Partial Characterization of Human C5a Anaphylatoxin

Abstract

Human C5a was isolated from complement-activated serum and was characterized for protein and carbohydrate content. The purified C5a was judged to be homogeneous by both polyacrylamide gel electrophoresis and immunologic techniques. The polypeptide moiety of C5a contains 73 amino acid residues which represent a m.w. of 8,200. Analysis of the carbohydrate moiety in C5a indicated 4 moles of glucosamine, 3 to 4 moles of sialic acid, 4 moles of mannose and 2 moles of galactose. The total carbohydrate content in C5a, therefore, amounts to approximately 25% of the apparent m.w. of the anaphylatoxin molecule. The protein and carbohydrate portions of C5a together equal a m.w. of approximately 11,000 which is considerably less than the 15 to 16,000 indicated by physical measurements. Human C5a contains a COOH-terminal arginine which is essential for anaphylatoxin activity and a sequence of Gln-Leu-Gly-Arg-COOH at the COOH-terminus which compares favorably with that of human C3a (Gly-Leu-Ala-Arg-COOH). Additional similarities between the C3a and C5a molecules include length of the polypeptide chain, number of disulfide bonds and an absence of tryptophan residues. A major chemical difference does exist between these two human anaphylatoxins, namely that carbohydrate is associated with C5a but is absent in the C3a molecule. The partial NH₂-terminal sequence of C5a was determined as NH₂-Thr-Leu-Glx-Lys-Lys-Ile-Glx-Glx-Ile-Ala- and direct comparison with the known sequence of human C3a shows little homology.