High‐Molecular‐Weight Kininogen from Horse Plasma

High‐molecular‐weight (high‐ M r ) kininogen was purified from horse plasma by chromatography on columns of DEAE‐Sephadex A‐50, CM‐Sephadex C‐50, p ‐chlorobenzylamine‐Sepharose and Sephadex G‐150. The yield was about 150 mg from 8 l of fresh plasma. The purified material gave a single band on sodium dodecylsulfate/ polyacrylamide gel electrophoresis and a single precipitin line on immunodiffusion and immunoelectrophoresis. The molecular weight of horse high‐ M r kininogen was estimated to be 78000 by dodecylsulfate gel electrophoresis using the Ferguson plot. Its polypeptide content was determined to be 86% by amino acid analysis arid there was a total of 581 amino acid residues/molecule of protein. The kininogen contained a total of 13.9% carbohydrates, consisting of hexoses (7.8%), glucosamine (1.9%], galactosamine (0.6%) and sialic acid (3.6%). On incubation of horse high‐ M r kininogen with bovine and horse plasma kallikreins, several fragments which contained extremely high levels of histidine, were liberated, in addition to kinin. After the liberation of kinin and histidine‐rich fragments, a protein free of kinin and its fragments was isolated. This protein consisted of two polypeptide chains, heavy chain and light chain, which are bridged by disulfide bonds. The molecular weight and amino acid composition of the heavy chain and the light chain from horse high‐ M r kininogen were very similar to those of the heavy and light chains from bovine high‐ M r kininogen, respectively. From these results, it was revealed that horse high‐ M r kininogen is quite similar to bovine high‐ M r kininogen in terms of their physicochemical and chemical properties, although they are immunologically distinguishable.