This study was initiated to investigate the properties and mechanism of action of the C3 inactivator, a serum protein which inhibits the hemolytic activity of cell bound C3. The C3 inactivator was found to be inseparable from the C3 proactivator on purification, and the final preparations, homogeneous on disc- and on immunoelectrophoresis, possessed both activities. Five separate preparations were isolated which differed slightly in charge and markedly in functional properties, suggesting the

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... istence of several distinct functional sites in the proactivator molecule. These preparations are: 1) 5–6S β-pseudoglobulins; 2) antigenically indistinguishable on Ouchterlony analysis with specific antisera; 3) able to hydrolyze certain synthetic amino acid esters, such as acetyl glycyl-l-lysine methyl ester; 4) fully able to complex with cobra factor. These preparations, however, differ markedly in: 1) ability to cleave C3 in free solution in the presence or absence of cobra factor, 2) activity/protein ratios in the C3 inactivator assay and 3) ability to inhibit the hemolytic activity of C3 proactivator depleted serum.