The Inactivator of the First Component of Human Complement (C1INA): The Complex Formation with Plasmin

The reaction of C1INA with plasmin was followed by the stoichiometric inactivation of both activities. On acrylamide gel electrophoresis, the reaction mixture revealed 3 new substances. One formed a precipitin band against anti-C1INA and its molecular weight was 103,000 daltons, 14,000 less than that of C1INA, indicating that a portion of C1INA was partially cleaved by the proteolytic activity of plasmin. Each of the other two substances formed precipitin bands against anti-C1INA as well as against anti-plasminogen. Molecular weights of these two substances were 200,000 and 179,000 daltons, whereas the molecular weights of C1INA and plasmin are 117,000 and 82,000 daltons, respectively. From these results, it was concluded that a portion of C1INA in the reaction mixture was partially cleaved by plasmin, and the partially cleaved C1INA as well as the native C1INA form 1:1 molecular complexes with plasmin, leading to inactivation of these activities.