The purification of human C8 in milligram quantities from outdated human serum was achieved by ammonium sulfate precipitation (37.5-50% saturation) and ion exchange column chromatography employing CM-32 cellulose and QAE-Sephadex. The yield of C8 activity ranged from 2-9%, and the average purification was 1,700-fold. Fully reduced C8 was shown by SDS polyacrylamide gel electrophoresis to have three polypeptide chains which were present in equimolor ratios: alpha, 77,000 daltons; beta, 63,000 daltons; and gamma, 13,700 daltons. C8 denaturation by SDS and urea in the absence of reducing agents revealed two noncovalently linked subunits: alpha-gamma, 99,000 daltons, and beta, 75,000 daltons.

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