The isolation and characterization of bovine C4a, an activation fragment of the fourth component of complement

The fourth component of bovine complement, C4, was cleaved specifically by subcomponent C1s to produce two fragments, C4a and C4b. The smaller, C4a, was isolated in pure form and is a peptide of 9500 mol.wt. containing approx. 84 amino acids and no detectable carbohydrate. C4a has an amino acid composition that is comparable with the anaphylatoxins C3a and C5a, containing six cysteine residues/mol and a high proportion of basic residues. The amino acid sequence of the first thirteen residues shows four identities with the porcine C3a sequence. There is almost complete identity between the C4a sequence and that of the alpha-chain of human C4, indicating that this region is highly conserved. This evidence also clearly establishes that C4a is cleaved from the N-terminal of the alpha-chain of C4.