The ninth component of human complement: purification and physicochemical characterization.

A procedure for the isolation of the human complement (C) protein C9 is described. The procedure allowin. The purified protein has the electrophoretic mobility of an alpha-globulin, and is a single polypeptide chain with a m.w. of 71,000. No impurities were detected either on gel electrophoretic or immunochemical examination. C9 is a glycoprotein containing 7.8% carbohydrate, and in terms of residues per mole, 3.0 glucosamine, 17.6 neutral hexose, and 7.4 sialic acid. Its amino acid composition is typical of a globular serum protein. Upon automated Edman degradation of reduced and alkylated C9, no amino acid residues were released, suggesting a blocked N-terminus. The concentration of C9 in normal human serum is 58 +/- 8 microgram/ml. A high titer rabbit antiserum was produced and employed to immunochemically deplete serum of C9. The CH50 of the C9-depleted serum was identical to that of whole human serum; however, membrane fragments of erythrocytes lysed by C9-depleted serum lacked the typical ultrastructural C lesions, which constitute the dimeric membrane attack complex.