The active site of C3a anaphylatoxin

The essential active site responsible for the inflammatory activities of C3a, an anaphylatoxin derived from the serum complement system, has been elucidated using C3a peptides synthesized by the solid phase method and assayed for their ability to contract guinea pig ileal tissue and to produce a wheal and flare response in human skin. The COOH-terminal C3a pentapeptide (Leu-Gly-Leu-Ala-Arg) common to rat, pig, and man shows vascular and smooth muscle activity as well as specificity similar to natural human C3a. The porcine C3a octapeptide is 3 times more active than the common pentapeptide, but the human octapeptide (Ala(70)-Ser-His-Leu(73)-Gly-Leu(75)-Ala-Arg(77) is 12 times more active than the pentapeptide. Replacement of the serine and histidine by alanine or acetylation of the NH2 terminus provides analogues with the same activity as the octapeptide. Thus, the increased activity of the human C3a octapeptide over the pentapeptide appears to be related to the backbone of residues 70 to 72 and is not due to the presence of the hydroxyl group of serine-71, the imidazole ring of histidine-72, or a positive charge at or near the NH2 terminus. Since the COOH-terminal tetrapeptide is 40 times less active than the pentapeptide, an adequate model of the essential active site of C3a anaphylatoxin is the common COOH-terminal pentapeptide region. A C3a active site analogue containing a COOH-terminal lysyl residue is devoid of ileal activity. In addition, the [alanine-73]pentapeptide is 9 times less active and the [alanine-75]pentapeptide is at least 70 times less active than the active site pentapeptide in the ileal assay. Thus, the hydrophobic side chains of leucine-73 and leucine-75 and the guanidinium group of arginine-77 are important for the contractile activity of the active site COOH-terminal pentapeptide of human C3a anaphylatoxin.