A method has been described for the purification and isolation of the second component of complement (C'2) from human serum. The protein is a ß1-globulin with an approximate molecular weight of 117,000. Immunochemical analysis using a variety of specific antisera, including a monospecific antiserum to the isolated protein, indicate that the C'2 protein represents a heretofore unrecognized human serum constituent. Isolated C'2 contained 2 x 109 "effective molecules" per microgram and 1000 hemolytically active C'2 molecules were required to produce a single hemolytically effective C'2 site on erythrocytes undergoing immune cytolysis. C'1 esterase treatment of C'2 resulted in reduction of both its electrophoretic mobility and its molecular size, the latter observation indicating fragmentation of the molecule. Direct evidence was presented for the physical presence of C'2 as an integral part of the enzyme C'3 convertase.

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