A Stoichiometric Assay for the Fourth Component of Complement in Whole Human Serum Using Eac′lagp and Functionally Pure Human Second Component

Abstract

Summary: The fourth component of complement (C′4hu) in whole human serum has been measured by a three-step titration based on its specific interaction with a stable intermediate of the hemolytic system, EAC′1agp. In this interaction the conversion of SAC′1agp to SAC′1agp4hu sites is stoichiometrically related to the relative C′4hu concentration in the reaction mixture; the reciprocal of the serum dilution which produces one SAC′1agp4hu per cell is arbitrarily defined as the number of C′4hu units per ml of serum. The specificity of the assay for C′4hu was supported by the finding that the assay of two preparations of C′4hu, purified by entirely different methods, was stoichiometric and optimal without alteration of the procedure. The application of the C′4hu titration to the study of patients with hereditary angioedema has shown it to be a reproducible and sensitive index of the in vivo activation of C′1.