The ninth component of human complement: isolation, description and mode of action.

A method is described which permits isolation of C′9 from human serum. The final product was homogeneous by polyacrylamide gel electrophoresis and the distribution of C′9 haemolytic activity corresponded to the position of the protein disc. C′9 is an α-globulin which has a sedimentation coefficient of 4.5S, a diffusion coefficient of 5.1 × 10 cm/sec and a calculated molecular weight of 79,000. The reaction of C′9 with EAC′1∼8 was found to be temperature dependent and to proceed optimally at pH 7. In the course of this reaction C′9 activity declined in the fluid phase and C′9 protein became specifically attached to the cells, as revealed by the use of radioactively labelled C′9. The haemolytic activity of C′9 was mimicked by bipyridine which exhibited the previously reported phenanthroline effect. Kinetic experiments showed that 0.09 M EDTA, which inhibits haemolysis, acts subsequent to the C′9 reaction step. Possible modes of action of C′9 are discussed.