title: Synthetic peptides as antagonists of the anaphylatoxin C3a.
abstract: 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 [YRRGRCGGLCLAR, YRRGRXCGGLCLAR and YRRGRXCGALCLAR (X = 6-aminohexanoyl)] because their Ki were smaller than their ED50 (Ki/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.
authors.affiliation: Institut für Medizinische Mikrobiologie, Medizinische Hochschule Hannover, Federal Republic of Germany.
authors.forename: T
authors.initials: T
authors.lastname: Kretzschmar
authors.forename: M
authors.initials: M
authors.lastname: Pohl
authors.forename: M
authors.initials: M
authors.lastname: Casaretto
authors.forename: M
authors.initials: M
authors.lastname: Przewosny
authors.forename: W
authors.initials: W
authors.lastname: Bautsch
authors.forename: A
authors.initials: A
authors.lastname: Klos
authors.forename: D
authors.initials: D
authors.lastname: Saunders
authors.forename: J
authors.initials: J
authors.lastname: Köhl
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001667:Binding, Competitive; D002478:Cells, Cultured; D015926:Complement C3a / Q000037:antagonists & inhibitors* / Q000378:metabolism / Q000494:pharmacology; D006168:Guinea Pigs; D006801:Humans; D007457:Iodine Radioisotopes; D008969:Molecular Sequence Data; D010455:Peptides / Q000494:pharmacology*; D015539:Platelet Activation / Q000187:drug effects
chemical_list: D007457:Iodine Radioisotopes; D010455:Peptides; D015926:Complement C3a