextran sulphate precipitation, affinity chromatography on heparin-Sepharose and ion-exchange chromatography on DEAE-Sephadex A-50. Rabbit immunoglobulins against human AT III were isolated by affinity chromatography using purified human AT III coupled to CNBr-activated Sepharose 4B. Trace amounts of immunoglobulin against human albumin, IgG and IgM were removed by solid phase immunoadsorption. The highly purified immunoglobulins against human AT III were coupled to CNBr-activated Sepharose 4B. This anti-AT III-Sepharose was used for single-step purification of AT III from plasma. Elution was performed by Na-citrate buffer at pH 3.0 and the eluted fractions immediately neutralized. The recovery was more than 60%.The purified AT III appeared as a single protein band in SDS-poly-acrylamide gel electrophoresis with or without reduction. Affinity purified AT III and AT III purified by the three-step procedure were indistinguishable when analyzed by crossed immunoelectrophoresis in the absence and the presence of heparin isoelectrical focusing in polyacrylamid gel at a pH 4-6.5 gradient, and SDS-polyacrylamide gel electrophoresis. AT III antigen concentration was determined by electroimmunoassay and the reactive site concentration determined by titration with purified human thrombin using Phe-Pip-Arg-Nan (S-2238) as substrate. The ratio (active site conc.)/(antigen conc.) was identical in the two AT III preparations. It is concluded that this single-step immuno-affinity chromatography gives a high recovery from plasma of a highly purified functionally intact AT III molecule. The purification method is independent of the heparin binding capacity of AT III. This is of particular importance for the purification and characterization of abnormal AT III molecules with impaired heparin-binding site.