Further studies on the structural requirements for polypeptide-mediated histamine release from rat mast cells

MRGPRX2 activation as a rapid, high-throughput mechanistic-based approach for detecting peptide-mediated human mast cell degranulation liabilities

Mast cells play key roles in allergy, anaphylaxis/anaphylactoid reactions, and defense against pathogens/toxins. These cells contain cytoplasmic granules with a wide spectrum of pleotropic mediators that are released upon activation. While mast cell degranulation (MCD) occurs upon clustering of the IgE receptor bound to IgE and antigen, MCD is also triggered through non-IgE-mediated mechanisms, one of which is via Mas-related G protein-coupled receptor X2 (MRGPRX2). MRGPRX2 can be activated by many basic biogenic amines and peptides. Consequently, MRGPRX2-mediated MCD is an important potential safety liability for peptide therapeutics. To facilitate peptide screening for this liability in early preclinical drug development, a rapid, high-throughput engineered CHO-K1 cell-based MRGPRX2 activation assay was evaluated and compared to histamine release in CD34⁺ stem cell-derived mature human mast cells as a reference assay, using 30 positive control and 29 negative control peptides for MCD. Both G protein-dependent (Ca²⁺ endpoint) and -independent (β-arrestin endpoint) pathways were assessed in the MRGPRX2 activation assay. The MRGPRX2 activation assay had a sensitivity of 100% for both Ca²⁺ and β-arrestin endpoints and a specificity of 93% (β-arrestin endpoint) and 83% (Ca²⁺ endpoint) compared to histamine release in CD34⁺ stem cell-derived mature human mast cells. These findings suggest that assessing MRGPRX2 activation in an engineered cell model can provide value as a rapid, high-throughput, economical mechanism-based screening tool for early MCD hazard identification during preclinical safety evaluation of peptide-based therapeutics.

Ligands and Signaling of Mas-Related G Protein-Coupled Receptor-X2 in Mast Cell Activation

Mas-related G protein-coupled receptor-X2 (MRGPRX2) is known as a novel receptor to activate mast cells (MCs). MRGPRX2 plays a dual role in promoting MC-dependent host defense and immunomodulation and contributing to the pathogenesis of pseudo-allergic drug reactions, pain, itching, and inflammatory diseases. In this article, we discuss the possible signaling pathways of MCs activation mediated by MRGPRX2 and summarize and classify agonists and inhibitors of MRGPRX2 in MCs activation. MRGPRX2 is a low-affinity and low-selectivity receptor, which allows it to interact with a diverse group of ligands. Diverse MRGPRX2 ligands utilize conserved residues in its transmembrane (TM) domains and carboxyl-terminus Ser/Thr residues to undergo ligand binding and G protein coupling. The coupling likely initiates phosphorylation cascades, induces Ca²⁺ mobilization, and causes degranulation and generation of cytokines and chemokines via MAPK and NF-κB pathways, resulting in MCs activation. Agonists of MRGPRX2 on MCs are divided into peptides (including antimicrobial peptides, neuropeptides, MC degranulating peptides, peptide hormones) and nonpeptides (including FDA-approved drugs). Inhibitors of MRGPRX2 include non-selective GPCR inhibitors, herbal extracts, small-molecule MRGPRX2 antagonists, and DNA aptamer drugs. Screening and classifying MRGPRX2 ligands and summarizing their signaling pathways would improve our understanding of MRGPRX2-mediated physiological and pathological effects on MCs.

Rescuing the Last-Line Polymyxins: Achievements and Challenges

Antibiotic resistance is a major global health challenge and, worryingly, several key Gram negative pathogens can become resistant to most currently available antibiotics. Polymyxins have been revived as a last-line therapeutic option for the treatment of infections caused by multidrug-resistant Gram negative bacteria, in particular Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacterales. Polymyxins were first discovered in the late 1940s but were abandoned soon after their approval in the late 1950s as a result of toxicities (e.g., nephrotoxicity) and the availability of "safer" antibiotics approved at that time. Therefore, knowledge on polymyxins had been scarce until recently, when enormous efforts have been made by several research teams around the world to elucidate the chemical, microbiological, pharmacokinetic/pharmacodynamic, and toxicological properties of polymyxins. One of the major achievements is the development of the first scientifically based dosage regimens for colistin that are crucial to ensure its safe and effective use in patients. Although the guideline has not been developed for polymyxin B, a large clinical trial is currently being conducted to optimize its clinical use. Importantly, several novel, safer polymyxin-like lipopeptides are developed to overcome the nephrotoxicity, poor efficacy against pulmonary infections, and narrow therapeutic windows of the currently used polymyxin B and colistin. This review discusses the latest achievements on polymyxins and highlights the major challenges ahead in optimizing their clinical use and discovering new-generation polymyxins. To save lives from the deadly infections caused by Gram negative "superbugs," every effort must be made to improve the clinical utility of the last-line polymyxins. SIGNIFICANCE STATEMENT: Antimicrobial resistance poses a significant threat to global health. The increasing prevalence of multidrug-resistant (MDR) bacterial infections has been highlighted by leading global health organizations and authorities. Polymyxins are a last-line defense against difficult-to-treat MDR Gram negative pathogens. Unfortunately, the pharmacological information on polymyxins was very limited until recently. This review provides a comprehensive overview on the major achievements and challenges in polymyxin pharmacology and clinical use and how the recent findings have been employed to improve clinical practice worldwide.

Insulinoma-Associated-1: From Neuroendocrine Tumor Marker to Cancer Therapeutics

Insulinoma-associated-1 (IA-1 or INSM1) encodes a zinc-finger transcription factor, which was isolated from a human insulinoma subtraction library, with specific expression patterns, predominantly in developing neuroendocrine tissues and tumors. INSM1 is key in early pancreatic endocrine, sympatho-adrenal lineage, and pan-neurogenic precursor development. Insm1 gene ablation results in impairment of pancreatic β cells, catecholamine biosynthesis, and basal progenitor development during mammalian neocortex maturation. Recently, INSM1 has emerged as a superior, sensitive, and specific biomarker for neuroendocrine tumors. INSM1 regulates downstream target genes and exhibits extranuclear activities associated with multiple signaling pathways, including Sonic Hedgehog, PI3K/AKT, MEK/ERK^1/2, ADK, p53, Wnt, histone acetylation, LSD1, cyclin D1, Ascl1, and N-myc. Novel strategies targeting INSM1-associated signaling pathways facilitate the suppression of neuroendocrine tumor growth. In addition, INSM1 promoter-driven reporter assay and/or suicide gene therapy are promising effective therapeutic approaches for targeted specific neuroendocrine tumor therapy. In this review, the current knowledge of the biological role of INSM1 as a neuroendocrine tumor biomarker is summarized, and novel strategies targeting multiple signaling pathways in the context of INSM1 expression in neuroendocrine tumors are further explored. IMPLICATIONS: Neuroendocrine transcription factor (INSM1) may serve as a neuroendocrine biomarker for the development of novel cancer therapeutics against neuroendocrine tumors.

A Breath of Fresh Air in the Fog of Antimicrobial Resistance: Inhaled Polymyxins for Gram-Negative Pneumonia

Despite advancements in therapy, pneumonia remains the leading cause of death due to infectious diseases. Novel treatment strategies are desperately needed to optimize the antimicrobial therapy of patients suffering from this disease. One such strategy that has recently garnered significant attention is the use of inhaled antibiotics to rapidly achieve therapeutic concentrations directly at the site of infection. In particular, there is significant interest in the role of inhaled polymyxins for the treatment of nosocomial pneumonia, including ventilator-associated pneumonia, due to their retained activity against multi-drug resistant Gram-negative pathogens, including Acinetobacter baumannii and Pseudomonas aeruginosa. This review will provide a comprehensive overview of the pharmacokinetic/pharmacodynamic profile, clinical outcomes, safety, and potential role of inhaled polymyxins in clinical practice.

Hypotension Following Treatment with Aerosolized Colistin in a Patient with Multidrug-Resistant Pseudomonas aeruginosa

Objective:

To report a case of a man who developed temporary hypotension after aerosolized colistin administration.

Case Summary:

A 62-year-old Arab male was admitted to the intensive care unit for respiratory failure and septic shock. Simultaneous therapy using intravenous and aerosolized colistin was initiated for the management of multidrug-resistant Pseudomonas aeruginosa. A significant but transient drop in blood pressure occurred when aerosolized colistin was introduced. However, when it was stopped, but intravenous administration was continued, no hypotension was observed. Moreover, the combined use of aerosolized amikacin with intravenous colistin did not significantly affect blood pressure.

Discussion:

It is widely accepted that aerosolization allows safe administration of colistin in the absence of significant systemic adverse effects. However, in our patient, hypotension was observed with aerosolized colistin, but not with the systemic formulation. The lack of adverse effects with administration of aerosolized amikacin in this patient demonstrates the safety of the aerosolization technique. Use of the Naranjo scale indicated a probable relationship between hypotension and aerosolized colistin administration.

Conclusions:

This case suggests that hypotension may be induced with administration of aerosolized colistin. Although this effect is rare, clinicians should be aware that hypotension may develop in critically ill patients following aerosolized colistin treatment.

Phase I studies of CBP501, a G2 checkpoint abrogator, as monotherapy and in combination with cisplatin in patients with advanced solid tumors

PURPOSE

Two phase I dose-escalation studies were conducted to determine the maximum tolerated dose (MTD) and safety profile of the G(2) checkpoint abrogator CBP501, as a single agent and in combination with cisplatin.

EXPERIMENTAL DESIGN

Patients with advanced solid tumors were treated with CBP501 alone (D1/D8/D15, q4w, from 0.9 mg/m(2)), or with cisplatin (both on D1, q3w, from 3.6 mg/m(2) CBP501, 50 mg/m(2) cisplatin). Dose escalation proceeded if dose-limiting toxicity (DLT) was observed in 1 or less of 3 to 6 patients; CBP501 dose increments were implemented according to the incidence of toxicity. MTD was determined from DLTs occurring during the first two cycles.

RESULTS

In the combination study, the DLT was a histamine-release syndrome (HRS) occurring 10 to 60 minutes after initiating infusion that was attenuated by prophylaxis comprising dexamethasone, diphenhydramine, ranitidine, and loratadine. The MTD was 25 mg/m(2) CBP501 and 75 mg/m(2) cisplatin, with two patients at the highest dose (36.4 mg/m(2) CBP501, 75 mg/m(2) cisplatin) experiencing grade 3 HRS. The only DLT with monotherapy was transient G(3) rise of troponin in one patient. Grade 3 to 4 treatment-related events were rare. Promising activity was observed with CBP501/cisplatin, mainly in ovarian and mesothelioma patients who had previously progressed on platinum-containing regimens. Among ovarian cancer patients, low expression of DNA repair proteins was associated with partial response or stable disease.

CONCLUSIONS

CBP501 is well tolerated in patients as monotherapy and with cisplatin. At the recommended phase II dose (RP2D), the combination is feasible and HRS manageable with prophylaxis. Evidence of antitumor activity was observed in platinum-resistant patients.

The mast cell in innate and adaptive immunity

Mast cells (MCs) were once considered only as effector cells in pathogenic IgE- and IgG-mediated responses such as allergy. However, developments over the last 15 years have suggested that MCs have evolved in vertebrates as beneficial effector cells that are involved in the very first inflammatory responses generated during infection. This pro-inflammatory environment has been demonstrated to be important for initiating innate responses in many different models of infection and more recently, in the development of adaptive immunity as well. Interestingly this latter finding has led to the discovery that small MC-activating compounds can behave as adjuvants in vaccine formulations. Thus, our continued understanding of the MC in the context of infectious disease is likely to not only expand our scope of the MC in the normal processes of immunity, but provide new therapeutic targets to combat disease.

The nociceptive and anti-nociceptive effects of bee venom injection and therapy: a double-edged sword

Bee venom injection as a therapy, like many other complementary and alternative medicine approaches, has been used for thousands of years to attempt to alleviate a range of diseases including arthritis. More recently, additional theraupeutic goals have been added to the list of diseases making this a critical time to evaluate the evidence for the beneficial and adverse effects of bee venom injection. Although reports of pain reduction (analgesic and antinociceptive) and anti-inflammatory effects of bee venom injection are accumulating in the literature, it is common knowledge that bee venom stings are painful and produce inflammation. In addition, a significant number of studies have been performed in the past decade highlighting that injection of bee venom and components of bee venom produce significant signs of pain or nociception, inflammation and many effects at multiple levels of immediate, acute and prolonged pain processes. This report reviews the extensive new data regarding the deleterious effects of bee venom injection in people and animals, our current understanding of the responsible underlying mechanisms and critical venom components, and provides a critical evaluation of reports of the beneficial effects of bee venom injection in people and animals and the proposed underlying mechanisms. Although further studies are required to make firm conclusions, therapeutic bee venom injection may be beneficial for some patients, but may also be harmful. This report highlights key patterns of results, critical shortcomings, and essential areas requiring further study.

Structure, expression, and biological function of INSM1 transcription factor in neuroendocrine differentiation

Zinc-finger transcription factors are DNA-binding proteins that are implicated in many diverse biological functions. INSM1 (formerly IA-1) contains five zinc-finger motifs and functions as a transcription factor. INSM1 protein structure is highly conserved in homologues of different species. It is predominantly expressed in developing neuroendocrine tissues and the nervous system in mammals. INSM1 represents an important player in early embryonic neurogenesis. In pancreatic endocrine cell differentiation, Ngn3 first activates INSM1 and subsequently NeuroD/beta2. Conversely, INSM1 exerts a feedback mechanism to suppress NeuroD/beta2 and its own gene expression. INSM1 gene ablation in the mouse results in the impairment of pancreatic endocrine cell maturation. Further, deletion of INSM1 severely impairs catecholamine biosynthesis and secretion from the adrenal gland that results in early embryonic lethality. Genetically, INSM1 acts as a downstream factor of Mash 1 and Phox2b in the differentiation of the sympatho-adrenal lineage. In the developing neocortex, mouse embryos lacking INSM1 expression contain half the number of basal progenitors and show a reduction in cortical plate radial thickness. Cell signaling studies reveal that INSM1 contributes to the induction of cell cycle arrest/exit necessary to facilitate cellular differentiation. INSM1 is highly expressed in tumors of neuroendocrine origin. Hence, its promoter could serve as a tumor-specific promoter that drives a specific targeted cancer gene therapy for the treatment of neuroendocrine tumors. Taken together, all of these features of INSM1 strongly support its role as an important regulator during neuroendocrine differentiation.

Effects of human hair and nail proteins and their films on rat mast cells

Human hair and nail are valuable materials for producing individual corresponding biocompatible materials. A rapid and convenient protein extraction method (Shindai method) and novel procedures for preparing their protein films from their extracts have been developed using human hair and nail. The effects of the human hair and nail proteins and their films on histamine release from rat peritoneal mast cells were investigated. Both protein solutions and their films, mainly consisting of keratins and matrix proteins, did not induce histamine release from the mast cells. Scanning electron microscopy (SEM) also showed that the mast cells were only slightly affected by adding the human hair and nail proteins or by incubating on their protein films. The IgE-dependent histamine release was inhibited by the hair and nail proteins and their films. Incubation of the mast cells with the hair and nail proteins prior to the addition of the IgE serum resulted in a high inhibition (50%) of the histamine release, while the inhibition was approximately 10% when the protein solutions were mixed with the mast cells after incubation with the IgE serum. These results suggest that the human hair and nail proteins and their films will be useful materials for antiallergic actions.

The use of synthetic peptides in the delineation of immunoglobulin antigenic epitopes and Fc effector functions

As an alternative strategy to the use of proteolytic and chemical cleavage in the production of fragments of immunoglobulins retaining Fc effector functions, peptides representative of amino acid sequences constituting the putative active sites have been synthesized and assessed for biological activity in various in vitro systems. This approach has been adopted in attempts to define more precisely the autoantigenic epitope on human IgG against which anti-gamma-globulin antibodies (the so-called general 'rheumatoid factors'), found in the sera and joint fluids of patients with rheumatoid arthritis, are directed. Synthetic peptides representative of epsilon-chain sequences are being used in the production of antibodies (polyclonal and monoclonal) directed against specific epitopes within the Fc regions of human and rat IgE. The ability of these antisera to influence the in vitro functional properties of IgE anaphylactic antibodies is now under investigation, with particular attention being focused on cytophilicity and mast cell triggering. Preliminary findings suggest that certain of the antisera might be capable of inhibiting mast cell sensitization by IgE antibodies, and therefore might form the basis of a new type of anti-allergy compound.

Bronchial constriction and inhaled colistin in cystic fibrosis

STUDY OBJECTIVE

Inhaled colistin is used for the treatment of Pseudomonas aeruginosa infection in cystic fibrosis (CF) patients despite reports of chest tightness and bronchospasm. The main objective of the study was to assess whether bronchospasm occurred in pediatric CF patients with or without clinical evidence of airway hyperreactivity.

DESIGN AND METHODS

A prospective placebo-controlled clinical trial with crossover design was devised using challenge tests with 75 mg colistin in 4 mL saline solution and a placebo solution of the same osmolarity using a breath-enhanced nebulizer for administration. Subjects were recruited as follows: high risk (HR) for bronchospasm due to a personal history of recurrent wheezing, a family history of asthma and/or atopy, or bronchial lability, as demonstrated in pulmonary function tests; or low risk (LR) without these characteristics.

RESULTS

The mean FEV(1) (expressed as the mean [+/- SD] fall from baseline) of the HR group (n = 12) fell 12 +/- 9% after placebo was administered, and fell 17 +/- 10% after colistin was administered. For the LR group (n = 8), the mean FEV(1) fell 9 +/- 4% following placebo administration and 13 +/- 8% following colistin administration. There was a greater number of subjects in the HR group compared to the LR group, which had a mean fall in FEV(1) of >/= 15% (p < 0.01) after inhaling colistin. The differences between placebo and colistin therapy in the LR group were not significant.

CONCLUSION

The results demonstrated that colistin can cause bronchospasm, particularly in those patients with coexisting CF and asthma.

A melittin-related peptide from the skin of the Japanese frog, Rana tagoi, with antimicrobial and cytolytic properties

Two peptides with antimicrobial and cytolytic properties were purified from an extract of the skin of Tago's brown frog Rana tagoi. The primary structure of one peptide (FLPILGKLLS(10)GIL.NH(2)) identifies it as a member of the temporin family, whereas the second peptide (AIGSILGALA(10)KGLPTLISWI(20)KNR.NH(2)) displays 78% sequence identity to melittin from the venom of the honeybee Apis florea. Compared with melittin, the melittin-related peptide (MRP) was equipotent in inhibiting the growth of the Gram-positive bacterium Staphylococcus aureus, 5-fold less potent against the Gram-negative bacterium Escherichia coli and against the fungal pathogen, Candida albicans. MRP was 13-fold less hemolytic than melittin against human erythrocytes and 4- and 5-fold less cytolytic against mouse EL4 T-lymphoma-derived cells and L929 fibroblasts, respectively. However, at non-cytotoxic concentrations (<or=8 microM), MRP did not protect HeLa cells from cell death produced by human rhinovirus type 2 infection.

alpha-Melanocyte stimulating hormone acts as a selective inducer of secretory functions in human mast cells

In the present study, we have investigated the pro-opiomelanocortin (POMC)-derived neuropeptide alpha-MSH for its ability to modulate activation of human mast cells. The in vitro ability of purified human skin mast cells to secrete various types of mast cell mediators was monitored in response to alpha-MSH at the mRNA and at the protein level. Picomolar concentrations of alpha-MSH induced a dose-dependent release of histamine from isolated human skin mast cells and from skin punch biopsies. However, no effect of alpha-MSH was seen regarding the expression of IL-1, IL-6, IL-8, TGF-beta, and TNF-alpha. Melanocortin receptor MC-1 was identified at the transcriptional level by RT-PCR analysis but not at the protein level, whereas, in leukemic human mast cells (HMC-1), the mRNAs and the proteins for the MC-1 and MC-5 receptor were identified. These results suggest that alpha-MSH may selectively induce acute inflammatory effects via secretion of histamine.

Neutrophil Defensins Induce Histamine Secretion from Mast Cells: Mechanisms of Action

Defensins are endogenous antimicrobial peptides stored in neutrophil granules. Here we report that a panel of defensins from human, rat, guinea pig, and rabbit neutrophils all have histamine-releasing activity, degranulating rat peritoneal mast cells with EC50 ranging from 70 to 2500 nM, and between 45 and 60% of the total histamine released. The EC50 for defensin-induced histamine secretion correlates with their net basic charge at neutral pH. There is no correlation between histamine release and antimicrobial potency. Degranulation induced by defensins has characteristics similar to those of activation by substance P. The maximum percent histamine release is achieved in &lt;10 s, and it can be markedly inhibited by pertussis toxin (100 ng/ml) and by pretreatment of mast cells with neuraminidase. These properties differ from those for degranulation induced by IgE-dependent Ag stimulation and by the calcium ionophore A23187. GTPase activity, a measure of G protein activation, was induced in a membrane fraction from mast cells following treatment with defensin. Thus, neutrophil defensins are potent mast cell secretagogues that act in a manner similar to substance P and 48/80, through a rapid G protein-dependent response that is mechanistically distinct from Ag/IgE-dependent mast cell activation. Defensins may provide important pathways for communication between neutrophils and mast cells in defenses against microbial agents and in acute inflammatory responses.

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.

Drugs interacting with G protein alpha subunits: selectivity and perspectives

Extracellular signal molecules as diverse as hormones, neurotransmitters and photons use a signal transduction pathway involving a receptor, a G protein and effectors. Compounds that interact directly with G proteins can mimic the receptor-G protein interaction or can block the activation of G proteins by receptors. Several binding sites exist on the G alpha protein that may be exploited for the design of synthetic stimulatory or inhibitory ligands. The effector binding site is regulated by endogenous proteins and appears to be a target for selective exogenous ligands. The GTP binding site presents a large homology within the G protein families and therefore the nucleotide analogs might not be considered as a tool to discriminate between the G protein subclasses. In contrast, different experimental strategies have substantiated the specificity in the interaction between a receptor and a G protein, the receptor binding site of G proteins should be considered as potential drug targets. Drugs interfering with this site such as mastoparan and related peptides, GPAnt-2 and suramin, are lead compounds in the design of selective G protein antagonists. Benzalkonium chloride and methoctramine have agonist or antagonist properties, depending on G protein subtypes. Such compounds would be very useful to delineate the functions of G proteins and G protein-coupled receptors, to understand some side effects of drugs used in therapy and to develop new therapeutic agents.

Melittin, a metabostatic peptide inhibiting Gs activity

Some basic amphiphilic peptides are known to directly stimulate heterotrimeric GTP-binding proteins (G proteins). Mastoparan and melittin are known to stimulate Gi activities. Here, we found melittin inhibited guanine nucleotide-dependent adenylyl cyclase activity in synaptic membranes of the rat cerebral cortex. However, in insect cell membranes overexpressing specific heterotrimeric G proteins using baculovirus expression system, melittin showed unique effects different from those by mastoparan on G protein activities. This peptide markedly stimulated Gi1 and G11 activities, whereas it did inhibit Gs activities. Kinetic studies revealed that the inhibition of Gs activity by melittin is attributed to the inhibition of GDP release in exchange for added guanine nucleotides (or the association of guanine nucleotides). Thus, melittin may be the first metabostatic peptide inhibiting G protein (Gs) activity, and both mechanisms through the stimulation of Gi and inhibition of Gs might be involved in the melittin-induced inhibition of adenylyl cyclase.

Effect of tonicity of nebulised colistin on chest tightness and pulmonary function in adults with cystic fibrosis

BACKGROUND

Inhalation of hypertonic nebulised colistin causes chest tightness and is a reason for discontinuing the treatment. This study examines the relationship of chest tightness and change in lung function in response to the inhalation of a range of tonicities of nebulised colistin and their influence on patients' preference.

METHODS

Twenty seven adult patients with cystic fibrosis and a mean forced expiratory volume in one second (FEV1) of 54% predicted (range 24-98) were studied. They inhaled a nebulised solution of hypertonic, isotonic, and hypotonic colistin over three consecutive days in random order in a double blind fashion. Measurements of chest tightness, using a visual analogue scale (VAS), and FEV1 were recorded before and 0, 15, 30, 60, and 90 minutes following inhalation. The solution preferred by each patient was determined at the end of the three days.

RESULTS

All tonicities caused a significant fall in FEV1 % predicted and an increase in chest tightness, with no differences between the solutions. However, the mean (SE) time to the maximum fall in FEV1 % predicted was significantly different between the solutions (hypertonic 7.8 (2.1) min, isotonic 19.2 (5.5) min, and hypotonic 34.2 (5.9) min) with a mean difference (95% CI) between hypotonic and hypertonic solutions of 28.04 (14.6 to 41.5) min, between isotonic and hypertonic solutions of 12.0 (-0.1 to 24.1) min, and between hypotonic and isotonic solutions of 15.6 (1.8 to 29.4) min. Positive correlations existed for the maximum fall in FEV1 % predicted between the hypertonic and isotonic solutions (r = 0.62, p < 0.001) and between the hypotonic and isotonic solutions (r = 0.64, p < 0.001). There was no correlation between the objective and subjective measurements for any solution. The patients' preference varied.

CONCLUSIONS

All tonicities of colistin caused equal symptoms of chest tightness and reduction in pulmonary function. It is recommended that the patient is challenged with nebulised colistin before prescription of the drug and that the challenge is preceded by an inhaled bronchodilator. Most of the patients preferred the isotonic or hypotonic solutions. The isotonic solution reflects a fall in FEV1 representative of all the solutions. The fall in FEV1 to the hypotonic solution occurred over a longer period and may be better tolerated by some patients.

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.

Blockade of mast cell histamine secretion in response to neurotensin by SR 48692, a nonpeptide antagonist of the neurotensin brain receptor

1. Pretreatment of rat isolated mast cells with SR 48692, a nonpeptide antagonist of the neurotensin (NT) receptor, prevented histamine secretion in response to NT. 2. This inhibition was rapid in onset (approximately 1 min) and dependent upon the concentration of SR 48692 (IC50 approximately 1-10 nM). 3. SR 48692 (1-1000 nM) did not inhibit histamine secretion elicited by substance P, bradykinin or compound 48/80, or by anti-IgE stimulation of sensitized mast cells. 4. When SR 48692 was injected intradermally (5 pmol in 50 microliters) into anaesthetized rats, 15 min before the intradermal injection of NT, it reduced the effect of NT on vascular permeability. 5. When injected intravenously, SR 48692 attenuated the effects of NT on haematocrit and blood stasis. 6. These results demonstrate that SR 48692 selectively antagonizes the actions of NT on rat isolated mast cells as well as mast cells in vivo. Given the demonstrated specific interaction of SR 48692 with receptors for NT in brain, our results suggest the presence of specific NT receptors on mast cells.

Neuropeptide Y promotes GTP photo-incorporation into a 55 kDa protein

Incubation of bovine hippocampal membranes with [alpha-32P]GTP and exposure to ultraviolet light resulted in the labelling of seven species with apparent molecular masses of 200, 74, 55, 53, 50, 43 and 40 kDa. Labelling of the 55 kDa species was greatly enhanced in the presence of carboxyl terminal fragments [neuropeptide Y-(18-36)] of neuropeptide Y. Labelling occurred with [alpha-32P]GTP but not [alpha-32P]ATP. A group of putative direct G protein activating peptides including mastoparan, melittin, substance P and adrenocorticotropic hormone (ACTH)-(1-24), were also able to stimulate the labelling of this protein. Labelling of the 55 kDa protein could be demonstrated in bovine brain but not peripheral tissues. Western blot analysis using an antibody against the common alpha subunit of G proteins recognized a protein co-migrating with the 55 kDa GTP-binding protein. These findings demonstrate the existence of a previously uncharacterized neuronal protein, with an apparent molecular mass of 55 kDa, that binds GTP in response to neuropeptide Y and other peptides.

Peptidergic pathway in human skin and rat peritoneal mast cell activation

The common pathway of heterogenous mast cell activation as mediated by antigens is through the cross-linking of IgE bound to Fc epsilon RI receptors. The peptidergic pathway of mast cell activation, achieved by cationic secretagogues, is restricted to "serosal" mast cells, the experimental models being rat peritoneal and human skin mast cells. Cationic secretagogues include positively charged peptides but also various amines such as compound 48/80 and natural polyamines. An early intracellular event of this pathway is the activation of pertussis toxin-sensitive G proteins. The correlation observed between the ability of basic compounds to trigger mast cell exocytosis and their potency to activate purified G proteins strongly suggests that cationic compounds activate mast cell G proteins via a receptor-independent but membrane-assisted process. In this paper, alternative mechanisms are discussed. The consequence of G protein stimulation is the activation of phospholipase C with an increase in inositol triphosphates. Natural polyamines are relatively poor triggers of mast cells (10(-4) to 10(-2) M). Neuropeptides such as substance P, neuropeptide Y or vasoactive intestinal peptide, peptidic hormones such as kinins, and venoms such as mastoparan and mast cell degranulating peptide, are all active in a concentration range from 10(-7) to 10(-4) M. The cationic anaphylatoxin C3a also stimulates mast cells at concentrations below precursor complement C3 blood levels. The component C3 of the complement system is one of only a few plasma proteins having activation fragments (i.e. C3a) that can be generated at micromolar levels. The effects of basic secretagogues defines a peptidergic pathway of mast cell activation, which represents a potentially toxic process considering the tissue effects caused by exogenous basic compounds such as venom peptides and certain amine containing drugs. Peptidergic activation of mast cells may also be a pathophysiological process having an important role in neurogenic inflammation and in diseases involving extensive activation of the blood complement cascade.

Mast cell degranulating (MCD) peptide and its optical isomer activate GTP binding protein in rat mast cells

The MCD peptide in bee venom induces degranulation in mast cells. The internal calcium concentration of mast cells increased and remained high following MCD stimulation. This calcium increase was blocked by pertussis toxin (Ptx) treatment, suggesting that MCD peptide activates Ptx-sensitive G-protein. Even in the absence of external calcium in the incubation medium, the calcium concentration increased by MCD treatment, but soon returned to the original level. D-MCD, the optical isomer of the MCD peptide, also increased the internal calcium concentration through a Ptx-sensitive pathway. We suggest that cationic clusters at one side of the surface are more important in activating the G-protein than the alpha-helix conformation.

Mechanisms of mediator release from human skin mast cells upon stimulation by the bradykinin analog, [DArg0-Hyp3-DPhe7]bradykinin

We have used the bradykinin analog, [DArg0-Hyp3-DPhe7]-bradykinin, as a model stimulus with which to examine peptide-induced mediator release from human skin mast cells (SMC) and to compare it with IgE-mediated release from the same cells. The bradykinin analog was an effective histamine secretagogue, inducing a comparable maximal level of release to that observed for anti-IgE. By contrast to anti-IgE, however, [DArg0-Hyp3-DPhe7]-bradykinin did not stimulate marked release of prostaglandin D2 (PGD2) from these cells. In experiments where cells were exposed to both stimuli simultaneously, histamine release was additive, while PGD2 release was the same as that observed for anti-IgE alone. The kinetics of [DArg0-Hyp3-DPhe7]-bradykinin-stimulated histamine release were rapid, with 50% of maximal release being achieved within 30 sec, compared to 2-3 min for anti-IgE. Interestingly, when both stimuli were applied simultaneously, the kinetics of release were intermediate between those of either stimulus alone. Studies of the signal transduction pathways that may be involved in [DArg0-Hyp3-DPhe7]-bradykinin-induced histamine release revealed striking differences to results obtained with anti-IgE. While agents that increase intracellular cyclic AMP have a pronounced inhibitory effect on IgE-mediated release, forskolin, isobutylmethylxanthine and isoproterenol were all totally ineffective at inhibiting histamine release induced by the bradykinin analog. Similarly, staurosporine, a relatively selective inhibitor of protein kinase C, and the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA) an activator of this enzyme, both have pronounced effects on IgE-mediated histamine release from SMC but were completely inactive with regard to [DArg0-Hyp3-DPhe7]-bradykinin-stimulated release. SMC stimulated with this peptide showed characteristic changes in intracellular free calcium levels, as assessed by digital video microscopy. This response differs from that induced by anti-IgE in that it had a more rapd onset, achieved a lower peak, and decayed much more rapidly. Analysis at the single cell level showed that cells that responded in this fashion upon exposure to the bradykinin analog were capable of showing an additional response upon subsequent exposure to anti-IgE. We conclude that histamine release from SMC in response to [DArg0-Hyp3-DPhe7]-bradykinin occurs via a completely different mechanism from that in response to IgE-mediated stimuli. Peptide-induced release is rapid and is not susceptible to pharmacologic manipulation of intracellular cyclic AMP or protein kinase C but utilizes a rapid transient shift in intracellular calcium concentrations as part of its signal transduction pathway.

G protein activation: a receptor-independent mode of action for cationic amphiphilic neuropeptides and venom peptides

The neuropeptide substance P, the venom peptide mastoparan and the synthetic polyamine compound 48/80 activate rat peritoneal mast cells, leading to rapid histamine release by exocytosis. Although these effects are inhibited by pertussis toxin and involve a transient increase in IP3, no selective membrane receptors have been identified. However, it has recently been shown that these compounds activate G proteins in vitro. Here Yves Landry and colleagues discuss the proposal that direct activation of G protein is the physiological mechanism of action of substance P on rat peritoneal mast cells, this mechanism being mimicked by mastoparan and 48/80, and possibly by other cationic amphiphilic peptides such as kinins. These compounds might be of help in defining the interaction between membrane receptors and G proteins.

Mast cell degranulating peptide: a multi-functional neurotoxin

This review discusses our present knowledge of the structure and activities of the mast cell degranulating peptide (MCDP). This peptide is a basic, 22 amino acid residue component of honey bee venom with striking immunological and pharmacological activities. MCDP is a potent anti-inflammatory agent, but at low concentrations it is a strong mediator of mast cell degranulation and histamine release. MCDP is also an epileptogenic neurotoxin, an avid blocker of the potassium channels and can cause a significant lowering of the blood pressure in rats. Some of the biological activities of MCDP appear to have distinct mechanisms and may represent a good illustration of the structure-function relationship.

Bradykinin receptor antagonists

Bradykinin and its active metabolites are produced at the sites of their actions by kallikreins. They potently elicit a variety of biological effects: hypotension, bronchoconstriction, gut and uterine contraction, epithelial secretion in airway, gut, and exocrine glands, vascular permeability, pain, connective tissue proliferation, and eicosanoid formation. These effects are mediated by at least two broad classes of receptors. The most common is the B2 subtype. The Stewart and Vavrek peptides characterized by a DPhe7 substitution have provided powerful tools for study of bradykinin's actions by competitively and specifically blocking bradykinin B2 receptors. The significance of kinins in certain human diseases is being explored using these new tools and potential therapeutic agents. At present, human clinical trials are underway to test the usefulness of bradykinin receptor antagonists in the symptoms of the common cold and in the pain associated with severe burns. Trials for use in asthma will be initiated in 1990.

Role of the N-terminal arginine in the histamine-releasing activity of substance P, bradykinin and related peptides

A series of substance P (SP)- and bradykinin (BK)-related peptides have been compared for their histamine-releasing activities on rat peritoneal mast cells. Some of these peptides only differed in the N-acetylation of the N-terminal arginine residue or by the removal of charged residue at the N-terminal. The aim was to examine the role of the N-terminal positive charges in the histamine-releasing activity of compounds that are selective for the SP receptor (named NK-1) or for the B2 type bradykinin receptor. Only compounds with positive charges at the N-terminal caused non-cytotoxic histamine release from rat mast cells. It is suggested that SP- and BK-related peptides caused histamine release by a mechanism which appeared to be non-specific and not related to the activation of mast cell NK-1 or B2 receptors, respectively. Our results show that NK-1 agonists or B2 antagonists devoid of histamine-releasing activity, which could be of potential use in the clinic, can be obtained by removing the positively charged N-terminal aminoacids or by N-acetylation of the N-terminal arginine.

Solid phase synthesis and biological activity of mast cell degranulating (MCD) peptide: a component of bee venom

Mast cell degranulating (MCD) peptide, a 22 amino acid residue basic peptide from bee venom, was synthesized by stepwise solid phase synthesis on a benzhydrylamine resin support. N alpha-t-butyloxycarbonyl and benzyl type side chain protection was used. The two disulfide bridges were formed selectively by using S-acetamidomethyl protection for the cysteine residues in position 5 and 19 and S-methylbenzyl protection for the cysteine residues in positions 3 and 15. Crude synthetic MCD peptide was obtained following deprotection and cleavage from the resin by the low/high HF method. The peptide was isolated in pure form by ion exchange chromatography and gel filtration. The final product has physical, chemical, and biological properties identical with those reported for the natural product. The synthetic strategy utilized for MCD peptide will facilitate the availability of structurally similar analogs for evaluating antihistaminic and anti-inflammatory activities.

Induction of histamine release from human skin mast cells by bradykinin analogs

Kinins are potent proinflammatory peptides that induce histamine release from rodent mast cells. We examined the ability of bradykinin, lysylbradykinin and a series of kinin analogs to cause histamine release from human basophils, human lung mast cells and human skin mast cells. At concentrations ranging from 0.1 microM to 1 mM, bradykinin failed to cause histamine release from any of the human histamine-containing cells studied. Lysylbradykinin was also without effect on basophils and lung mast cells, but was a weak secretagogue for human skin mast cells, inducing 5.5 +/- 3% (mean +/- SD) of total cellular histamine release at a concentration of 10(-5) M. Similarly, when sixteen recently developed bradykinin antagonists were examined, these compounds had no effect on basophils or lung mast cells but all sixteen induced dose-dependent histamine release from skin mast cells. The release process was temperature dependent and, at a concentration of 10(-5) M, the antagonists induced 8-27% histamine release. Although preincubation of cells with 10(-3) M bradykinin or des(Arg9) bradykinin significantly inhibited antagonist-induced histamine release, the requirement for such high concentrations of these peptides to cause inhibition suggested that histamine release is not mediated by either B1 or B2 kinin receptors. To understand further the mechanism of histamine release, we examined a series of bradykinin analogs with single amino acid substitutions in the bradykinin sequence. Replacement of proline7 in the bradykinin sequence with D-phenylalanine is the essential change used to convert kinin analogs into antagonists, and 10(-5) M [DPhe7]-bradykinin induced 8-10% histamine release. Other analogs, devoid of antagonist activity, however, such as [DPhe6]-bradykinin and [LPhe7]-bradykinin were able to induce equivalent levels of histamine release. The ability to induce histamine release appears to be related, at least in part, to aromaticity, since [DTrp6]-bradykinin and [DTrp7]-bradykinin induced greater amounts of histamine release than equivalent [DPhe]-analogs, causing approximately 20% histamine release at 10(-5) M. By contrast, [DAla7]-bradykinin was an ineffective stimulus. In summary, a single amino acid substitution can convert bradykinin into a secretagogue for human skin mast cells. The ability of kinin analogs to induce histamine release from skin mast cells, but not lung mast cells or basophils, emphasizes the heterogeneity of human histamine-containing cells.

Characteristics of beta-endorphin-induced histamine release from rat serosal mast cells. Comparison with neurotensin, dynorphin and compound 48/80

Rat peritoneal mast cells were exposed to the neurohormone and basic opioid peptide beta-endorphin. beta-Endorphin induced a dose-dependent release of histamine from the mast cells. A significant histamine release was found at 5 mumol/l of beta-endorphin and maximal release (35% of total) at 20 mumol/l. The histamine release process was very rapid and terminated within 30 s at 37 C, and in this sense is very similar to the histamine release induced by compound 48/80 or neurotensin. The histamine release was temperature-dependent showing an optimum release around 30 C, and it was independent of available extracellular calcium, but was inhibited in the presence of high extracellular calcium concentrations. Naloxone, only in very high concentrations (10 mmol/l), inhibited the release, and the very same concentration also inhibited the neurotensin - as well as the compound 48/80-induced histamine release. Cromoglycate and benzalkoniumchloride, a 48/80 antagonist, both produced a progressive dose-dependent inhibition of beta-endorphin-, neurotensin- as well as compound 48/80-induced histamine release. Taken together, the findings indicate that the opioid peptide beta-endorphin induces a selective, energy-dependent release of histamine from peritoneal rat mast cells. The pattern of release has much in common with that of compound 48/80 and other basic peptides, such as neurotensin and substance P. In addition this pattern of release is similar to that induced by dynorphin.

Histamine release from rat peritoneal mast cells by kinin antagonists

Kinins are known to be potent releasers of histamine. In the present study, the B2 antagonist analogues of bradykinin (BK) and kallidin (KD): [( Thi6,9,D-Phe8]KD, [Thi5,8,D-Phe7]BK) were also found to be potent releasers of histamine from rat mast cells and not to exert any antagonistic effect. Similarly, two B1 receptor antagonists, des-Arg10-[Leu9]KD and des-Arg9-[Leu8]BK, were shown to promote histamine release and acted only as agonists. The order of potency of these analogues was the following: [Thi6,9,D-Phe8]KD greater than [Thi5,8,D-Phe7]BK greater than des-Arg10- [Leu9]KD greater than des-Arg9-[Leu8]BK. The high activity of some of these compounds suggests that (1) the potency of kinins for histamine release from rat mast cells not only increases in parallel with the number of positively charged amino acids (Lys, Arg) but also, because of others factors such as peptide conformation, receptor binding affinity or receptor accessibility, (2) the histamine release by kinins is either a non-specific effect not mediated by receptors or, if receptors are involved, these sites may be different from other kinin receptors (i.e. B1 or B2).

Desensitization of rat peritoneal mast cells to substance P

Rat peritoneal mast cells were pretreated for 10 min at 37 degrees C with either substance P (SP, 3 or 6 microM) or compound 48/80 (37.5, 50 or 75 ng/ml). The effect of this pretreatment on the subsequent responsiveness of the cells to SP was studied. Both SP and compound 48/80 pretreatment of rat peritoneal mast cells inhibited the subsequent response of the cells to SP. The degree of inhibition produced by either SP or compound 48/80 was dependent on the concentration used to pretreat the cells. Inhibition of the response of the cells to SP was observed whether or not the pretreating agent was removed or remained in contact with the cells during the subsequent stimulation with SP. It is concluded that compound 48/80 and SP desensitize the cells to subsequent stimulation by SP and possible mechanisms for this are discussed.

Effect of antianaphylactic agents on substance-P induced histamine release from rat peritoneal mast cells

Substance P is known to be a potent histamine liberator for mast cells. The influence of antianaphylactic agents, disodium cromoglycate (DSCG), ketotifen, and tranilast was studied on substance-P and compound 48/80-induced histamine release from rat peritoneal mast cells. Substance-P induced histamine release was inhibited by these agents, while compound 48/80-induced histamine release was not inhibited by tranilast. Our findings suggest that these antianaphylactic agents are assumed to be effective for cutaneous diseases which might be concerned with substance P and histamine.

Structural requirements for mast cell triggering by substance P-like peptides

Experiments were based on the hypothesis that the histamine releasing action of substance P and some other neuropeptides is not due to the interaction of the neuropeptide with a specific membrane receptor. Rather, this may be a property of many amphiphilic compounds having a minimal number of positives charges and a hydrophobic chain which is not necessarily a peptide. According to this hypothesis, 18 substance P derivatives and fragments were synthesized and tested on rat and hamster peritoneal mast cells, among them five compounds which contain a non-peptide chain instead of the C-terminal substance P heptapeptide. The results are in accord with the above hypothesis.

Epileptogenic properties of the mast cell degranulating peptide in CA3 hippocampal neurones

The epileptogenic properties of the mast cell degranulating peptide (MCD) have been investigated in the CA3 region of the hippocampal slice preparation. Brief (3-5 min) bath application of MCD (0.5-2 microM) to CA3 hippocampal neurones produced an enhancement of the spontaneous synaptic activity and the appearance of spontaneous bursts that persisted for several hours. These bursts were network driven and the underlying paroxysmal depolarizing shift met the criteria for a giant excitatory postsynaptic potential (EPSP), with a reversal potential close to 0 mV. Furthermore following the application of MCD, stimulation of the mossy fibres, commissural or temporo-ammonic pathway evoked an EPSP followed by an evoked network burst. The bursts which could be elicited for several hours were reversibly blocked by a brief application of tetrodotoxin (TTX; 1 microM) or cobalt (2 mM). In contrast, prior and concomitant treatment with TTX or cobalt prevented the occurrence of the bursts induced by MCD. The effects of MCD were not due to a blockade of GABAergic inhibition since the toxin did not reduce the fast and slow IPSP. Furthermore, the N-methyl-D-aspartate (NMDA) antagonists D-2-amino-phosphonovalerate (D-APV; 30 microM) or DL-amino-phosphoheptanoic acid (AP-7, 30 microM) did not block the action of MCD, suggesting that the activation of NMDA receptors are neither necessary nor sufficient for MCD-induced bursts. It is concluded that MCD induces in the CA3 region long-lasting changes in the synaptic responses which may be mediated through a presynaptic mechanism.

Brain coated vesicle destabilization and phosphorylation of coat proteins

Two basic polypeptides, bee venom melittin and poly-L-lysine, induced concentration-dependent destabilization of bovine brain coated vesicles. Ultrastructurally the changes observed were aggregation of clathrin coats and segregation of the vesicle membrane, concomitant with the appearance of elongated cisternae of various sizes. Changes in coated vesicle morphology induced by melittin and poly-L-lysine were concurrent with stimulation of phosphate incorporation in proteins of the coat lattice: Mr 33,000 and 100,000. Melittin-stimulated phosphorylation was Ca2+ sensitive and inhibited by EGTA. The initiation of vesicle membrane segregation by melittin, followed by fusion and formation of elongated membrane cisternae, paralleled an increase of endogenous phospholipase A2 activity. The data suggest that a correlation exists between the state of assembly of the coat proteins on coated vesicles and protein phosphorylation.

Mast cell activation--a receptor-independent mode of substance P action?

Substance P is a representative of a group of amphiphilic neuropeptides which act as mast cell secretagogues. Our experiments with some new substance P derivatives suggest that these effects are dependent on two structural elements: (i) a hydrophobic chain which is not essentially a peptide, and (ii) a hydrophilic part with two positively charged amino acids. The mast cell triggering effect is unlikely to be mediated by a selective substance P receptor, but has strong similarities to the mode of action of polycations.

The brain response to the bee venom peptide MCD. Activation and desensitization of a hippocampal target

The mast cell-degranulating peptide (MCD) isolated from bee venom has been found previously to have receptor sites in rat brain. Behavioral and electrocorticographic responses following intracerebroventricular injections of various doses of MCD have been analyzed. MCD produced a quasi-permanent hippocampal theta rhythm in the motionless animal alternating with epileptiform spike waves and paroxystic seizures. At a dose of 70 pmol seizures occurred for half of the treated rats. At a dose of 100 pmol generalized paroxystic crises were observed for all the rats. These effects were not antagonized by naloxone, morphine, diazepam and progabide. Rats recovered 24 h after a 100 pmol injection of MCD. A second ipsilateral injection to these rats showed the occurrence of a desensitization phenomenon. Desensitization was not observed when the second injection was contralateral. These physiological responses were studied in relation with a biochemical approach on membrane sites of action of MCD using [125I]MCD and their behavior in the desensitization process. The target of [125I]MCD is the ipsilateral hippocampus. Recovery from MCD effects was not due to MCD degradation. Desensitization was not due to down-regulation of the MCD receptor level.

Isolation and structure analysis of bee venom mast cell degranulating peptide

The venom of Apis mellifera was processed by gel permeation chromatography on Sephadex G-50 and by reversed-phase HPLC. The initial gel permeation step was carried out in the presence of phosphate ions (0.5 M). Ion pair reagents were required to resolve the strongly basic peptides, secapin, mast cell degranulating (MCD-) peptide and apamin, by reversed-phase (RP) HPLC. Using this relatively simple procedure it is possible to isolate these peptides essentially free of melittin (less than 1 in 10(7)) and phospholipase (less than 1 in 10(5] in high yield. The CD spectrum and secondary structure analysis are reported for MCD-peptide and on this basis a solution structure is proposed for this toxin.