Fifth component of guinea pig complement: purification and characterization

The 5th component of complement (C5) was purified from guinea pig serum. The 6-step procedure, involving removal of C1 by precipitation at pH 7.5, mu = 0.04, 2.0 M ammonium sulfate precipitation, acid precipitation at pH 5.6 mu = 0.1, and successive chromatographies on Sephadex G-200, DEAE-cellulose, and hydroxylapatite columns, yielded 1.6 to 4 mg of C5 from 250 ml of serum. Purified C5 gave 1 protein band on disc polyacrylamide gel electrophoresis (PAGE) and sodium dodecylsulfate-(SDS) PAGE. Guinea pig C5, like human C5, consisted of 2 polypeptide chains designated as alpha (m.w. of 108,000) and beta (m.w. of 79,000) linked together by disulfide bonds. The amino acid composition was also very similar to that of human C5. The amino-terminus of the alpha-chain was aspartic acid or asparagine, and that of the beta-chain was undetectable by the dansyl method. Limited proteolysis of C5 with trypsin caused virtually no alteration in its mobility on immunoelectrophoresis and SDS-PAGE without reduction. Cleavage with trypsin was restricted to the alpha-chain: the beta-chain was totally resistant to the digestion. The alpha-chain was split into at least 4 fragments of 58,000, 34,000, 29,000, and 27,000 daltons linked to one another and/or to the beta-chain by disulfide bonds.