Identification of the second alpha-2-antiprotease of equine serum as antithrombin III

The alpha-2-protease inhibitor, of 65,000 daltons molecular weight, described by several authors in horse plasma and also present as a contaminant in alpha-1-isoinhibitor isolates previously described by us (Pellegrini & von Fellenberg (1980) Biochim. biophys. Acta 616, 351-361) has now been isolated to purity and identified as antithrombin III. The inhibitor is composed of a single polypeptide chain as judged by SDS polyacrylamide gel electrophoresis. The inhibitor was effective only against trypsin and thrombin. Serological cross-reaction existed between the inhibitor and the antiserum to human antithrombin III. An antiserum to our isolate, however, did not react with human antithrombin III. This confirms the results reported by Kurachi et al. (1976, Biochemistry 15, 368-372).