Synthetic peptides with the biological activities and specificity of human C3a anaphylatoxin.

Abstract

Two peptides identical to the COOH-terminal sequence of human C3a anaphylatoxin and 2 analogs were synthesized by the solid-phase method and tested for biological activity. The synthetic COOH-terminal octapeptide, C3a-(70-77) or Ala-Ser-His-Leu-Gly-Leu-Ala-Arg, caused contraction of guinea pig ileum and uterus, release of vasoactive amines from rat mast cells, and increased vascular permeability in guinea pig and human skin. On a molar basis, the synthetic octapeptide possessed 1-2% of the biological activities of C3a and specifically desensitized smooth muscle to stimulation by C3a. Like natural C3a, the synthetic C3a-(70-77) was inactivated by digestion with carboxypeptidase B [peptidyl-L-lysine(-L-arginine) hydrolase, EC 3.4.12.3], which removed the essential COOH-terminal arginine. A synthetic nonapeptide [C3a-(70-77)-Gly], containing a glycyl instead of an arginyl COOH terminus, was approximately 1% as active as the octapeptide when assayed with smooth muscle. The COOH-terminal 13-residue peptide of C3a, C3a-(65-77), was equal in activity to C3a-(70-77); similarly, C3a-(65-77)-Gly expressed the same activity as C3a-(70-77)-Gly. The biological specificity and the activity of human C3a anaphylatoxin are determined by 8 or fewer residues located at the COOH terminus of the natural protein. Expression of full activity requires additional groups and the secondary conformational integrity of the C3a molecule.