Two adrenal opioid polypeptides: proposed intermediates in the processing of proenkephalin

Emerging roles of atypical chemokine receptor 3 (ACKR3) in normal development and physiology

The discovery that atypical chemokine receptors (ACKRs) can initiate alternative signaling pathways rather than classical G-protein coupled receptor (GPCR) signaling has changed the paradigm of chemokine receptors and their roles in modulating chemotactic responses. The ACKR family has grown over the years, with discovery of new functions and roles in a variety of pathophysiological conditions. However, the extent to which these receptors regulate normal physiology is still continuously expanding. In particular, atypical chemokine receptor 3 (ACKR3) has proven to be an important receptor in mediating normal biological functions, including cardiac development and migration of cortical neurons. In this review, we illustrate the versatile and intriguing role of ACKR3 in physiology.

Human Antimicrobial Peptides in Bodily Fluids: Current Knowledge and Therapeutic Perspectives in the Postantibiotic Era

Antimicrobial peptides (AMPs) are an integral part of the innate immune defense mechanism of many organisms. Due to the alarming increase of resistance to antimicrobial therapeutics, a growing interest in alternative antimicrobial agents has led to the exploitation of AMPs, both synthetic and isolated from natural sources. Thus, many peptide-based drugs have been the focus of increasing attention by many researchers not only in identifying novel AMPs, but in defining mechanisms of antimicrobial peptide activity as well. Herein, we review the available strategies for the identification of AMPs in human body fluids and their mechanism(s) of action. In addition, an overview of the distribution of AMPs across different human body fluids is provided, as well as its relation with microorganisms and infectious conditions.

Antimicrobial peptides: Possible anti-infective agents

Multidrug-resistant bacterial, fungal, viral, and parasitic infections are major health threats. The Infectious Diseases Society of America has expressed concern on the decrease of pharmaceutical companies working on antibiotic research and development. However, small companies, along with academic research institutes, are stepping forward to develop novel therapeutic methods to overcome the present healthcare situation. Among the leading alternatives to current drugs are antimicrobial peptides (AMPs), which are abundantly distributed in nature. AMPs exhibit broad-spectrum activity against a wide variety of bacteria, fungi, viruses, and parasites, and even cancerous cells. They also show potential immunomodulatory properties, and are highly responsive to infectious agents and innate immuno-stimulatory molecules. In recent years, many AMPs have undergone or are undergoing clinical development, and a few are commercially available for topical and other applications. In this review, we outline selected anion and cationic AMPs which are at various stages of development, from preliminary analysis to clinical drug development. Moreover, we also consider current production methods and delivery tools for AMPs, which must be improved for the effective use of these agents.

Structure and mode of action of microplusin, a copper II-chelating antimicrobial peptide from the cattle tick Rhipicephalus (Boophilus) microplus

Microplusin, a Rhipicephalus (Boophilus) microplus antimicrobial peptide (AMP) is the first fully characterized member of a new family of cysteine-rich AMPs with histidine-rich regions at the N and C termini. In the tick, microplusin belongs to the arsenal of innate defense molecules active against bacteria and fungi. Here we describe the NMR solution structure of microplusin and demonstrate that the protein binds copper II and iron II. Structured as a single alpha-helical globular domain, microplusin consists of five alpha-helices: alpha1 (residues Gly-9 to Arg-21), alpha2 (residues Glu-27 to Asn-40), alpha3 (residues Arg-44 to Thr-54), alpha4 (residues Leu-57 to Tyr-64), and alpha5 (residues Asn-67 to Cys-80). The N and C termini are disordered. This structure is unlike any other AMP structures described to date. We also used NMR spectroscopy to map the copper binding region on microplusin. Finally, using the Gram-positive bacteria Micrococcus luteus as a model, we studied of mode of action of microplusin. Microplusin has a bacteriostatic effect and does not permeabilize the bacterial membrane. Because microplusin binds metals, we tested whether this was related to its antimicrobial activity. We found that the bacteriostatic effect of microplusin was fully reversed by supplementation of culture media with copper II but not iron II. We also demonstrated that microplusin affects M. luteus respiration, a copper-dependent process. Thus, we conclude that the antibacterial effect of microplusin is due to its ability to bind and sequester copper II.

Interactions of an anionic antimicrobial peptide with Staphylococcus aureus membranes

The antimicrobial activity of the anionic peptide, AP1 (GEQGALAQFGEWL), was investigated. AP1 was found to kill Staphylococcus aureus with an MLC of 3mM and to induce maximal surface pressure changes of 3.8 mN m(-1) over 1200s in monolayers formed from lipid extract of S. aureus membranes. FTIR spectroscopy showed the peptide to be alpha-helical (100%) in the presence of vesicles formed from this lipid extract and to induce increases in their fluidity (Deltanu circa 0.5 cm(-1)). These combined data show that AP1 is able to function as an alpha-helical antimicrobial peptide against Gram-positive bacteria and suggest that the killing mechanism used by the peptide involves interactions with the membrane lipid headgroup region. Moreover, this killing mechanism differs strongly from that previously reported for AP1 against Gram-negative bacteria, indicating the importance of considering the effects of membrane lipid composition when investigating the structure/function relationships of antimicrobial peptides.

Proenkephalin A 119-159, a stable proenkephalin A precursor fragment identified in human circulation

In this report, we describe a newly developed sandwich immunoassay using antibodies against the proenkephalin A 119-159 peptide (PENK A 119-159). PENK A 119-159 immunoreactivity was detectable in the circulation of human blood donors and in cerebrospinal fluid (CSF) of patients without a neurologic disorder. The concentration was about 100 times higher in CSF than in serum. Analytical reversed phase HPLC revealed that PENK A 119-159 is the main immunoreactivity in human circulation and CSF. Moreover, PENK A 119-159 is stable in vitro for at least 48 h at room temperature as compared to the low stability of the peptides methionine- and leucine-enkephalin. This suggests the use of PENK A 119-159 measurement as surrogate molecule for the release of the mature peptides derived from proenkephalin A.

Synthesis and binding characteristics of a novel enkephalin analogue, [3H]Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe

The endogenous opioid heptapeptide (Tyr-Gly-Gly-Phe-Met-Arg-Phe; MERF) has been shown to interact with multiple opioid as well as non-opioid sites in mammalian brain membranes. To increase the stability and bioavailability of MERF, new synthetic derivatives with D-amino acid substitutions were prepared and studied. One of the new compounds in this series, Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe (DADN), had only moderate affinity in competing with [3H]MERF, whereas it displayed the highest potency in producing antinociception following intrathecal administration. DADN was radiolabeled with 41Ci/mmol specific activity. Specific binding of [3H]DADN was saturable, stereoselective and of high affinity. Chemical stability, increased micro-receptor selectivity, and hydrophobicity of the peptide all contribute to the effectiveness observed in biochemical and pharmacological studies.

Peptidomics-based discovery of novel neuropeptides

Modern proteomic methodologies have significantly improved the possibilities of large-scale identification of proteins. However, these methodologies are limited by their inability to reliably detect endogenously expressed peptides. We describe a novel approach of combining sample preparation, comprising focused microwave irradiation and mass spectrometric peptide profiling that has enabled us to simultaneously detect more than 550 endogenous neuropeptides in 1 mg of hypothalamic extracts. Automatic switching tandem mass spectrometry and amino acid sequence determination of the peptides showed that they consist of both novel and previously described neuropeptides. The methodology includes virtual visualization of the peptides as two- and three-dimensional image maps. In addition, several novel and known post-translational modifications of the neuropeptides were identified. The peptidomic approach proved to be a powerful method for investigating endogenous peptides and their post-translational modifications in complex tissues such as the brain. It is anticipated that this approach will complement proteomic methods in the future.

Capillary LC-MS2 at the attomole level for monitoring and discovering endogenous peptides in microdialysis samples collected in vivo

Fused-silica capillary LC columns (25-microm i.d.) with 3-microm-i.d. integrated electrospray emitters interfaced to a quadrupole ion trap mass spectrometer were evaluated for high-sensitivity LC-MS2. Column preparation involved constructing frits by in situ photopolymerization of glycidyl methacrylate and trimethylolpropane trimethacrylate, preparing the electrospray emitter by pulling the column outlet to a fine tip with a CO2 laser puller, and slurry-packing the column with 5-microm reversed-phase particles. Large-volume injections were facilitated by an automated two-pump system that allowed high-flow rates for sample loading and low-flow rates for elution. Small electrospray emitters, low elution flow rates, and optimization of gradient steepness allowed a detection limit of 4 amol, corresponding to 2 pM for 1.8 microL injected on-column, for a mixture of peptides dissolved in artificial cerebral spinal fluid. The system was coupled on-line to microdialysis sampling and was used to monitor and discover endogenous neuropeptides from the globus pallidus of anesthetized male Sprague-Dawley rats. Time-segmented MS2 scans enabled simultaneous monitoring of Met-enkephalin, Leu-enkephalin, and unknown peptides. Basal dialysate levels of Met-enkephalin and Leu-enkephalin were 60 +/- 30 and 70 +/- 20 pM while K+-stimulated levels were 1,900 +/- 500 and 1,300 +/- 300 pM, respectively (n = 7). Data-dependent and time-segmented MS2 scans revealed several unknown peptides that were present in dialysate. One of the unknowns was identified as peptide I(1-10) (SPQLEDEAKE), a novel product of preproenkephalin A processing, using MS2, MS3, and database searching.

The neuroendocrine system of annelids

In vertebrates the neuroendocrine system is based on chemical signaling between neural and endocrine structures. Final outcomes may be realized via chemical messengers traveling through circulatory conduits to their specific target sites. This process may rely, in part, on neurosecretion of the signaling molecules. The complexity of this system can be readily visualized when one considers the way in which interactions among classical neurotransmitters, cytokines, growth factors, and neuroendocrine hormones, in combination with autocrine and paracrine communication, can regulate cells and tissues. Apart from the neuroendocrine system there is also neuroimmune communication, consisting of reciprocal signaling between neuroendocrine and immune cells, which use the same molecules to coordinate their activity. Thus, our concept of the neuroendocrine system is constantly growing, despite its complexity, but it may be simply summarized as allowing bidirectional communication between neural and endocrine structures over distances greater than that achieved by synaptic communication. In the light of this, I demonstrate in this review that annelids, which are considered "simple" animals, also possess a neuroendocrine system.

The presence of antibacterial and opioid peptides in human plasma during coronary artery bypass surgery

Antibacterial peptides, found in both invertebrates and vertebrates, represent a potential innate defense mechanism against microbial infections. However, it is unknown whether this process occurs in humans during surgery. We looked for evidence of release of antibacterial peptides during coronary artery bypass grafting (CABG). We used immunological techniques and antibacterial assays combined with high-performance gel-permeation chromatography, reverse-phase HPLC, N-terminal sequencing and comparison with synthetic standards to characterize the peptide B/enkelytin. We show the presence of anionic antibacterial peptide, the peptide B/enkelytin which correspond to the C-terminal part of proenkephalin A, from the plasma of patients undergoing CABG. Our studies show that peptide B/enkelytin is initially present at low levels in plasma and is then released in increased amounts just after skin incision. Antibacterial assays confirmed that the peptides specifically target gram-positive bacteria. We also demonstrate that peptide B/enkelytin is metabolized in vivo to the opioid peptides methionine-enkephalin-Arg-Phe and methionine-enkephalin, peptides that we show have granulocyte chemotactic activity. These findings suggest that in humans, surgical incision leads to the release of antibacterial peptides. Furthermore, these antibacterial peptides can be metabolized into compounds that have immune-activating properties.

Characterization of antibacterial COOH-terminal proenkephalin-A-derived peptides (PEAP) in infectious fluids. Importance of enkelytin, the antibacterial PEAP209-237 secreted by stimulated chromaffin cells

Proenkephalin-A (PEA) and its derived peptides (PEAP) have been described in neural, neuroendocrine tissues and immune cells. The processing of PEA has been extensively studied in the adrenal medulla chromaffin cell showing that maturation starts with the removal of the carboxyl-terminal PEAP209-239. In 1995, our laboratory has shown that antibacterial activity is present within the intragranular chromaffin granule matrix and in the extracellular medium following exocytosis. More recently, we have identified an intragranular peptide, named enkelytin, corresponding to the bisphosphorylated PEAP209-237, that inhibits the growth of Micrococcus luteus (Goumon, Y., Strub, J. M., Moniatte, M., Nullans, G., Poteur, L., Hubert, P., Van Dorsselaer, A., Aunis, D., and Metz-Boutigue, M. H. (1996) Eur. J. Biochem. 235, 516-525). As a continuation of this previous study, in order to characterize the biological function of antibacterial PEAP, we have here examined whether this COOH-terminal fragment is released from stimulated chromaffin cells and whether it could be detected in wound fluids and in polymorphonuclear secretions following cell stimulation. The antibacterial spectrum shows that enkelytin is active against several Gram-positive bacteria including Staphylococcus aureus, but it is unable to inhibit the Gram-negative bacteria growth. In order to relate the antibacterial activity of enkelytin with structural features, various synthetic enkelytin-derived peptides were tested. We also propose a computer model of synthetic PEAP209-237 deduced from 1H NMR analysis, in order to relate the antibacterial activity of enkelytin with the three-dimensional structure. Finally, we report the high phylogenetic conservation of the COOH-terminal PEAP, which implies some important biological function and we discuss the putative importance of enkelytin in the defensive processes.

Antibacterial peptides are present in chromaffin cell secretory granules

1. Antibacterial activity has recently been associated with the soluble matrix of bovine chromaffin granules. Furthermore, this activity was detected in the contents secreted from cultured chromaffin cells following stimulation. 2. The agents responsible for the inhibition of Gram+ and Gram- bacteria growth are granular peptides acting in the micromolar range or below. In secretory granules, these peptides are generated from cleavage of chromogranins and proenkephalin A and are released together with catecholamines into the circulation. 3. Secretolytin and enkelytin are the best characterized; these two peptides share sequence homology and similar antibacterial activity with insect cecropins and intestinal diazepam-binding inhibitor. For some of the peptides derived from chromogranin A, posttranslational modifications were essential since antibacterial activity was expressed only when peptides were phosphorylated and/or glycosylated. 4. The significance of this activity is not yet understood. It may be reminiscent of some primitive defense mechanism or may serve as a first barrier to bacteria infection during stress, as these peptides are secreted along with catecholamines.

Opiate tolerance and dependence: receptors, G-proteins, and antiopiates

Despite the existence of a large body of information on the subject, the mechanisms of opiate tolerance and dependence are not yet fully understood. Although the traditional mechanisms of receptor down-regulation and desensitization seem to play a role, they cannot entirely explain the phenomena of tolerance and dependence. Therefore, other mechanisms, such as the presence of antiopiate systems and the coupling of opiate receptors to alternative G-proteins, should be considered. A further complication of studies of opiate tolerance and dependence is the multiplicity of endogenous opiate receptors and peptides. This review will focus on the endogenous opioid system--peptides, receptors, and coupling of receptors to intracellular signaling via G-proteins--in the context of their roles in tolerance and dependence. Opioid peptides include the recently discovered endomorphins and those encoded by three known genes--pro-opiomelanocortin, pro-enkephalin, and pro-dynorphin. They bind to three types of receptors--mu, delta, and kappa. Each of the receptor types is further divided into multiple subtypes. These receptors are widely known to be coupled to G-proteins of the Gi and Go subtypes, but an increasing body of results suggests coupling to other G-proteins, such as Gs. The coupling of opiate receptors to Gs, in particular, has implications for tolerance and dependence. Alterations at the receptor and transduction level have been the focus of many studies of opiate tolerance and dependence. In these studies, both receptor down-regulation and desensitization have been demonstrated in vivo and in vitro. Receptor down-regulation has been more easily observed in vitro, especially in response to morphine, a phenomenon which suggests that some factor which is missing in vitro prevents receptors from down-regulating in vivo and may play a critical role in tolerance and dependence. We suggest that antiopiate peptides may operate in vivo in this capacity, and we outline the evidence for the antiopiate properties of three peptides: neuropeptide FF, orphanin FQ/nociceptin, and Tyr-W-MIF-1. In addition, we provide new results suggesting that Tyr-W-MIF-1 may act as an antiopiate at the cellular level by inhibiting basal G-protein activation, in contrast to the activation of G-proteins by opiate agonists.

Identification of a missense mutation and several polymorphisms in the proenkephalin A gene of schizophrenic patients

Schizophrenia is a complex and severe disorder of unknown cause and pathophysiology. In this study, we examined the opioid hypothesis for schizophrenia at the molecular level, focusing on the dopamine-regulated proenkephalin A gene (chromosome 8q11.23-q12). We have screened 150 schizophrenic patients for sequence variations within the promoter region, entire coding sequence, and 3'-untranslated region. We find one sequence change in a conserved amino acid that may be of functional significance. This mutation was found in a single schizophrenia patient but not in controls. Although several new, race-specific polymorphisms were identified, all other sequence changes appeared to be common polymorphisms, unlikely to contribute to the etiology of schizophrenia.

The C-terminal bisphosphorylated proenkephalin-A-(209-237)-peptide from adrenal medullary chromaffin granules possesses antibacterial activity

The chromaffin granules have been shown to be an excellent model to study the processing of proenkephalin-A and chromogranins. Recently, we reported a study dealing with the processing of chromogranin B/secretogranin I and the occurrence of the C-terminal chromogranin B-derived peptide 614-626 which was shown to have antibacterial activity [Strub, J.M., Garcia-Sablone, P., Looning, K., Taupenot, L., Hubert, P., Van Dorsselaer, A., Aunis, D. & Metz-Boutigue, M.H. (1995) Eur. J. Biochem. 229, 356-368]. We also observed that this new antibacterial activity present in chromaffin granules was associated with other endogenous protein-derived fragments yet to be characterized. The present study reports the isolation and characterization of a peptide which possesses antibacterial activity and which corresponds to the C-terminal 209-237 sequence of proenkephalin-A. A detailed study using microsequencing and matrix-assisted-laser-desorption time-of-flight mass spectrometry (MALD-TOF MS) allowed us to correlate the antibacterial activity of this peptide named enkelytin (FAEPLPSEEEGESYSKEVPEMEKRYGGFM) with post-translational modifications. Endogenous bisphosphorylated proenkephalin-A-(209-237) was active on Micrococcus luteus and Bacillus megaterium killing bacteria in the 0.2 - 0.4 microM range but was inactive in similar conditions towards Escherichia coli. Enkelytin shares sequence and structural similarities with the antibacterial C-terminal domain of diazepam-binding inhibitor. According to this similarity, a prediction of secondary structure is proposed for enkelytin and discussed in relationship to its biological activity.

Distribution, Isolation and Sequence Analysis of the C-Terminal Heptapeptide of Pro-Enkephalin A (YGGFMRF) from the Ovine Median Eminence

Abstract Using a polyclonal antiserum raised against the C-terminal heptapeptide of pro-enkephalin A, we have isolated the opioid heptapeptide Tyr-Gly-Gly-Phe-Met-Arg-Phe (MERF) from ovine median eminence and mapped its distribution in that structure. MERF-immunoreactivity was confined to the pars externa (neurosecretory zone) where it colocalized with corticotrophin-releasing factor in the majority of terminals. No larger, N-terminally extended forms of MERF were detected in median eminence extracts suggesting that pro-enkephalin is fully processed to its constituent enkephalin congeners, and that the bioactive products, including MERF, act at the level of the hypothalamus in regulating anterior pituitary function.

Secondary structure characteristics of proenkephalin peptides E, B, and F

The conformations of three adrenal medullary enkephalin containing polypeptides (ECPs) were investigated to gain an understanding of their potential structure-activity relationships. Secondary structure characteristics of peptides E, B, and F were examined by circular dichrosim (CD) under conditions designed to mimic both the soluble state and the anisotropic environment which exists at the biological effector site. Conformational differences between the three peptides were further examined by Fourier Transform Infrared Spectroscopy (FTIR) and by empirical predictions for conformation and hydrophobic periodicity. Although all three peptides have a similar structure, existing in random configurations in aqueous solutions, they do exhibit unique individual potentials to assume secondary structure in less polar environments. These conformational differences may be important factors in determining their unique individual biological activities.

Plasma native and peptidase-derivable Met-enkephalin responses to restraint stress in rats. Adaptation to repeated restraint

Met-enkephalin and related proenkephalin A-derived peptides circulate in plasma at picomolar concentration as free, native pentapeptide and at nanomolar concentration in cryptic forms. We have optimized conditions for measurement of immunoreactive Met-enkephalin in plasma and for generation by trypsin and carboxypeptidase B of much greater amounts of total peptidase-derivable Met-enkephalin in plasma of rats, dogs, and humans. Free Met-enkephalin (11 pM) is constituted by native pentapeptide and its sulfoxide. Characterization of plasma total Met-enkephalin derived by peptidic hydrolysis revealed a small amount (38 pM) of Met-enkephalin associated with peptides of molecular mass less than 30,000 D, and probably derived from proenkephalin A, but much larger amounts of Met-enkephalin associated with albumin (1.2 nM) and with a globulin-sized protein (2.8 nM). Thus, plasma protein precursors for peptidase-derivable Met-enkephalin differ structurally and chemically from proenkephalin A. Met-enkephalin generated from plasma by peptidic hydrolysis showed naloxone-reversible bioactivity comparable to synthetic Met-enkephalin. Prolonged exposure of adult, male rats to restraint stress produced biphasic plasma responses, with peaks occurring at 30 s and 30 min in both free native and total peptidase-derivable Met-enkephalin. Repeated daily exposure to this 30-min stress resulted in adaptive loss of responses of both forms to acute restraint. Initial plasma responses of Met-enkephalin paralleled those of epinephrine and norepinephrine, but subsequently showed divergence of response. In conclusion, Met-enkephalin circulates in several forms, some of which may be derived from proteins other than proenkephalin A, and plasma levels of both free native, and peptidase-derivable Met-enkephalin are modulated physiologically.

Synthesis and biological evaluation of human preproenkephalin (100-111) and its analogs

The dodecapeptide sequence, Tyr-Gly-Gly-Phe-Met-Lys-Arg-Tyr-Gly-Gly-Phe-Met (BI), which is totally conserved in the primary structures of human, bovine, rat and toad preproenkephalins, has been synthesized by the solid-phase method. Coupling reactions were achieved by using symmetrical anhydrides of tert.-butyloxycarbonylamino acids performed with N-tert.-butyl,N'-methylcarbodiimide. 6-Arg and 7-Lys analogs have also been obtained. The peptides show opiate activity in both GPI and MVD assay, and possess antinociceptive properties as estimated by the hot-plate test in mice when applied intracisternally.

Proenkephalin A processing in the upper digestive tract: isolation and characterisation of phosphorylated N-terminally extended Met-enkephalin Arg6Phe7 variants

Previous studies suggest the processing of proenkephalin A in the porcine upper digestive tract might differ from that in the brain. To characterise more precisely some of the products, we have used antibodies to Met-enkephalin Arg6Phe7 (MERF) in radioimmunoassay to monitor the isolation of immunoreactive peptides from extracts of porcine pyloric antral muscle, antral mucosa, and duodenum. Sephadex G50 gel filtration of each extract produced a single broad peak of high-molecular-weight MERF-immunoreactivity. On anion-exchange chromatography the antral muscle MERF-immunoreactivity fractionated into two major peaks, and that from the antral mucosa and duodenum each into four major peaks, suggesting tissue specific processing of proenkephalin A within the porcine gut. Reverse-phase HPLC and Edman degradation analysis revealed that the least acidic antral muscle peptide was a 31-residue N-terminally extended form of MERF that is equivalent to proenkephalin A 209-239. Alkaline phosphatase digestion of the N-terminally extended MERF variants indicated that some of these peptides were modified by phosphorylation. We conclude that there are complex patterns of proenkephalin A processing in the porcine gut, which in part are due to phosphorylation.

Peptide E and its products, BAM 18 and Leu-enkephalin, in bovine adrenal medulla and cultured chromaffin cells: release in response to stimulation

Peptide E is a 25 amino acid opioid peptide which, if cleaved at the sole double basic (Lys-Arg) typical processing site, would generate two opioid fragments, the amino-terminal fragment BAM 18 and the carboxy-terminal fragment Leu-enkephalin. We have analysed extracts of bovine adrenal medulla in order to quantify these three opioid peptides (peptide E, BAM 18, and Leu-enkephalin). Here we present evidence that BAM 18 and Leu-enkephalin were present in similar amounts, whereas peptide E was present at a higher concentration. This is consistent with previous observations showing a preferential accumulation of larger peptides in the bovine adrenal, and also with the Lys-Arg bond being the principal site of cleavage of peptide E. However, when bovine adrenal chromaffin cells were maintained in culture for several days, Leu-enkephalin was found to be present in much greater amounts than was BAM 18-like immunoreactivity. The molar amounts of peptide E still exceeded the estimated levels of BAM 18 and Leu-enkephalin. We provide evidence that under conditions of basal release BAM 18 and peptide E were released, whereas Leu-enkephalin was released in much smaller amounts, if at all. On stimulation with nicotine results were consistent with an increased release of all three peptides with a preferential stimulation of Leu-enkephalin release. Under all conditions, the molar amounts of peptide E released apparently exceeded that of the other peptides. The results are discussed in terms of the regulation of partial proteolysis and the fate of peptide E.

Distribution pattern of metorphamide compared with other opioid peptides from proenkephalin and prodynorphin in the bovine brain

Metorphamide is a [Met]-enkephalin-containing opioid octapeptide with a C-terminal alpha-amide group. It is derived from proenkephalin and is, so far, the only endogenous opioid peptide with a particularly high affinity for mu opioid (morphine) receptors, a somewhat lesser affinity for kappa opioid receptors, and a relatively low affinity for delta opioid receptors. The concentrations of metorphamide in the bovine caudate nucleus, the hypothalamus, the spinal cord, and the neurointermediate pituitary were determined by radioimmunoassay and chromatography separation procedures. Metorphamide concentrations were compared with the concentrations of eight other opioid peptides from proenkephalin and prodynorphin in identical extracts. The other opioid peptides were [Met]-enkephalyl-Arg6-Phe7 and [Met]-enkephalyl-Arg6-Gly7-Leu8 from proenkephalin; alpha-neoendorphin, beta-neoendorphin, dynorphin A(1-8), dynorphin A(1-17), and dynorphin B from prodynorphin; and [Leu]-enkephalin, which can be derived from either precursor. All opioid peptides were present in all four bovine neural tissues investigated. Metorphamide concentrations were lower than the concentrations of the other proenkephalin-derived opioid peptides. They were, however, similar to the concentrations of the prodynorphin-derived opioid peptides in the same tissues. Marked differences in the relative ratios of the opioids derived from prodynorphin across brain regions were observed, a finding suggesting differential posttranslational processing. Differences in the ratios of the proenkephalin-derived opioids across brain regions were less pronounced. The results from this study together with previous findings on metorphamide's mu opioid receptor binding and bioactivities suggest that the amounts of metorphamide in the bovine brain are sufficient to make this peptide a candidate for a physiologically significant endogenous mu opioid receptor ligand.

Reactivity of consecutive basic amino acid residues in peptides

Different tetrapeptides of general formula L-Ala-X-X-Gly, possessing a basic doublet in the second and third position (X = Arg or Lys), have been synthesized as free or N-acetylated molecules. The chemical reactivity of the arginine guanidino group and of the lysine epsilon-amino group were studied using respectively the Sakaguchi and the ortho-diacetylbenzene reactions, in the tetrapeptides as well as in related molecules. In both cases, the colour yield is markedly influenced by the length of the polypeptide chain and by the relative positions of the arginine and lysine residues, suggesting the occurrence of intramolecular bonds within the tetrapeptide molecule. Tryptic hydrolysis of the tetrapeptides was followed by evaluating the amino acids or peptides which appear to be specific for the different possible cleavages at the arginyl or at the lysyl bonds. The susceptibility to trypsin of the carboxylic group of the second basic amino acid decreases progressively in the order Lys-Arg greater than Arg-Arg much greater than Lys-Lys greater than Arg-Lys, which shows a fair correlation with the intra-cellular cleavage of the bonds observed during the processing of preproteins of of the precursors of several physiologically active peptides.

Opioids and opioid receptors in peripheral tissues

Opioid peptides belonging to the enkephalin, beta-endorphin or dynorphin family, acting on specific opiate receptors may be found in peripheral tissues. Enkephalins have a widespread peripheral distribution, while beta-endorphin and dynorphin may be found locally in the enteric nervous system. The peptides of the various families are formed from specific precursor molecules. Apart from the enteric nervous system, opioids are also found in the adrenal medulla as well as in several autonomic ganglia. There is some evidence of three different classes of opioid receptors in peripheral tissues, i.e. mu-, delta- and kappa-receptors. These receptors are not only found on enteric nervous and mucosa cells but also on various cells in the immune system where opioid peptides seem to have important actions and appear to link the neuroendocrine and immune systems to control immunological functions. The physiological as well as the pathophysiological role of opioid peptides in the periphery is gradually being elucidated and, based on such knowledge, new therapeutic implications in gastrointestinal or immune diseases may be developed.

Structure-activity relationships of the yeast alpha-factor

The yeast Saccharomyces cerevisiae produces a peptide pheromone, termed the alpha-factor, as a prelude to sexual conjugation. Haploid MAT alpha-cells, but not haploid MAT a-cells or MAT a/alpha-diploids, produce this tridecapeptide of the structure: Trp-His-Trp-Leu-Gln-Leu-Lys-Pro-Gly-Gln-Pro-Met-Tyr. Structural analogues of the alpha-factor have been prepared with alterations in many of the residues, derivatized peptides have been synthesized, and truncated and elongated peptides have been studied. These peptides have been analyzed for their biological activities by various assays. Mutants of S. cerevisiae have been isolated that do not respond to alpha-factor or are supersensitive to the pheromone and its analogues. The mating system of S. cerevisiae provides a powerful model in which genetics, biochemistry, and molecular biology can be used to unravel the mysteries of peptide hormone structure and function.

Alpha-factor structural gene mutations in Saccharomyces cerevisiae: effects on alpha-factor production and mating

The role of alpha-factor structural genes MF alpha 1 and MF alpha 2 in alpha-factor production and mating has been investigated by the construction of mf alpha 1 and mf alpha 2 mutations that totally eliminate gene function. An mf alpha 1 mutant in which the entire coding region is deleted shows a considerable decrease in alpha-factor production and a 75% decrease in mating. Mutations in mf alpha 2 have little or no effect on alpha-factor production or mating. The mf alpha 1 mf alpha 2 double mutants are completely defective in mating and alpha-factor production. These results indicate that at least one alpha-factor structural gene product is required for mating in MAT alpha cells, that MF alpha 1 is responsible for the majority of alpha-factor production, and that MF alpha 1 and MF alpha 2 are the only active alpha-factor genes.

Occurrence of met-enkephalin, met-enkephalin-Arg6-Phe7 and met-enkephalin-Arg6-Gly7-Leu8 in gastrin cells of hog antral mucosa

Region-specific antisera to three enkephalins: met-enkephalin, met-enkephalin-Arg6-Phe7 and met-enkephalin-Arg6-Gly7-Leu8, together with four region specific antisera to progastrin: C-terminal G17 specific, N-terminal G34 specific, cryptic peptides A- and B-specific, were used in immunohistochemical studies of hog antral mucosa. A sub-population (6-10%) of the gastrin-containing endocrine cells (G-cells) was found to react with antisera to met-enkephalin, met-enkephalin-Arg6-Phe7 and met-enkephalin-Arg6-Gly7-Leu8. About 30% of all the enkephalin-containing cells were identified as G-cells. The results indicate that a fraction of G-cells produces both enkephalin-like peptides and gastrin.

Alpha-factor structural gene mutations in Saccharomyces cerevisiae: effects on alpha-factor production and mating

The role of alpha-factor structural genes MF alpha 1 and MF alpha 2 in alpha-factor production and mating has been investigated by the construction of mf alpha 1 and mf alpha 2 mutations that totally eliminate gene function. An mf alpha 1 mutant in which the entire coding region is deleted shows a considerable decrease in alpha-factor production and a 75% decrease in mating. Mutations in mf alpha 2 have little or no effect on alpha-factor production or mating. The mf alpha 1 mf alpha 2 double mutants are completely defective in mating and alpha-factor production. These results indicate that at least one alpha-factor structural gene product is required for mating in MAT alpha cells, that MF alpha 1 is responsible for the majority of alpha-factor production, and that MF alpha 1 and MF alpha 2 are the only active alpha-factor genes.

Nucleotide sequence of the extracellular glucoamylase gene STA1 in the yeast Saccharomyces diastaticus

The complete nucleotide sequence of the extracellular glucoamylase gene STA1 from the yeast Saccharomyces diastaticus has been determined. A single open reading frame codes for a 778-amino-acid protein which contains 13 potential N-glycosylation sites. In the 5'- and 3'-flanking regions of the gene, there are striking sequence homologies to the corresponding regions of ADH1 for alcohol dehydrogenase and MAT alpha 2 for mating type control in the yeast Saccharomyces cerevisiae. The putative precursor begins with a hydrophobic segment that presumably acts as a signal sequence for secretion. The presumptive signal sequence showed a significant homology to that of Bacillus subtilis alpha-amylase precursor. The next segment, of ca. 320 amino acids, contains a threonine-rich tract in which direct repeat sequences of 35 amino acids exist, and is bordered by a pair of basic amino acid residues (Lys-Lys) which may be a proteolytic processing signal. The carboxy-terminal half of the precursor is a presumptive glucoamylase which contains several peptide segments showing a high degree of homology with alpha-amylases from widely diverse organisms including a procaryote (B. subtilis) and eucaryotes (Aspergillus oryzae and mouse). Analysis of both the nucleotide sequence of the STA1 gene and the amino acid composition of the purified glucoamylase suggested that the putative precursor is processed to yield subunits H and Y of mature enzyme by both trypsin-like and chymotrypsin-like cleavages.

Immunoreactive Met-enkephalin Arg6 in rat brain, and bovine brain, gut, and adrenal

Antibodies directed against the Met-enkephalin-related hexapeptide, Met-enk Arg6, have been used in radioimmunoassays in the characterization of material in rat brain, and bovine striatum, colon, and adrenal medulla. Met-enk Lys6 reacted 0.27 relative to Met-enk Arg6, but Leu-enk Arg6 and C-terminal extensions or deletions of Met-enk Arg6 showed less than 0.02 immunoreactivity. In rat brain, the concentration of Met-enk Arg6-like immunoreactivity was less than 20 pmol X g-1 in all regions, but after trypsinization of tissue extracts there were up to 80-fold increases in immunoreactivity as a result of cleavage of C-terminally extended forms. The tryptic product eluted as Met-enk Arg6 on gel filtration. In control extracts of rat brain there were at least three immunoreactive forms of Met-enk Arg6; one eluted in the position of the hexapeptide standard on gel filtration and HPLC while the others had properties of N-terminally extended forms. In bovine striatum and colon the hexapeptide-like material predominated; but in bovine adrenal extracts, there were relatively low concentrations of the hexapeptide and, instead, the dominant immunoreactive forms corresponded to two components that were probably N-terminally extended variants. Trypsin again produced marked increases in immunoreactivity. HPLC studies indicated that Met-enk Arg6Phe7- and Met-enk Arg6Gly7Leu8-like immunoreactive peptides were important substrates in bovine brain for the production of hexapeptide immunoreactivity after trypsin. The differences in the patterns of immunoreactive forms in bovine adrenal, colon, and brain are consistent with tissue variations in the pathways of posttranslational processing of the precursor molecules.

Enkephalin convertase localization by [3H]guanidinoethylmercaptosuccinic acid autoradiography: selective association with enkephalin-containing neurons

Enkephalin convertase, an enkephalin-forming carboxypeptidase, is potently inhibited by guanidinoethylmercaptosuccinic acid (GEMSA). We have localized enkephalin convertase in rat brain by in vitro autoradiography with [3H]GEMSA. [3H]GEMSA-associated silver grains are highly concentrated in the median eminence, bed nucleus of the stria terminalis, lateral septum, dentate gyrus, hippocampus, central nucleus of the amygdala, preoptic hypothalamus, magnocellular nuclei of the hypothalamus, interpeduncular nucleus, dorsal parabrachial nucleus, locus coeruleus, nucleus of the solitary tract, and the substantia gelatinosa of the spinal trigeminal tract. This distribution corresponds closely with immunocytochemical localizations of enkephalin-containing cells and axons, indicating that enkephalin convertase is selectively involved in enkephalin biosynthesis.

Purification from brain of synenkephalin, the N-terminal fragment of proenkephalin

The primary sequence of adrenal proenkephalin was recently deduced from the structure of the cloned cDNA that codes for this protein. Several enkephalin-containing proteins with molecular weights between 8,000 and 20,000 daltons were purified from the bovine adrenal medulla. These proteins appear to represent intermediates in the processing of proenkephalin into physiologically active opioid peptides. While the concentrations of these large processing intermediates in the adrenal medulla are quite high, similar proteins have not yet been shown to be present in brain, and there is some question as to whether the brain synthesizes an enkephalin precursor similar to adrenal proenkephalin. We report here the purification from bovine caudate nucleus of synenkephalin, the N-terminal fragment of adrenal proenkephalin. The amino acid composition of synenkephalin indicates that the protein represents residues 1-70 of adrenal proenkephalin. Thus the brain and adrenal glands appear to utilize a similar precursor for enkephalin biosynthesis.

Distribution and characterization of opioid peptides derived from proenkephalin A in human and rat central nervous system

In various areas of rat and human brain and spinal cord the distributions of opioid peptides derived from the proenkephalin A precursor, the heptapeptide [Met]enkephalin-Arg6-Phe7 (MERF), the octapeptide [Met]enkephalin-Arg6-Gly7-Leu8 (MERGL), and bovine adrenal medulla dodecapeptide (BAM-12P), were determined by a combination of radioimmunoassay, gel filtration, and high-performance liquid chromatography. In the human central nervous system the highest concentrations were seen in the striatum (pallidum greater than caudate nucleus greater than putamen) and in substantia nigra, hypothalamus, and periaqueductal gray. Similarly, in rat brain high levels were found in striatum and hypothalamus. Bovine adrenal medulladocosa peptide (BAM-22P) only occurred in the rat brain, but could not be detected in human brain. No MERF, MERGL, BAM-12P, or BAM-22P could be found in either rat or human pituitary. In contrast to MERF, MERGL and BAM-12P, peptides derived from the proenkephalin B precursor, dynorphin1-8 and dynorphin B, showed high concentrations only in substantia nigra and pallidum, but quite low levels in the other regions of human brain and spinal cord. The present study provides evidence that the proenkephalin A precursor known from adrenal medulla also exists in the rat and human central nervous system. Moreover, the identification of BAM-12P in these tissues indicates that cleavage of the precursor molecule must also involve sites different from those with paired basic amino acids.

Changes in rat adrenal catecholamines and proenkephalin metabolism after denervation

Adrenal enkephalin-containing peptides are known to increase 10- to 15-fold in the rat after surgical denervation of the gland. In this report we show that the increase is preceded by a lag of several hours, which is indicative of stimulation of protein synthesis at the transcriptional level. The major species of newly appearing enkephalin-containing peptide appears to be the intact precursor, proenkephalin. Processing of proenkephalin to smaller enkephalin-containing peptides in the denervated glands is slow and limited. The only product that accumulates in the process is a peptide of 3-4 kilodaltons that is derived from the carboxyl terminus of proenkephalin. An interesting observation was the dissociation between the effects of denervation on enkephalin-containing peptides and catecholamines. This is surprising because both are localized in the chromaffin granules of the gland.

Further characterization of an enkephalin-generating enzyme from adrenal medullary chromaffin granules

An adrenomedullary protease capable of generating Met5-enkephalin from endogenous precursor(s) has been purified 1,000-fold using affinity chromatography in combination with gel filtration. This trypsin-like enzyme has an apparent molecular weight of 20,000 daltons by gel filtration. The reactivity of the enzyme toward several fluorogenic peptides, Peptides E and F, and the heptapeptides, Met5-enkephalin-Arg6-Phe7 and Met5-enkephalin-Arg6-Arg7, was examined. The two heptapeptides and the fluorogenic compounds were poor substrates for the adrenal enzyme; in contrast, Peptides E and F were cleaved. The low molecular weight products of Peptide F digestion were identified by HPLC as Arg1-Met6-enkephalin, Met5-enkephalin, and Met5-enkephalin-Lys6, while digestion of Peptide E resulted in the production of Leu5-enkephalin and Met5-enkephalin-Arg6-Arg7. [3H]-beta m-Lipotropin was not hydrolyzed by the adrenal enzyme. These results indicate that this adreno-medullary protease is capable of cleaving adrenal opioid peptides at the paired basic sites and thus represents a possible candidate for a proenkephalin-converting enzyme.

Metorphamide: isolation, structure, and biologic activity of an amidated opioid octapeptide from bovine brain

Acid acetone extracts of caudate nucleus from bovine brain were found to contain an amidated opioid octapeptide with the following structure: Tyr-Gly-Gly-Phe-Met-Arg-Arg-Val-NH2. The peptide has been named metorphamide. Bovine metorphamide appears to be derived by proteolytic cleavage from proenkephalin, the common precursor to [Met5]enkephalin and [Leu5]enkephalin. The cleavage within the precursor giving rise to the carboxyl terminus of metorphamide occurs at a single arginine residue and is followed by transformation of a carboxyl-terminal glycine into an amide group. Metorphamide was detected in bovine caudate nucleus extracts by radioimmunoassay, and it was purified to homogeneity by gel filtration and reversed-phase high performance liquid chromatography. Amino acid composition analysis and automated Edman degradation in the gas-phase sequencer confirmed the postulated amino acid sequence. Carboxyl-terminal amidation of bovine metorphamide was shown by stability to carboxypeptidase A digestion and full crossreactivity in a radioimmunoassay that required the carboxyl-terminal amide as part of the recognition site. A synthetic replicate of metorphamide as well as several synthetic analogs were tested for opioid activity in several bioassays and binding assays, and metorphamide was found to have a high mu-binding activity. Metorphamide is the only known naturally occurring opioid peptide that has a high mu-binding activity. The kappa-binding activity is approximately equal to 50% that of the mu-binding activity, but delta-binding activity is negligible.

Processing of enkephalin-containing peptides in isolated bovine adrenal chromaffin granules

Intact chromaffin granules isolated from bovine adrenal medulla were incubated at 37 degrees C for up to 22 hr. Processing of enkephalin-containing (EC) peptides in the granules was followed by the change in their size distribution as shown by chromatography on Sephadex G-75 columns. A gradual shift toward lower molecular weight EC peptides was observed during the incubation, indicating processing of the higher molecular weight to lower molecular weight EC peptides. The total amount of [Met]-enkephalin, free and in peptide linkage, remained constant indicating that little or no nonspecific degradation occurred during the experiment. HPLC resolution of the fraction containing the low molecular weight EC peptides showed that free enkephalins as well as [Met]enkephalin-Arg6-Phe7 and [Met]enkephalin-Arg6-Gly7-Leu8 accumulated while [Met]enkephalin-Arg6 and [Met]enkephalin-Lys6 disappeared. All the above data indicate the presence of an atypical trypsin activity and the presence of a carboxypeptidase B-like activity within the granules. From the rates of accumulation of the low molecular weight EC peptides and the disappearance of the higher molecular weight EC peptides, a processing rate of 65-70 pmol/g tissue per hr was estimated, which calculates to a lifetime of 6-8 days for EC peptides in the granules. Under steady-state conditions this rate of processing appears to be too low to produce significant amounts of free enkephalins from larger EC peptides. This is well in accord with previous observations that relatively small amounts of free enkephalins are found in bovine adrenal medulla.