C4a: the third anaphylatoxin of the human complement system.

The activation peptide C4a was isolated from C1s-cleaved C4, the fourth component of complement. The peptide appeared to be homogeneous by electrophoresis on cellulose acetate and by polyacrylamide gel electrophoresis. C4a has a molecular weight of 8650 and an electrophoretic mobility at pH 8.6 of +2.1 x 10(-5) cm2V-1 sec-1. Carboxypeptidase B released approximately 1 mol of arginine per mol of C4a. The partial COOH-terminal sequence was determined to be Leu-Gln-Arg-COOH. The isolated C4a was spasmogenic for guinea pig ileum at a concentration of 1 microM and it desensitized the muscle (i.e., produced tachyphylaxis) with respect to human C3a anaphylatoxin (at 0.33 microM) but not with respect to human C5a anaphylatoxin. Increased vascular permeability was observed in human skin after intradermal injection of 1 nmol of C4a, as evidenced by immediate erythema and edema formation. The spasmogenic, tachyphylactic, and vascular activities of C4a were abrogated by removal of the COOH-terminal arginine, a property that is characteristic also of the C3a and C5a anaphylatoxins. Contamination of C4a with either C3a or C5a has been ruled out by using radioimmunoassays for these peptides. Although C4a is considerably less active than are C3a and C5a, the present observations suggest that C4a constitutes a heretofore unrecognized anaphylatoxin that is related biologically and chemically to the activation peptides of C3 and C5.