Synthetic peptides as antagonists of the anaphylatoxin C3a

Peptide compounds resembling the receptor‐binding C‐terminal domain of the anaphylatoxic peptide C3a were synthesized to examine two kinds of C3a antagonism: (a) specific desensitization of C3a‐sensitive cells and (b) competitive binding to the C3a receptor. We used guinea‐pig platelets, which express a C3a receptor and specifically release ATP upon stimulation, to evaluate the actions of the C3a analogues. The ATP liberation can be inhibited by pretreatment (i.e. desensitization) of the guinea‐pig platelets with substimulatory concentrations of C3a or its analogues. Compared to C3a, several peptides were found with at least a tenfold greater difference between the required concentrations for C3a‐specific half‐maximal desensitization (DD₅₀) and half‐maximal platelet activation (ED₅₀). The most potent compounds were YAAALKLAR and Fmoc‐EAALKLAR (Fmoc: 9‐fluorenylmethoxycarbonyl) with an ED₅₀/DD₅₀ of 140 ± 28 and 80 ± 17, respectively (mean ± standard deviation). The ED₅₀/DD₅₀ of human C3a was found to be only 6 ± 2. Some C3a derivatives were also tested in competitive binding studies for their ability to compete with C3a for receptor sites on guinea‐pig platelets. Three of them were considered partial antagonists [YRRGRLAR, YRRGRXLAR and YRRGRXLAR (X=6‐aminohexanoyl)] because their K_i were smaller than their ED₅₀ (K_i/ED₅₀= 0.6 ± 0.3, 0.5 ± 0.1 and 0.4 ± 0.2, respectively). Interestingly, the last two compounds also had ED₅₀/DD₅₀ values greater than 60. Common to all three peptides are N‐terminal arginine‐rich sequences and intramolecular disulfide bridges which introduce conformational constraint.