Studies on Human C′1-Esterase

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Abstract
Summary: Partially purified human C′1-esterase is antigenic in the rabbit. The immunizing antigen contained seven components by immunoelectrophoresis, only two of which were demonstrable in highly purified preparations of C′1-esterase. Antigenic similarity of C′1s and C′1-esterase was observed both by Ouchterlony gel diffusion and immunoelectrophoresis, providing additional evidence for the identification of C′1s as C′1-proesterase. Anti-C′1-esterase in suitable concentrations completely inhibited the inactivation of C′4 by C′1-esterase, but only partially inhibited hydrolysis of ATE, a low molecular weight substrate. C′1-esterase completely inhibited by DFP was still capable of precipitating with anti-C′1-esterase. These data suggested that the antigenic and catalytic sites of C′1-esterase are distinct. The mechanism of inhibition by antibody is probably referable to steric hindrance or distortion of the catalytic site. Anti-C′1-esterase inhibited hemolytic function of whole complement. Studies with the intermediate complexes EAC′1, EAC′1,4, and EAC′4 indicated that most of the antibody activity was directed toward C′1 and further implied the functional participation of C′1-esterase in immune hemolysis. At high concentrations of antibody, some agglutination of complexes which contained C′4 occurred, probably as a result of low titers of anti-C′4 in the antiserum. Anti-C′1-esterase did not inhibit the activity of the complex EAC′1,4,2, a finding consistent with previous work indicating that C′1 was not necessary for hemolysis once C′4 and C′2 had attached to the sensitized cell.

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