Circular dichroism (CD) studies on biological activity of mast cell degranulating (MCD) peptide analogs

[Ala12]MCD peptide: a lead peptide to inhibitors of immunoglobulin E binding to mast cell receptors1

An effort was made to discover mast cell degranulating (MCD) peptide analogs that bind with high affinity to mast cell receptors without triggering secretion of histamine or other mediators of the allergic reaction initiated by immunoglobulin E (IgE) after mast cell activation. Such compounds could serve as inhibitors of IgE binding to mast cell receptors. An alanine scan of MCD peptide reported previously showed that the analog [Ala12]MCD was 120-fold less potent in histamine-releasing activity and fivefold more potent in binding affinity to mast cell receptors than the parent MCD peptide. Because this analog showed marginal intrinsic activity and good binding affinity it was subsequently tested in the present study as an IgE inhibitor. In contrast to MCD peptide, [Ala12]MCD showed a 50% inhibition of IgE binding to the Fc epsilon RI alpha mast cell receptor by using rat basophilic leukemia (RBL-2H3) mast cells and fluorescence polarization. Furthermore, in a beta-hexosaminidase secretory assay, the peptide also showed a 50% inhibition of the secretion of this enzyme caused by IgE. An attempt was made to relate structural changes and biologic differences between the [Ala12]MCD analog and the parent MCD peptide. The present results show that [Ala12]MCD may provide a base for designing agents to prevent IgE/Fc epsilon RI alpha interactions and, consequently, allergic conditions.

Further studies on the structural requirements for mast cell degranulating (MCD) peptide-mediated histamine release

Mast cell degranulating (MCD) peptide was modified in its two disulfide bridges and in the two arginine residues in order to measure the ability of these analogs to induce histamine release from mast cells in vitro. Analogs prepared were [Ala(3,15)]MCD, [Ala(5,19)]MCD, [Orn(16)]MCD, and [Orn(7,16)]MCD. Their histamine-releasing activity was determined spectrofluorometrically with peritoneal mast cells. The monocyclic analogs in which the cysteine residues were replaced pairwise with alanine residues showed three-to ten-fold diminished histamine-releasing activity respectively, compared with the parent MCD peptide. Substantial increases in activity were observed where arginine residues were replaced by ornithines. The ornithine-mono substituted analog showed an almost six-fold increase and the ornithine-doubly substituted analog three-fold increase in histamine-releasing activity compared with the parent MCD peptide. The structural changes associated with these activities were followed by circular dichroism (CD) spectroscopy. Changes in the shape and ellipticity of the CD spectra reflected a role for the disulfide bonds and the two arginine residues in the overall conformation and biological activity of the molecule.

Mast cell degranulating peptide binds to RBL-2H3 mast cell receptors and inhibits IgE binding

Fluorescent and biotinylated analogs of mast cell degranulating (MCD) peptide were synthesized and the labels fluoresceinisothiocyanate and N-hydroxysuccinimidobiotin were conjugated at position 1 in the MCD peptide sequence. The analogs with these moieties retained histamine-releasing activity as high as that of the parent MCD peptide in rat peritoneal mast cell assays. These labeled analogs were used in rat basophilic leukemia cells (RBL-2H3) to demonstrate by confocal microscopy and flow cytometry the specific binding of MCD peptide to mast cell receptors. Consequently MCD peptide was found to compete with and inhibit the binding of fluorescent IgE on RBL cells as monitored by flow cytometry. Thus MCD peptide may prove to be useful in the study of IgE receptor-bearing cells.

Mast cell degranulating (MCD) peptide: a prototypic peptide in allergy and inflammation

The solid phase synthesis of mast degranulating peptide (MCD peptide) raised the possibility of preparing analogs and examining the pharmacology and the proposed role of this peptide as a potential agent in allergy and inflammation. MCD peptide, a cationic 22-amino acid residue peptide with two disulfide bridges, causes mast cell degranulation and histamine release at low concentrations and has anti-inflammatory activity at higher concentrations. Because of these unique immunologic properties, MCD peptide may serve as a useful tool for studying secretory mechanisms of inflammatory cells such as mast cells, basophils, and leukocytes, leading to the design of compounds with therapeutic potential.

Bioactivities and secondary structure of mast cell degranulating (MCD) peptide analogs

Three analogs of Mast Cell Degranulating (MCD) peptide with C-terminal and one analog with N- and C-terminal deletions were synthesized and assayed for histamine-releasing activity in mast cells. Des(20-22)-MCD and des(21)-MCD markedly decreased this activity. In des(16-17,21)-MCD this activity was completely abolished. By contrast, when the C- and N-termini were truncated in des(1-2,20-22)-MCD, the full activity of MCD peptide was restored. The possibility that these analogs trigger or inhibit oxygen radicals from neutrophils was examined in a cell-free system with a chemiluminescence assay. None of the above analogs exhibited inflammatory or anti-inflammatory activity. Changes in biological activities were correlated with structural changes, as seen by circular dichroism (CD) spectroscopy.

Influence of alpha-helicity, amphipathicity and D-amino acid incorporation on the peptide-induced mast cell activation

Mast cell activation by polycationic substances is believed to result from a direct activation of G protein alpha subunits and it was suggested that the adaption of amphipathic, alpha-helical conformations would allow the peptide to reach the cytosolic compartment to interact with G proteins (Mousli et al., 194, Immunopharmacology 27, 1, for review). We investigated the histamine-releasing activity of model peptides as well as analogues of magainin 2 amide and neuropeptide Y with different amphipathicities and alpha-helix content on rat peritoneal mast cells. Amphipathic helicity is not a prerequisite for mast cell activation. Moreover, non-helical magainin peptides with high histamine-releasing activity were less active in the liberation of carboxyfluoresceine from negatively charged liposomes, indicating that peptide-induced mast cell activation and peptide-induced membrane perturbation do not correlate. In contrast to the negligible influence of the secondary structure, amino acid configuration may exert a striking influence on peptide-induced mast cell activation. Thus histamine-release by substance P was markedly impaired when the L-amino acids in the positively charged N-terminal region were replaced by D-amino acids, with [D-Arg1)substance P being the most inactive substance P diastreoisomer.