C3a 70-77 (Complement 3a (70-77))

Modulation of human lymphocyte function by C3a and C3a(70-77)

Human C3a and the synthetic octapeptide C3a (70-77), which retains the activities of an anaphylatoxin, inhibit in a concentration-dependent manner the generation of leukocyte inhibitory factor (LIF) activity by human mononuclear leukocytes and T lymphocytes cultured with the mitogens phytohemagglutinin (PHA) or concanavalin A (Con A) or the antigen streptokinase-streptodornase (SK-SD). The generation of LIF activity was inhibited by 50% by 10(-8) M C3a or C3a(70-77) with PHA or Con A as the stimulus, whereas a more than 10-fold higher concentration of C3a(70-77) than C3a was required to achieve the same level of suppression with SK-SD as the stimulus. Similar concentrations of C3a(70-77) inhibited to the same extent the migration of T lymphocytes stimulated by alpha-thioglycerol of Con A. Neither C3a nor C3a(70-77) altered significantly the uptake of [3H]thymidine by human mononuclear cells exposed to PHA, Con A, or SK-SD. The capacity of C3a(70-77)-Sepharose,m but not Sepharose alone, to adsorb or inactivate mononuclear leukocytes required for the generation of LIF activity established a direct interaction. Analysis of the lymphocytes in the effluent from C3a(70-77)-Sepharose columns, using monoclonal antibodies to surface antigens, showed a selective depletion of the helper/inducer population of lymphocytes. C3a might represent an important mediator of the functionally selective regulation of human T lymphocyte activities by the complement system.

Contraction of guinea pig lung by synthetic oligopeptides related to human C3a

Complement-derived human C3a is a 77 residue protein whose biological activities include the contraction of guinea pig ileum and parenchymal lung strips. The C3a molecule is active at submicromolar concentrations and the spasmogenic activities are absolutely dependent on a carboxy-terminal arginyl residue. Studies with synthetic peptide analogues of C3a have localized the active site for all spasmogenic functions at the carboxy-terminal portion of the native molecule. Studies reported here demonstrate that the spasmogenic action of C3a on guinea pig parenchymal lung tissue is mimicked by synthetic peptides based on the carboxy-terminal sequence of C3a. Synthetic peptides with sequences corresponding to the 5, 8, 13 and 21 carboxy-terminal residues of C3a all possess spasmogenic activity on lung tissue. Molar activities of the synthetic peptides relative to that of C3a increase as the length of the peptide increases. The activity of the pentapeptide C3a 73-77 is only 0.5% that of C3a, while those of C3a 70-77 and C3a 65-77 are 3.8 and 7.8%, respectively. A 21 residue peptide, C3a 57-77, exhibits activity equivalent to native C3a. The synthetic peptides, unlike C3a, fail to produce tachyphylaxis. We compared C3a reactivity of guinea pig parenchymal lung strips with those of the synthetic C3a peptides in the presence of various inhibitor combinations. Responses of lung strips to C3a or the C3a peptides were not significantly inhibited by the antihistamine pyrilamine. However, lung responses to synthetic C3a peptides, like those to C3a, were inhibited by indomethacin. Complete inhibition of responses to C3a or the synthetic C3a peptides was produced in the presence of indomethacin, FPL55712 and pyrilamine.

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

Two peptides identical to the COOH-terminal sequence of human C3a anaphylatoxin and two 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. It is concluded that both the biological specificity and the activity of human C3a anaphylatoxin are determined by eight or fewer residues located at the COOH terminus of the natural protein. However, expression of full activity requires additional groups and the secondary conformational integrity of the C3a molecule.

Studies on the ileum-contracting mechanisms and identification as a complement C3a receptor agonist of oryzatensin, a bioactive peptide derived from rice albumin

Oryzatensin (Gly-Tyr-Pro-Met-Tyr-Pro-Leu-Pro-Arg) is an ileum-contracting and immunostimulating peptide derived from rice albumin. The mechanisms for the ileal contraction that it induces, consisting of rapid and slow components, were examined. The rapid contraction was mediated by histamine release and the slow contraction by a prostaglandin E2-like substance, judging from the effects of various pharmacological inhibitors and antagonists on ileal contraction and titration of histamine release. The contractile profile was very similar to that of human complement C3a(70-77), which is the COOH-terminal octapeptide of C3a and has, although less potent, qualitatively the same biological activities as C3a. Oryzatensin showed homology with C3a(70-77) and affinity for C3a receptors (IC50 = 44 microM) by radioreceptor assay. This is the first report of a food-derived bioactive peptide acting through complement C3a receptors.

Identification of casoxin C, an ileum-contracting peptide derived from bovine kappa-casein, as an agonist for C3a receptors

Casoxin C (Tyr-Ile-Pro-Ile-Gln-Tyr-Val-Leu-Ser-Arg) is a bioactive peptide that was isolated from a tryptic digest of bovine kappa-casein as an anti-opioid peptide in longitudinal strips of guinea pig ileum. Casoxin C also evokes contraction of the ileal strips, and we found that this process was biphasic with rapid and slow components. The contractile profile was very similar to that of human complement C3a(70-77), which is the COOH-terminal octapeptide of C3a and has, although less potent, qualitatively the same biological activities as C3a. Casoxin C also has homology with C3a(70-77). The rapid contraction was mediated by histamine release and the slow contraction was mediated by a prostaglandin E2-like substance, judging from the effects of various pharmacological inhibitors and antagonists on the ileal contraction. Casoxin C had affinity for C3a receptors (IC50 = 40 microM) in the radioreceptor assay. In addition, casoxin C showed phagocyte-stimulating activities. Casoxin C is therefore the first milk-derived peptide identified, that acts through complement C3a receptors.