Selective protection of methionine enkephalin released from brain slices by enkephalinase inhibition

Involvement of neuronal nitric oxide synthase in desensitisation of µ-opioid receptors in the rat locus coeruleus

Nitric oxide (NO) has been recently shown to enhance µ-opioid receptor (MOR) desensitisation in locus coeruleus (LC) neurons. The aim of this study was to evaluate by single-unit extracellular recordings in rat brain slices whether the neuronal NO synthase is involved in MOR desensitisation in LC neurons. As expected, a high concentration of the opioid agonist Met(5)-enkephalin (ME; 10 µM, 10 min) strongly desensitised the inhibition induced by a test application of ME (0.8 µM, 1 min), whereas lower ME concentrations (1 and 3 µM) only weakly desensitised it. The neuronal NO synthase inhibitors 7-nitroindazole (10-100 µM), S-methyl-L-thiocitrulline (0.01-10 µM) and N(ω)-propyl-L-arginine (1-10 µM) attenuated ME (10 µM)-induced opioid desensitisation, although the endothelial NO synthase inhibitor N(5)-(1-iminoethyl)-L-ornithine (3-30 µM) failed to change it. The NO donor sodium nitroprusside (1 mM), but not its inactive analog potassium ferricyanide (1 mM), enhanced the ME (3 µM)-induced desensitisation and prevented the effect of S-methyl-L-thiocitrulline (10 µM). Sodium nitroprusside (1 mM) failed to change the desensitisation of α2-adrenoceptors by noradrenaline (100 µM, 10 min). These results suggest the contribution of NO and a neuronal type of NO synthase in homologous MOR desensitisation in rat LC neurons.

Multiple roles of angiotensin in colorectal cancer

Colorectal cancer (CRC) cells express renin and chymase through which they can activate angiotensin. Renin expression is induced by hyperglycemic conditions. As angiotensinogen is produced in the liver, CRC cells that can activate angiotensin have an enhanced ability to metastasize to this organ. In human CRC cases, patients with diabetes have higher activities of rennin and angiotensin-II in primary tumors, and on average, have a more progressed disease stage, especially with respect to liver metastasis. These patients exhibit a stronger association with Hemoglobin A1c levels and metastasis compared to patients without diabetes. In a combined diabetes/CRC liver metastasis mouse model, concurrent treatment with anti-angiotensin and hypoglycemic agents shows a synergic effect in terms of reduced liver metastasis and improved survival. The effect of anti-angiotensin treatment and blood sugar control as a baseline management for colon cancer patients with diabetes needs to be examined in clinical trials to establish whether it can prevent liver metastasis.

Brain-Specific Aminopeptidase: From Enkephalinase to Protector Against Neurodegeneration

The major breakthrough discovery of enkephalins as endogenous opiates led our attempts to determine their inactivation mechanisms. Because the NH2-terminal tyrosine is absolutely necessary for the neuropeptides to exert analgesic effects, and aminopeptidase activities are extraordinarily high in the brain, a specific "amino-enkephalinase" should exist. Several aminopeptidases were identified in the central nervous system during the search. In fact, our laboratory found two novel neuron-specific aminopeptidases: NAP and NAP-2. NAP is the only functionally active brain-specific enzyme known. Its synaptic location coupled with its limited substrate specificity could constitute a "functional" specificity and contribute to enkephalin-specific functions. In addition, NAP was found to be essential for neuron growth, differentiation, and death. Thus, aminopeptidases are likely important for mental health and neurological diseases. Recently, puromycin-sensitive aminopeptidase (PSA) was identified as a modifier of tau-induced neurodegeneration. Because the enzymatic similarity between PSA and NAP, we believe that the depletion of NAP in Alzheimer's disease (AD) brains plays a causal role in the development of AD pathology. Therefore, use of the puromycin-sensitive neuron-aminopeptidase NAP could provide neuroprotective mechanisms in AD and similar neurodegenerative diseases.

Neuropeptidases

Neuropeptides are neurotransmitters and modulators distributed in the central nervous system (CNS) and peripheral nervous system. Their abnormalities cause neurological and mental diseases. Neuropeptidases are enzymes crucial for the biosynthesis and biodegradation of neuropeptides. We here focus on the peptidases involved in the metabolism of the well-studied opioid peptides. Bioactive enkephalins are formed from propeptides by processing enzymes—prohormone thiol protease, prohormone convertase 1 and 2 (PC 1 and 2), carboxypeptidase H/E, and Arg/Lys aminopeptidase. After they exert their biological effects, enkephalins are likely to be inactivated by degrading enzymes—angiotensin-converting enzyme (ACE), aminopeptidase N (APN), puromycin-sensitive aminopeptidase (PSA), and endopeptidase 24.11. Recently, a neuron-specific aminopeptidase (NAP), which was a putative enkephalin-inactivating enzyme at the synapses, was found. Neuropeptidases are useful drug targets and their inhibitors can be therapeutic. Synthetic anti-enkephalinases and anti-aminopeptidases are being developed. They are potent analgesics but have fewer side effects than the opiates.

Caenorhabditis elegans neprilysin NEP-1: an effector of locomotion and pharyngeal pumping

The control of signal peptide activity by cell surface proteases is one of the main factors that regulate the development and behaviour of organisms. In mammals, neprilysins (NEPs) are known to play a key role in these processes and their inactivation can initiate cellular disorganisation, which in turn may lead to prostate cancer or Hirschsprung disease. Although the proteome of the nematode Caenorhabditis elegans has been intensively studied, very little is known about the function of neprilysins. ZK20.6 (NEP-1), the C.elegans protein with highest identity to mammalian neprilysins, is a 753 amino acid residue protein that displays all neprilysin-typical characteristics, including a short intracellular domain, a transmembrane domain and a long extracellular active domain. Here we show that the expression pattern of nep-1 is limited to pharyngeal cells and a single head neuron. Compared to wild-type, the locomotion of nep-1 knockout animals is significantly impaired, a phenotype that can be rescued by the extrachromosomal re-introduction of nep-1. This suggests that this enzyme plays an important role in the regulation of nematode locomotion. Finally, electrophysiological recording of the pharyngeal activity showed a high sensitivity of the nep-1 pharynx to serotonin (5-HT) and to the neuropeptide AF1 (C.elegans FLP-8), indicating that NEP-1 is a central component that controls the neuronal innervation of pharyngeal pumping in C.elegans.

The Kell protein of the common K2 phenotype is a catalytically active metalloprotease, whereas the rare Kell K1 antigen is inactive. Identification of novel substrates for the Kell protein

The Kell blood group is a highly polymorphic system containing over 20 different antigens borne by the protein Kell, a 93-kDa type II glycoprotein that displays high sequence homology with members of the M13 family of zinc-dependent metalloproteases whose prototypical member is neprilysin. Kell K1 is an antigen expressed in 9% of the Caucasian population, characterized by a point mutation (T193M) of the Kell K2 antigen, and located within a putative N-glycosylation consensus sequence. Recently, a recombinant, non-physiological, soluble form of Kell was shown to cleave Big ET-3 to produce the mature vasoconstrictive peptide. To better characterize the enzymatic activity of the Kell protein and the possible differences introduced by antigenic point mutations affecting post-translational processing, the membrane-bound forms of the Kell K1 and Kell K2 antigens were expressed either in K562 cells, an erythroid cell line, or in HEK293 cells, a non-erythroid system, and their pharmacological profiles and enzymatic specificities toward synthetic and natural peptides were evaluated. Results presented herein reveal that the two antigens possess considerable differences in their enzymatic activities, although not in their trafficking pattern. Indeed, although both antigens are expressed at the cell surface, Kell K1 protein is shown to be inactive, whereas the Kell K2 antigen binds neprilysin inhibitory compounds such as phosphoramidon and thiorphan with high affinity, cleaves the precursors of the endothelin peptides, and inactivates members of the tachykinin family with enzymatic properties resembling those of other members of the M13 family of metalloproteases to which it belongs.

A Three-dimensional Model of the Neprilysin 2 Active Site Based on the X-ray Structure of Neprilysin

Neprilysin 2 (NEP2), a recently identified member of the M13 subfamily of metalloproteases, shares the highest degree of homology with the prototypical member of the family neprilysin. Whereas the study of the in vitro enzymatic activity of NEP2 shows that it resembles that of NEP as it cleaves the same substrates often at the same amide bonds and binds the same inhibitory compounds albeit with different potencies, its physiological role remains elusive because of the lack of selective inhibitors. To aid in the design of these novel compounds and better understand the different inhibitory patterns of NEP and NEP2, the x-ray structure of NEP was used as a template to build a model of the NEP2 active site. The results of our modeling suggest that the overall structure of NEP2 closely resembles that of NEP. The model of the active site reveals a 97% sequence identity with that of NEP with differences located within the S'(2) subsite of NEP2 where Ser(133) and Leu(739) replace two glycine residues in NEP. To validate the proposed model, site-directed mutagenesis was performed on a series of residues of NEP2, mutants expressed in AtT20 cells, and their ability to bind various substrates and inhibitory compounds was tested. The results confirm the involvement of the conserved Arg(131) and Asn(567) in substrate binding and catalytic activity of NEP2 and further show that the modifications in its S'(2) pocket, particularly the presence therein of Leu(739), account for a number of differences in inhibitor binding between NEP and NEP2.

Ontogeny, regional and cellular distribution of the novel metalloprotease neprilysin 2 in the rat: a comparison with neprilysin and endothelin-converting enzyme-1

The localisation of the gene transcripts of a recently discovered peptidase, neprilysin 2 (NEP2), was established by in situ hybridisation in rat tissues during development and adulthood. It was compared with those of neprilysin (NEP), a closely related enzyme in terms of sequence homology or substrate specificity, and of endothelin-converting enzyme 1 (ECE-1) which, like the other two, belongs to the M-13 sub-family of zinc-dependent metallopeptidases. The ontogeny of the three enzymes differed markedly, the expression of NEP2 being restricted to developing and differentiating fields of the CNS, whereas NEP and ECE-1 genes were broadly expressed early on in the CNS and periphery. In contrast to the wide expression of NEP and ECE-1 in peripheral adult tissues and in CNS, NEP2 was almost exclusively expressed in selected neuronal populations of the brain and spinal cord. The only exceptions were the intermediate and anterior lobes of the pituitary as well as the choroid plexuses, where NEP2 was also strongly expressed. These localisations as well as those in the hypothalamic nuclei, together with the previously established pattern of cleaved peptides, suggest the involvement of NEP2 in the metabolism of neurohormones of the hypothalamo-pituitary axis.Complementary distributions of NEP and NEP2 mRNAs were observed in a large number of brain areas with, for instance the former being highly expressed in the striatum in which NEP2 transcripts were almost undetectable. In contrast, NEP2 was highly expressed in numerous thalamic, hypothalamic and brainstem nuclei from which NEP was absent. Since both peptidases are able to cleave the same neuropeptides, this pattern may suggest a complementary role in their peptide inactivation functions in the CNS. Finally, ECE-1 mRNAs were generally observed in neuronal populations known to express the pre-proendothelin-1 gene, confirming the function of the metallopeptidase in endothelin-1 generation.

Cell-specific activity of neprilysin 2 isoforms and enzymic specificity compared with neprilysin

Neprilysin (NEP) 2 is a recently cloned glycoprotein displaying a high degree of sequence identity with neprilysin (EC 3.4.24.11), the prototypical member of the M13 subfamily of metalloproteases. Whereas NEP is involved in the metabolism of several bioactive peptides by plasma membranes of various cells, the enzymic properties and physiological functions of NEP2 are unknown. Here we characterize the cell-expression modalities and enzymic specificity of two alternatively spliced isoforms of NEP2 in Chinese hamster ovary and AtT20 cells. In the two cell lines, both isoforms are type II glycoproteins inserted in the endoplasmic reticulum as inactive precursors. Maturation detected by Western-blot analysis of glycosidase digests was cell-specific and more efficient in the endocrine cell line. The enzymic activity of both isoforms semi-purified from AtT20 cells reveals comparable specificities in terms of model substrates, pH optima and inhibitory patterns. NEP2 activity was compared with that of NEP regarding potencies of transition-state inhibitors, modes of hydrolysis, maximal hydrolysis rates and apparent affinities of bioactive peptides. Although all transition-state inhibitors of NEP inhibited NEP2 activity, albeit with different potencies, and many peptides were cleaved at the same amide bond by both peptidases, differences could be observed, i.e. in the hydrolysis of gonadotropin-releasing hormone and cholecystokinin, which occurred at different sites and more efficiently in the case of NEP2. Differences in cleavage of bioactive peptides, in cell-trafficking patterns and in tissue distribution indicate that NEP and NEP2 play distinct physiological roles in spite of their high degree of sequence identity.

Neutral endopeptidase knockout induces hyperalgesia in a model of visceral pain, an effect related to bradykinin and nitric oxide

Neutral endopeptidase (EC3.4.24.11, NEP, enkephalinase) is a zinc-metalloendopeptidase, cleaving a variety of substrates like enkephalins, substance P, and bradykinin. In the brain, NEP is a key enzyme in the degradation of enkephalins. Pharmacological inhibition of NEP-activity causes analgesia resulting from enhanced extracellular enkephalin concentrations. Recently, transgenic mice lacking the enzyme NEP have been developed (Lu, 1995). The present study was designed to investigate the nociceptive behavior of these NEP-knockout mice. Interestingly, NEP-deficient mice did not respond with decreased pain perception, but exhibited hyperalgesia in the hot-plate jump, warm-water tail-withdrawal, and mostnotablyin theacetic-acid writhing test. Inhibition of aminopeptidase N by bestatin reduced writhing in both strains, whereas NEP-inhibition by thiorphan reduced writhing selectively in wild-type mice. Naloxone increased writhing in wild-type but not in knockouts, whereas the bradykinin B2-receptor antagonist HOE140 reduced writhing selectively in NEP-knockouts. Similarly, the nitric oxide synthase inhibitor L-NAME reduced writhing in NEP-knockouts. These results indicate that genetic elimination of NEP, in contrast to pharmacological inhibition, leads to bradykinin-induced hyperalgesia instead of enkephalin-mediated analgesia. Nitric oxide (NO) is suggested to be involved in this process.

Inhibitors of tripeptidyl peptidase II. 2. Generation of the first novel lead inhibitor of cholecystokinin-8-inactivating peptidase: a strategy for the design of peptidase inhibitors

The cholecystokinin-8 (CCK-8)-inactivating peptidase is a serine peptidase which has been shown to be a membrane-bound isoform of tripeptidyl peptidase II (EC 3.4.14.10). It cleaves the neurotransmitter CCK-8 sulfate at the Met-Gly bond to give Asp-Tyr(SO(3)H)-Met-OH + Gly-Trp-Met-Asp-Phe-NH(2). In seeking a reversible inhibitor of this peptidase, the enzymatic binding subsites were characterized using a fluorimetric assay based on the hydrolysis of the artificial substrate Ala-Ala-Phe-amidomethylcoumarin. A series of di- and tripeptides having various alkyl or aryl side chains was studied to determine the accessible volume for binding and to probe the potential for hydrophobic interactions. From this initial study the tripeptides Ile-Pro-Ile-OH (K(i) = 1 microM) and Ala-Pro-Ala-OH (K(i) = 3 microM) and dipeptide amide Val-Nvl-NHBu (K(i) = 3 microM) emerged as leads. Comparison of these structures led to the synthesis of Val-Pro-NHBu (K(i) = 0.57 microM) which served for later optimization in the design of butabindide, a potent reversible competitive and selective inhibitor of the CCK-8-inactivating peptidase. The strategy for this work is explicitly described since it illustrates a possible general approach for peptidase inhibitor design.

Immunolocalization of tripeptidyl peptidase II, a cholecystokinin-inactivating enzyme, in rat brain

Tripeptidyl peptidase II (EC 3.4.14.10) is a serine peptidase apparently involved in the inactivation of cholecystokinin octapeptide [Rose C. et al. (1996) Nature 380, 403-409]. We have compared its distribution with that of cholecystokinin in rat brain, using a polyclonal antibody raised against a highly purified preparation for immunohistochemistry at the photon and electron microscope levels. Tripeptidyl peptidase II-like immunoreactivity was mostly detected in neurons, and also in ependymal cells and choroid plexuses, localizations consistent with a possible participation of the peptidase in the inactivation of cholecystokinin circulating in the cerebrospinal fluid. Immunoreactivity was mostly detected in cell bodies, large processes and, to a lesser extent, axons of various neuronal populations. Their localization, relative to that of cholecystokinin terminals, appears to define three distinct situations. The first corresponds to neurons with high immunoreactivity in areas containing cholecystokinin terminals, as in the cerebral cortex or hippocampal formation, where pyramidal cell bodies and processes surrounded by cholecystokinin axons were immunoreactive. A similar situation was encountered in many other areas, namely along the pathways through which cholecystokinin controls satiety, i.e. in sensory vagal neurons, the nucleus tractus solitarius and hypothalamic nuclei. The second situation corresponds to cholecystokinin neuronal populations containing tripeptidyl peptidase II-like immunoreactivity, as in neurons of the supraoptic or paraventricular nuclei, axons in the median eminence or nigral neurons. In both situations, localization of tripeptidyl peptidase II-like immunoreactivity is consistent with a role in cholecystokinin inactivation. The third situation corresponds to areas with mismatches, such as the cerebellum, a region devoid of cholecystokinin, but in which Purkinje cells displayed high tripeptidyl peptidase II-like immunoreactivity, possibly related to a role in the inactivation of neuropeptides other than cholecystokinin. Also, some areas with cholecystokinin terminals, e.g., the molecular layer of the cerebral cortex, were devoid of tripeptidyl peptidase II-like immunoreactivity, suggesting that processes other than cleavage by tripeptidyl peptidase II may be involved in cholecystokinin inactivation. Tripeptidyl peptidase II-like immunoreactivity was also detected at the ultrastructural level in the cerebral cortex and hypothalamus using either immunoperoxidase or silver-enhanced immunogold detection. It was mainly associated with the cytoplasm of neuronal somata and dendrites, often in the vicinity of reticulum cisternae, Golgi apparatus or vesicles, and with the inner side of the dendritic plasma membrane. Hence, whereas a fraction of tripeptidyl peptidase II-like immunoreactivity localization at the cellular level is consistent with its alleged function in cholecystokinin octapeptide inactivation, its association with the outside plasma membrane of neurons remains to be confirmed.

Characterization and inhibition of a cholecystokinin-inactivating serine peptidase

A cholecystokinin (CCK)-inactivating peptidase was purified and identified as a membrane-bound isoform of tripeptidyl peptidase II (EC 3.4.14.10), a cytosolic subtilisin-like peptidase of previously unknown functions. The peptidase was found in neurons responding to cholecystokinin, as well as in non-neuronal cells. Butabindide, a potent and specific inhibitor, was designed and shown to protect endogenous cholecystokinin from inactivation and to display pro-satiating effects mediated by the CCKA receptor.

On the physiology of metazoa

The problem on integration and control of the various processes of the metazoan organism is a major challenge to the physiologist. The traditional research strategy in dealing with the problem is neuron-oriented and its roots extend back into the last century when knowledge of hormones was lacking. In the present article, the traditional strategy is analyzed in the light of available data and its logical basis is questioned. Different levels of communication are supposed to occur in the animal or human body. Circulating hormones are responsible for the highest level of communication that occurs between organs or tissues. The central concept in the article is that regulation of circulating hormones constitutes a higher level of control relative to regulation of intercellular hormones. This is regardless of whether the latter occurs in the nervous system or elsewhere. The approach is utilized in defining the mechanism that integrates and controls the part processes of the body. The mechanism is defined as endothelial; the vascular endothelial system is the controlling part and the nervous system is one of the subordinate parts. Thanks to the new approach, meaningful biological explanations of major psychiatric disorders are now possible.

Endogenous opioid regulation of hippocampal function

Endogenous opioid peptides modulate neural transmission in the hippocampus. Procnkephalin-derived peptides have been demonstrated to act at mu and delta opioid receptors to inhibit GABA release from inhibitory interneurons, resulting in increased excitability of hippocampal pyramidal cells and dentate gyrus granule cells. Prodynorphin-derived peptides primarily act at presynaptic kappa opioid receptors to inhibit excitatory amino acid release from perforant path and mossy fiber terminals. Opioid receptors reduce membrane excitability by modulating ion conductances, and in this way they may decrease voltage-dependent calcium influx and transmitter release. Synaptic plasticity in the hippocampus also is modulated by endogenous opioids. Enkephalins facilitate long-term potentiation, whereas dynorphins inhibit the induction of this type of neuroplasticity. Further, opioids may play important roles in hippocampal epilepsy. Recurrent seizures induce changes in the expression of opioid peptides and receptors. Also, enkephalins have proconvulsant effects in the epileptic hippocampus, whereas dynorphins may function as endogenous anticonvulsants.

Detection of neutral endopeptidase (NEP, enkephalinase, E.C.3.4.24.11) in relation to dopaminergic and gonadoliberinergic nerve endings in the median eminence of the male rat: a double labeling ultrastructural study

The existence of neutral endopeptidase (Enkephalinase, NEP, E.C.3.4.24.11) in membranes of nerve endings in the rat median eminence suggests that some neuropeptides have paracrine and/or autocrine actions in this region. In vitro, neutral endopeptidase is capable of hydrolysing a variety of regulatory peptides but in vivo, many works indicate that in the central nervous system this enzyme is highly implicated in the biological inactivation of enkephalins and tachykinins. In addition there is evidence that NEP is also involved in the inactivation of neurotensin in vivo. The modulation of the release of gonadotrophin releasing hormone (GnRH) is one of the documented actions of enkephalins within the median eminence. However, it is at present unclear whether enkephalins act on dopamine endings, on GnRH endings or on both. As the technical parameters and particularly the tissue fixation used to detect neutral endopeptidase are compatible with immunocytochemical detection of GnRH and tyrosine-hydroxylase (the rate limiting enzyme in the synthesis of catecholamines), two double immunolabelings were realised at the ultrastructural level to determine if GnRH and dopamine nerve endings have the enzyme inserted within their plasma membrane. Our study shows the presence of neutral endopeptidase on tyrosine-hydroxylase-immunoreactive nerve endings while presence of the enzyme on GnRH-immunoreactive nerve endings is not demonstrated. Consequently, our results provide morphological arguments for possibilities of paracrine and/or autocrine actions by neuropeptides inactivated by neutral endopeptidase on tuberoinfundibular dopaminergic nerve endings. Conversely, action of the same peptides on GnRH boutons seems more unlikely.

Enkephalinase inhibition and hippocampal excitatory effects of exogenous and endogenous opioids

1. The relationships between the in vivo and in vitro epileptogenic effects of opioids or enkephalins and the electrophysiological activity of inhibitors of endogenous enkephalinase were analyzed. 2. The functional effects of the inhibition of the endogenous enkephalinase has been compared with the role of the endogenous opioid peptidergic system in the control of neuronal excitability.

Concomitant variation in immunoreactivity for enkephalins and neutral endopeptidase in the median eminence of the lactating rat

In the brain, neutral endopeptidase (NEP) is implicated in the biological inactivation of enkephalins and substance P. NEP, enkephalins and neurokinins were detected by immunocytochemistry in the median eminence (ME) of lactating female rats either freely nursing or separated from their pups for 48 h. A significant decrease in enkephalin and NEP immunoreactivity was observed in the external zone of the ME after pup removal. In both animal groups, neurokinin immunoreactivity was absent from this region. These results indicate that: (1) a concomitant variation in immunoreactivity for enkephalins and NEP occur in the ME of the lactating rat; (2) in the ME during lactation, NEP may essentially be implicated in the biological degradation of enkephalins.

The effects of methionine-enkephalin and its related metabolites upon the duration of the dorsal immobility response in rats

The effects of SC injections of methionine-enkephalin (Met1-5-Enk) and its N-terminal and C-terminal fragments upon the duration of the dorsal immobility response (DIR) over a 60-min time course were investigated. Experiment 1 analyzed the effects of various dosages (0.00-100.0 micrograms/kg) on DIR resulting in a potentiation of the duration in a dose-time course function. The effects of various fragments of Met1-5-Enk (10.0 micrograms/kg) from the N-terminal in Experiment 2 and from the C-terminal in Experiment 3 on the DIR resulted in the potentiation of the duration with the Met2-5-Enk and Met1-3-Enk fragments. All other fragments were not significant. The results were discussed in reference to the processing and metabolism of Met1-5-Enk.

Identification and characterization of a neutral endopeptidase activity in Aplysia californica

Kidney plasma membranes of Aplysia californica were shown to contain an endopeptidase activity which cleaved [Leu]enkephalin (Tyr-Gly-Gly-Phe-Leu) and [Leu]enkephalinamide (Tyr-Gly-Gly-Phe-Leu-NH2) at the Gly3-Phe4 bond, as determined by reverse-phase h.p.l.c. analysis of metabolites. The optimal pH was shown to be 6.5. The bivalent cation chelating agent, 1,10-phenanthroline protected [Leu]enkephalin from degradation, suggesting that this enzyme is a metallopeptidase. The degradation of [Leu]enkephalin was also abolished by the neutral endopeptidase-24.11 inhibitors RB104 (2-[(3-iodo-4-hydroxyl)-phenylmethyl]-4-N-[3-(hydroxyamino-3-oxo-1- phenylmethyl)-propyl]amino-4-oxobutanoic acid), HABCO-Gly [(3-hydroxy-aminocarbonyl-2-benzyl-1-oxypropyl)glycine], phosphoramidon and thiorphan, with IC50 values of 1 nM, 1 microM, 20 microM and 30 microM respectively. By contrast, the angiotensin-converting enzyme inhibitor captopril and the serine proteinase inhibitor phenylmethanesulphonyl fluoride were without effect. Phase separation experiments using Triton X-114 showed that about 64% of the neutral endopeptidase activity in the Aplysia kidney membrane corresponds to an integral membrane protein. A specific radioiodinated inhibitor ([125I]RB104) was shown to bind the Aplysia endopeptidase with high affinity; the KD and Bmax. values were 21 +/- 5 pM and 20.3 +/- 5 fmol/mg of proteins respectively. This inhibitor was used to determine the molecular form of the enzyme, after separation of solubilized membrane proteins on SDS/PAGE and transfer on to nitrocellulose membranes. A single protein band with an apparent molecular mass of 140 kDa was observed. The labelling was abolished by specific neutral endopeptidase inhibitors. This study provides the first biochemical characterization of an endopeptidase with catalytic properties similar to those of neutral endopeptidase-24.11 in the mollusc Aplysia californica.

Radioimmunocytochemical distribution of neutral endopeptidase (enkephalinase E.C.3.4.24.11) at the ultrastructural level in the rat median eminence

Neutral endopeptidase (E.C.3.4.24.11) was visualized at the ultrastructural level in the external zone of the rat median eminence by using 125I-labelled IgG of a monoclonal serum. A precise analysis of the localization of the immunolabelling, which appears in the form of individual stray silver grains, was undertaken. Among the 1,045 grains counted, 82% were localized over membrane appositions involving nerve endings only and nerve endings plus tanycytes. The difference between the real and a randomly generated population of grains was statistically significant. Our results provide morphological arguments in support of the view of a paracrine action of neuropeptides present in the median eminence especially enkephalins but possibly, substance P, angiotensin, cholecystokinin and neurotensin. These neuropeptides are known to be inactivated by neutral endopeptidase. The action of these peptides may be exerted on nerve endings (autocrine or paracrine) but an intervention on tanycytes cannot be excluded.

Identification and characterization of aminopeptidases from Aplysia californica

Aminopeptidase activities were identified in extracts of kidney, ovotestis, head ganglia, heart and haemolymph of Aplysia californica. These enzyme preparations hydrolysed [3H][Leu]enkephalin at the Try-1-Gly-2 bond as determined by h.p.l.c. analysis of cleavage products. In all these tissues, enkephalin-degrading aminopeptidase activities were present both in membrane-bound and cytosolic fractions. The bivalent-cation-chelating agent, 1,10-phenanthroline, inhibited kidney membrane aminopeptidase activity with an IC50 of 30 microM, suggesting that this enzyme is a metalloproteinase. The aminopeptidase inhibitor amastatin was the most potent inhibitor of [Leu]enkephalin degradation (IC50 25 nM) by membrane-bound aminopeptidase, and bacitracin, bestatin and puromycin were about 100-1000 times less potent. In contrast with membrane-bound aminopeptidase, the cytosolic form is sensitive to puromycin. Angiotensin-converting enzyme inhibitor had no effect on [Leu]enkephalin degradation by kidney membranes, while the neutral endopeptidase inhibitors were poor inhibitors of the enzymes in this preparation. The Km values of the aminopeptidase in the kidney membranes and cytosolic fractions for the [Leu]enkephalin substrate were 2.4 and 7.4 microM respectively. The aminopeptidase present in the kidney membranes also hydrolysed endogenous Phe-Met-Arg-Phe-amide peptide at the Phe-1-Met-2 bond as well as synthetic alanine p-nitroanilide and leucine p-nitroanilide. When used in a competition assay, these substrates inhibited hydrolysis of [3H][Leu]enkephalin, suggesting that the same enzyme degraded all these substrates. Taken together, these results suggest that Aplysia tissues contain both a membrane-bound aminopeptidase related to the mammalian aminopeptidase N and a cytosolic puromycin-sensitive aminopeptidase.

Effects of thiorphan, bestatin and a novel metallopeptidase inhibitor JMV 390-1 on the recovery of neurotensin and neuromedin N released from mouse hypothalamus

The effects of the endopeptidase 24.11 ('enkephalinase') inhibitor thiorphan, the aminopeptidase inhibitor bestatin and a novel metallopeptidase inhibitor JMV 390-1 on the K(+)-evoked release of immunoreactive neurotensin and neuromedin N (iNT and iNN) from mouse hypothalamic slices were examined. (JMV 390-1 inhibits several metallopeptidases including endopeptidases 24.11, 24.15 and 24.16, and aminopeptidase N equipotently with Ki values around 50 nM.) Thiorphan increased the recovery of released iNT nearly 2-fold and had no effect on iNN. Bestatin produced a 4-fold increase in iNN recovery and was inactive on iNT. Finally, iNT and iNN recoveries were increased up to 4- and 5-fold, respectively, by JMV 390-1. These results show that in the mouse hypothalamus endopeptidase 24.11 participates with other metalloendopeptidases to the degradation of endogenously released NT while endogenously released NN is principally degraded by aminopeptidase(s).

Clinical and biochemical aspects of depressive disorders: II. Transmitter/receptor theories

The present document is the second of three parts in a review that focuses on recent data from clinical and animal research concerning the biochemical bases of depressive disorders, diagnosis, and treatment. Various receptor/transmitter theories of depressive disorders are discussed in this section. Specifically, data supporting noradrenergic, serotonergic, cholinergic, dopaminergic, GABAergic, and peptidergic theories, as well as interactions between noradrenergic and serotonergic, or cholinergic and catecholaminergic systems are presented. Problems with the data and future directions for research are also discussed. A previous publication, Part I of this review, dealt with the classification of depressive disorders and research techniques for studying the biochemical mechanisms of these disorders. A future publication, Part III of this review, discusses treatments for depression and some of the controversies in this field.

An in vitro study on the hippocampal electrophysiological properties of enkephalinase inhibitors in rats

The effects of two enkephalinase inhibitors were studied on the CA1 and dentate hippocampal extracellular field potentials (FPs). The enkephalinase inhibitors thiorphan and SCH 32615, at a concentration of 1-500 microM, failed to significantly affect CA1 and dentate FPs. Thiorphan and SCH 32615, at a concentration of 150 microM, were able to potentiate the enkephalin-induced epileptiform bursting, inducing an increase in the bursting duration and in the number of spikes per burst due to 3.5 microM DAEAM or 0.20 microM DAGO. The results suggest: 1) the potentiation of an electrophysiological opiate receptor-mediated response by enkephalinase inhibitors; 2) the inability to show a direct effect on the basal CA1FP as a result of the inhibition of the endogenous enkephalinase.

Dopaminergic regulation of enkephalin release

The effects of dopamine receptor stimulation on enkephalin release were evaluated in vitro and in vivo by measuring the changes in the levels of [Met5]enkephalin (YGGFM) and Tyr-Gly-Gly (YGG), a characteristic extracellular enkephalin metabolite produced under the action of enkephalinase. In rat striatal slices, D1-receptor agonists or antagonists did not modify enkephalin release. By contrast, D2-receptor agonists enhanced the potassium-induced release of YGGFM and YGG without affecting spontaneous release from nondepolarized slices. This response was prevented by the D2-receptor antagonists haloperidol and RIV 2093, the latter compound being more potent, which suggested the involvement of a putative D2-receptor subtype. Acute administration of apomorphine or selective D2-receptor agonists, but not that of a D1-receptor agonist, enhanced the steady-state level of YGG without affecting the YGGFM level in rat striatum. The effect was blocked selectively by D2-receptor antagonists which, administered alone, had no effect. These observations indicate that D2-receptor stimulation in vitro or in vivo facilitates enkephalin release from striatal neurons, but that endogenous dopamine does not exert any tonic influence upon the opioid peptide neuron activity under basal conditions. However, chronic administration of haloperidol resulted in increases in striatal YGGFM and YGG, an effect presumably reflecting a long-term adaptive process.

Tissue distribution of neutral endopeptidase 24.11 ('enkephalinase') activity in guinea pig trachea

The distribution of neutral endopeptidase 24.11 (NEP; 'enkephalinase') activity was studied on tissue sections of the guinea pig trachea using a histochemical method based on the catalytic activity of the enzyme. The specificity for NEP of the histochemical reaction was verified by application of an array of peptidase inhibitors. NEP activity was most prominent on the respiratory epithelium, but occurred also in submucous glands, connective tissue of the lamina propria, perichondrium and chondrocytes. The findings suggest that NEP in the trachea is involved in various functions, cleavage of neurally released peptides being only one of them.

An in vitro study on the hippocampal epileptogenic properties of enkephalins and enkephalinase inhibitors in rats

1. The effects of enkephalins and enkephalinase inhibitors were studied in CA1 area in rat hippocampal slices. 2. The data demonstrate a prevalent involvement of mu opiate receptors in the epileptogenic properties of enkephalins. 3. A potentiation of the mu opiate receptor-mediated epileptogenic response by enkephalinase inhibitors has been shown. 4. The results also show an inability to affect basal CA1 field potentials by inhibition of endogenous endopeptidase.

Neurotensin high affinity binding sites and endopeptidase 24.11 are present respectively in the meningothelial and in the fibroblastic components of human meningiomas

The presence of neurotensin receptors and endopeptidase 24.11 (E-24.11) in 16 human meningioma specimens, obtained at surgery, was assessed by measuring the binding of 125I-[tyrosyl3]neurotensin(1-13) (125I-NT) and the inhibitor 3H-N(2RS)-3-hydroxyaminocarbonyl-2-benzyl-1-oxopropyl)glycine (3H-HACBO-Gly), for the receptor and enzyme, respectively. E-24.11 activity was also measured. Autoradiography, on the 16 meningiomas, showed that specific 125I-NT labeling (nonspecific labeling was assessed in the presence of excess NT) was exclusively located in the meningothelial regions. In contrast, specific 3H-HACBO-Gly labeling (nonspecific labeling was assessed in the presence of an excess of the E-24.11 inhibitor thiorphan) was exclusively found in fibroblastic regions. No specific labeling of either ligand was found on collagen or blood vessels. In vitro binding assays were performed on membranes of 10 of the 16 meningiomas. In the 4 meningiomas rich in meningothelial cells, 125I-NT specifically bound to one population of sites with Bmax ranging from 57 to 405 fmol/mg protein and Kd around 0.3 nM. These sites share common properties with the brain NT receptor, since the carboxy terminal acetyl NT(8-13) fragment bound to the same sites but with a higher affinity. The carboxy terminal analogue of NT, neuromedin N, also bound to the same sites with a 10-fold lower affinity and the sites were bradykinin and levocabastine insensitive. In the 4 meningiomas rich in fibroblastic cells, 3H-HACBO-Gly specifically bound to one population of sites with Bmax ranging from 251 to 739 fmol/mg protein and Kd around 2.8 nM.(ABSTRACT TRUNCATED AT 250 WORDS)

Release in vivo of Met-enkephalin and encrypted forms of Met-enkephalin from brain and spinal cord of the anesthetized cat

Processing of the proenkephalin molecule will result in peptide fragments in which the pentapeptide YGGFM is included. We have employed a molecular sieve (2 kDa) separation, enzyme hydrolysis radioimmunoassay (RIA) treatment sequence which permits concurrent measurement of Met-enkephalin (Enk) and several enkephalin-encrypting (X-Enk) peptides in a single sample. Using this protocol, the release of Enk and X-Enk (total Enk-Enk) greater and less than 2 kDa from spinal cord and the mesencephalic aqueductal grey was assessed under resting conditions and during stimulation of the sciatic nerve in the chloralose-urethane anesthetized cat. Under resting conditions measurable levels of Enk (10.5 +/- 4.7; 9.1 +/- 2.1 pg/min) and X-Enk (47.8 +/- 19.7; 45.7 +/- 12.3 pg/min) are found in aqueductal and spinal superfusates, respectively. The X-Enk measured under resting and evoked conditions in aqueductal and spinal perfusates is associated almost exclusively (greater than 90-95%) with fragments greater than 2 kDa in size. There results, showing the relative absence of detectable levels of X-Enk forms greater than 2 kDa, were confirmed by reverse phase chromatography. During sciatic nerve stimulation, the levels of both Enk and X-Enk were mildly elevated in spinal and ventricular perfusates. With the addition of thiorphan (10(-5) M), though there was no effect on the resting release of either Enk or X-Enk, the levels of Enk measured under evoked conditions were significantly augmented in both ventricular and spinal perfusates.

The antinociceptive effects of SCH-32615, a neutral endopeptidase (enkephalinase) inhibitor, microinjected into the periaqueductal, ventral medulla and amygdala

The local effects of SCH-32615, an inhibitor of enkephalinase (EC 3.4.24.11) on the hot-plate (HP) and tail-flick (TF) responses were examined following unilateral intracerebral microinjection into the periaqueductal brain (PAG), the medial ventral medulla (VM) and bilateral microinjection into the amygdala (AM) of the rat. In the PAG and VM, SCH-32615 resulted in a dose-dependent increase in HP and TF response latencies over a dose range of 1-30 micrograms with the ED50 values (micrograms) being PAG-TF = 17; PAG-HP = 11; VM-TF = 7; VM-HP = 6. In the AM, dose-dependent increases were only observed on the HP. (ED50 (micrograms) HP = 17). Peak effects were observed within 10 min and response latencies remained elevated for 45-60 min. Injections of SCH-32615 at sites outside of the PAG or VM were considerably less effective. All antinociceptive effects were antagonized by naloxone (1 mg/kg, i.p.). Twenty-four hours following the microinjection of beta-funaltrexamine (an irreversible opioid antagonist) into the PAG or the VM, the effects of SCH-32615 in the PAG were virtually abolished while in the VM, its effects were only moderately reduced. These data suggest that in the presence of a strong thermal stimulus, the behavioral response is subject to a tonically active or stimulus-evoked modulation by the local release in the PAG, VM and AM of an agent, presumably an enkephalin peptide, the degradation of which is altered by enkephalinase inhibition.

Haloperidol-induced increase in striatal concentration of the tripeptide, Tyr-Gly-Gly, provides an index of increased enkephalin release in vivo

A sensitive and specific radioimmunoassay has been developed for the tripeptide, Tyr-Gly-Gly, which has been shown previously to be an extraneuronal metabolite of opioid peptides derived from proenkephalin A. Using this assay, we found a regional variation in Tyr-Gly-Gly immunoreactivity in rat brain, with highest levels in striatum and lowest in cerebral cortex. Intracerebroventricular administration of the aminopeptidase inhibitor, bestatin; produced a threefold increase in Tyr-Gly-Gly immunoreactivity in rat striatum, whereas thiorphan, an enkephalinase inhibitor, produced a 45% reduction in striatal Tyr-Gly-Gly immunoreactivity. These data suggest that the tripeptide, Tyr-Gly-Gly, is in a dynamic state in the brain, and provide further support for the hypothesis that its concentration in specific brain areas may reflect the release of endogenous enkephalins in these brain areas. Further confirmation of the validity of measurements of brain Tyr-Gly-Gly as indices of enkephalin release under conditions of altered neuronal activity was provided by our demonstration that chronic dopamine receptor blockade with haloperidol increased striatal concentrations of both Met-enkephalin and Tyr-Gly-Gly.

Role of delta opioid receptors in the effects of inhibitors of enkephalin-degrading peptidases on the horizontal and vertical components of locomotion in mice

In the present study we report the effects of inhibitors of enkephalin-degrading peptidases on spontaneous locomotion in mice and the involvement of delta opioid receptors in these effects. Animals received intracerebroventricularly (i.c.v.) or intravenously (i.v.) enkephalinase inhibitors (thiorphan and acetorphan), aminopeptidase inhibitors (bestatin and carbaphethiol) or mixed peptidase inhibitors (kelatorphan). The i.c.v. co-administration of bestatin and thiorphan (50 micrograms + 50 micrograms) induced an increase in both the horizontal and vertical components of locomotion. A similar pattern was observed after the i.c.v. administration of kelatorphan (8.5-50 micrograms) or the i.v. co-administration of acetorphan and carbaphethiol (5 mg/kg + 10 mg/kg). The opiate antagonist naltrexone (1 mg/kg, s.c.) failed to reverse the excitolocomotor effects of kelatorphan or of bestatin and thiorphan and antagonized only partially the effects of acetorphan and carbaphethiol. Naloxone (2 mg/kg-10 mg/kg, s.c.) partially reversed the increase in locomotion elicited by bestatin and thiorphan. The pretreatment with the delta opioid antagonists ICI 154,129 (20 micrograms, i.c.v.) or ICI 174,864 (2-4 micrograms, i.c.v.) strongly decreased the effects of all the peptidase inhibitors we tested. These results suggest that endogenous enkephalins may control via delta opioid receptors the horizontal and vertical components of locomotor activity in mice.

Studies on the effect of SCH-34826 and thiorphan on [Met5]enkephalin levels and release in rat spinal cord

SCH-34826 and thiorphan are inhibitors of the neutral endopeptidase (NEP; E.C. 3.4.24.11;) that cleaves the opiate peptides [Met5]- and [Leu5]enkephalin at the glycinylphenylalanine bond. These compounds were evaluated for their ability to affect the levels of [Met5]enkephalin-like immunoreactivity (MELI) in the brain and the spinal cord and the release into the extracellular space under resting and K+-evoked conditions. The results showed that oral administration of SCH-34826 (30-100 mg/kg p.o.) or thiorphan (10-30 mg/kg p.o.) had no effect on tissue levels of MELI. In contrast, both agents caused a dose-dependent increase in both the resting and the K+-evoked levels in spinal perfusates, which reached up to 10 times the control values. These data indicate that tissue (presumably intracellular) stores of [Met5]enkephalin are not affected by NEP inhibition and that it is the extracellular effects of the peptide that are potentiated by enzyme blockade. This agrees with the prior results demonstrating that NEP inhibitors require a nociceptive stimulus sufficient to release endogenous stores of [Met5]enkephalins for their actions to be observed.

The release of Leu5-enkephalin-like immunoreactivity from chicken retina is reduced by light in vitro

A superfusion system was established to examine the efflux of endogenous Leu5-enkephalin-like immunoreactivity (LE-LI) from isolated chicken retinas. Superfusion with buffer containing high concentration of K+ (60 mM KCl) increased the efflux of LE-LI by 96%. This effect was not observed when Co2+ (4 mM CoCl2) was present. Exposing the retinas to light decreased the efflux of LE-LI by 59% compared to that observed during superfusion in the dark. No effect of ambient light could be detected in the presence of Co2+. Upon reverse-phase high-performance liquid chromatography the material released by the retina comigrated with synthetic Leu5-enkephalin. These results demonstrate that the release of LE-LI from retinal neurons is increased during the dark, and it is concluded that the lighting conditions exert their effects by modifying the state of polarization of the LE-LI amacrine cells and hence the release of LE-LI from these neurons.

Changes in levels of the tripeptide Tyr-Gly-Gly as an index of enkephalin release in the spinal cord: effects of noxious stimuli and parenterally-active peptidase inhibitors

The tripeptide Tyr-Gly-Gly (YGG), representing the product of enkephalin hydrolysis by enkephalinase (EC 3.4.24.11), was characterized and its levels measured in spinal cord perfusates of halothane-anaesthetized rats. During noxious pinching of the muzzle, which is known to trigger enkephalin release, YGG levels were enhanced more markedly and for longer than were those of [Met5]enkephalin (YGGFM), in the same samples. By contrast, neither YGG nor YGGFM levels were affected by pinching the tail. Treatment with carbaphethiol, a parenterally-active aminopeptidase inhibitor, markedly increased YGG levels and lengthened the duration of the increase produced by pinching the muzzle. Treatment with acetorphan, a parenterally-active enkephalinase inhibitor, given alone or in combination with carbaphethiol, completely prevented the rise in YGG triggered by noxious stimulation. By contrast, [Met5]enkephalin levels in the perfusates were increased by the combined administration of the two peptidase inhibitors but these levels were not further enhanced by noxious stimulation. Thus, spinal cord YGG appears to be formed under the influence of enkephalinase and to constitute a sensitive index of enkephalin release.

Fluorescent histochemical localization of neutral endopeptidase-24.11 (enkephalinase) in the rat spinal cord

The localization of neutral endopeptidase-24.11 (E.C. 3.4.24.11; enkephalinase) in rat spinal cord was investigated by a novel fluorescent histochemical method. Enkephalinase was localized by using a coupled enzyme assay based upon the sequential cleavage of the synthetic peptide substrate glutaryl-ala-ala-phe-4-methoxy-2-naphthylamide by enkephalinase and exogenous aminopeptidase M. Enzyme distribution was examined in segments from cervical, thoracic, lumbar, and sacral cord. At all spinal cord levels, enkephalinase was localized to discrete regions of the gray matter. The substantia gelatinosa displayed rich enkephalinase staining which overlapped the inner and outer zones of lamina II. A staining pattern similar to that observed in lamina II was observed in the spinal trigeminal nucleus in the medulla. In lamina III the enzyme was associated with small and medium-sized cells. Lamina IV showed staining associated with medium-sized and large cell bodies. The medial boundary of the dorsal gray of laminae IV and V had medium-sized fusiform cells which stained for enkephalinase. In the lateral reticulated areas of lamina V, enkephalinase reaction product was localized to scattered medium-sized and large cells compressed against the white matter of axon bundles. Staining in lamina VI was similar in appearance to lamina V. Enkephalinase reaction product was widely distributed in the ventral horn. Numerous ventral horn motor neurons of varied size and morphology in laminae VIII and XI stained richly for the enzyme. The enzyme was also localized to medium-sized and large cells in lamina X and to cells of the central cervical nucleus. The size and morphology of the cell types associated with the enzyme supported a neuronal association for enkephalinase. The regional distribution of the enzyme overlapped that of enkephalin- and substance-P rich regions of the spinal cord. These findings support a role for enkephalinase in the metabolic regulation of centrally acting neuropeptides.

Role of endogenous enkephalins in locomotion and nociception studied with peptidase inhibitors in two inbred strains of mice (C57BL/6J and DBA/2J)

Acetorphan, a parenterally active enkephalinase inhibitor, induced dose-dependently a naloxone-reversible analgesia on the hot-plate jump test in DBA/2J (DBA2) mice but was devoid of effects in C57BL/6J (C57) mice. By contrast, acetorphan increased locomotion in both strains; however, the DBA2 strain was much more sensitive than C57 mice to the locomotor stimulant effect. The increased locomotion was antagonized by naloxone in both strains. These data suggest that endogenous enkephalins modulate nociception and locomotion in the two inbred strains differently.

Detailed immunoautoradiographic mapping of enkephalinase (EC 3.4.24.11) in rat central nervous system: comparison with enkephalins and substance P

The metallopeptidase enkephalinase known to participate in the inactivation of endogenous enkephalins and, possibly, other neuropeptides such as tachykinins, was visualized by autoradiography using a [125I]iodinated monoclonal antibody. A detailed mapping of the enzyme in rat brain and spinal cord was established on 10-micron serial sections prepared in a frontal plane as well as a few sections in a sagittal plane. On adjacent sections, and for the purpose of comparison, substance P-like and enkephalin-like immunoreactivities were also visualized by autoradiography using a 125I-monoclonal antibody and a polyclonal antibody detected by a secondary 125I-anti-rabbit antibody respectively. Histological structures were identified on adjacent Nissl-stained sections. Using the highly sensitive 125I-probe, enkephalinase immunoreactivity was found to be distributed in a markedly heterogeneous manner in all areas of the central nervous system. Immunoreactivity was undetectable in white matter areas, for example the corpus callosum or fornix, and had a laminar pattern in, for example, the cerebral cortex or hippocampal formation. Hence, although immunodetection was not performed at the cellular level, a major neuronal localization of the peptidase is suggested. The latter is consistent with the detection of a strong immunoreactivity in a pathway linking the striatum to the globus pallidum, the entopeduncular nucleus and the substantia nigra, as well as with a series of biochemical and lesion data. The strong immunoreactivity also present in choroid plexuses and ependymal cells as well as in the intermediate lobe and in scattered cells of the anterior lobe of the pituitary suggests that populations of glial and endocrine cells also express the peptidase. The highest density of enkephalinase immunoreactivity was observed in basal ganglia and limbic areas (caudate putamen, globus pallidus, nucleus accumbens, olfactory tubercles) as well as in areas involved in pain control mechanisms (superficial layers of the spinal nucleus of the trigeminal nerve or of the dorsal horn of the spinal cord) which also display the highest immunoreactivities for both enkephalins and substance P (except in globus pallidus for the latter). These localizations account for the opioid-like analgesic and motor effects of enkephalinase inhibitors inasmuch as a selective or predominant participation of the peptidase in enkephalin inactivation is assumed. A number of other areas appear richly endowed in both enkephalinase and enkephalins whereas substance P is hardly detectable. This is particularly the case for the olfactory bulb, bed nucleus of the accessory olfactory tract, the cerebellum (where enkephalinase mainly occurs in the molecular layer) and the hippocampal formation (namely in the molecular layer of the dentate gyrus).(ABSTRACT TRUNCATED AT 400 WORDS)

Novel inhibitors of enkephalin-degrading enzymes. I: Inhibitors of enkephalinase by penicillins

Several penicillins have been found to have pro-antinociceptive properties and also to be enkephalinase (neutral endopeptidase-24.11) inhibitors, carfecillin being the most potent. Carfecillin i.c.v. (but not i.p.) had significant antinociceptive activity in the mouse tail immersion test and completely suppressed abdominal constrictions (acetic acid) in mice (IC50 = 23 micrograms/animal). In combination with (D-Ala2-D-leu5)-enkephalin (DADL) i.c.v. in the abdominal constriction test the complete protection observed was reversed by the opioid receptor antagonist naltrexone. Carfecillin was a competitive inhibitor of enkephalinase from mouse brain striata (IC50 = 207 + 57 nM, cf thiorphan 10.6 +/- 1.9 nM) but did not inhibit other known enkephalin- degrading enzymes. Carfecillin provides a new lead structure for the development of more potent enkephalinase inhibitors.

Novel inhibitors of enkephalin-degrading enzymes. II: N5'-substituted-4-thioxohydantoic acids as aminopeptidase inhibitors

Some 2-substituted-(2'-aminophenyl)-4-thioxohydantoic acids (o-amino PTC-amino acids) have antinociceptive activity when administered (icv) alone (IC50 = 0.04-0.87 microM/animal) and show a striking prolongation of the antinociceptive action of (D-Ala-2 D-Leu5)-enkephalin (DADL) in combination. The effects are thought to be mediated via opioid receptors since they are naloxone-reversible. Although inhibitors of the enkephalin degrading puromycin-insensitive, bestatin-sensitive aminopeptidase (possibly aminopeptidase M) their action is weak (IC50 = 32 microM leucine, 536 microM, glycine) and they might be considered to have a direct antinociceptive effect on opioid receptors. The titled compounds constitute novel 'lead' compounds for the development of potent aminopeptidase M inhibitors.

Protection by serine peptidase inhibitors of endogenous cholecystokinin released from brain slices

Endogenous cholecystokinin immunoreactivity released by depolarization of slices of rat cerebral cortex undergoes extensive degradation (85% of released immunoreactivity) before reaching the incubation medium. In order to identify the responsible peptidases, a large number of inhibitors of the four catalytic classes were tested for their protective effects. Inhibitors of metallopeptidases (bestatin, amastatin, puromycin, Thiorphan, captopril, o-phenantroline), thiol-peptidases, (leupeptin, antipain, p-hydroxymercuribenzoate) or carboxyl-peptidases (pepstatin) had generally low if any protective effect. By contrast, several serine peptidase inhibitors, i.e. diisopropyl-fluorophosphate, phenylmethylsulphonylfluoride or the chloromethylketone Ala-Ala-Pro-Val-CH2Cl, doubled the recovery of cholecystokinin immunoreactivity and the effect was amplified in the co-presence of bestatin, an aminopeptidase inhibitor and/or Thiorphan, an enkephalinase inhibitor. High-performance liquid chromatographic analysis of the cholecystokinin immunoreactivity recovered in medium in the absence of any inhibitor showed cholecystokinin-8 to be the major peak, representing 8% of the released immunoreactive material. Non-sulphated cholecystokinin-8 represented less than 1%, indicating that desulphation does not constitute a major inactivation pathway for the endogenous octapeptide. Cholecystokinin-5 was the major clearly identifiable immunoreactive fragment, representing 9% of released immunoreactivity in the absence of inhibitors. Its formation was decreased by about 50% in the presence of either diisopropyl-fluorophosphate or bestatin and Thiorphan and abolished when they were associated, suggesting that it resulted from the actions of a serine peptidase(s) and an aminopeptidase(s). Cholecystokinin-6 (or cholecystokinin-7) was less abundant, representing 4% of the released immunoreactivity, and its level was augmented in the presence of diisopropyl-fluorophosphate. Hence a serine endopeptidase cleaving the Met3-Gly4 bond of cholecystokinin-8 may represent a major inactivating peptidase for the endogenous neuropeptide. Additional metabolic pathways not blocked by serine peptidase inhibitors and resulting in the formation of cholecystokinin-6 (or cholecystokinin-7) and, possibly, cholecystokinin-4, are also suggested by the present approach.

A serine peptidase responsible for the inactivation of endogenous cholecystokinin in brain

A serine endopeptidase was characterized as a major inactivating enzyme for endogenous cholecystokinin (CCK) in brain. CCK-8 released by depolarization of slices of rat cerebral cortex, as measured by its immunoreactivity (CCK-ir), undergoes extensive degradation (approximately 85% of the amount released) before reaching the incubation medium. However, recovery of CCK-ir is enhanced up to 3-fold in the presence of serine-alkylating reagents (i.e., phenylmethylsulfonyl fluoride) as well as selected active site-directed inactivators (i.e., peptide chloromethyl ketones) or transition-state inhibitors (i.e., peptide boronic acids) of serine peptidases. Among these compounds, elastase inhibitors were the most potent protecting agents, whereas trypsin or chymotrypsin inhibitors were ineffective. HPLC analysis of endogenous CCK-ir recovered in media of depolarized slices indicated that endogenous CCK-5 [CCK-(29-33)-pentapeptide] was the most abundant fragment and that its formation was strongly decreased in the presence of an elastase inhibitor. HPLC analysis of fragments formed upon incubation of exogenous CCK-8 [CCK-(26-33)-octapeptide] with brain slices showed CCK-5, Gly-Trp-Met, and Trp-Met to be major metabolites of CCK-8 whose formation was prevented or at least diminished in the presence of the elastase inhibitor. It is concluded that there is an elastase-like serine endopeptidase in brain that cleaves the two peptide bonds of CCK-8 where the carboxyl group is donated by a methionine residue and constitutes a major inactivation ectoenzyme for the neuropeptide.

Labelling and exploration of the active site of enkephalinase (EC 3.4.24.11) in kidney membranes with [3H]thiorphan as ligand

[3H]Thiorphan, a potent inhibitor of enkephalinase (EC 3.4.24.11), was used to label the enzyme in membranes from rat kidney cortex an to explore its specificity at the active site. [3H]Thiorphan binding occurred reversibly, with low non-specific binding and to a single class of sites. The dissociation constant, determined by either kinetics or saturation studies was approximately 0.4 nM. The ratio of the maximal velocity of enkephalinase with enkephalins as substrates to the maximal binding of [3H]thiorphan was consistent with the catalytic constant of the enzyme. Enkephalinase inhibitors competed with [3H]thiorphan and had inhibitory constants in agreement with the corresponding values derived from measurement of the enzyme catalytic activity, whereas inhibitors of other metallopeptidases were ineffective. The inhibitory potencies of a series of systematically varied oligopeptides regarding [3H]thiorphan binding and enkephalinase activity were also highly correlated. Structure-activity relationships among competitors indicated that the main subsites of enkephalinase are: (1) the hydrophobic pocket in P'1, the requirements of which are best satisfied by aromatic amino acid side chain residues (2) the P'2 subsite, the requirements of which are best satisfied by amino acids with a short, uncharged side chain and a free terminal carboxyl group. This novel binding assay should facilitate the exploration of the active site of enkephalinase and the development of new inhibitors.