Biological inactivation of enkephalins and the role of enkephalin-dipeptidyl-carboxypeptidase ("enkephalinase") as neuropeptidase

Inhibition of aminopeptidase M by alkyl D-cysteinates

Ethyl D-cysteinate is a potent competitive inhibitor (Ki = 3.5 x 10(-7) M) of aminopeptidase M. D-cysteine and ethyl L-cysteinate inhibit more than two orders of magnitude less effectively. Inhibition studies on several n-alkyl esters of D-cysteine reveal an optimum at the n-butyl ester (Ki = 1.8 x 10(-7) M). The results are consistent with the hypothesis that the thiol group coordinates to Zn+2 at the active site and the alkyl group occupies the hydrophobic binding site for the side chain of the amino-terminal residue of substrates. Cytosolic leucine aminopeptidase is not significantly inhibited by ethyl D-cysteinate.

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)

Differential down-regulation of delta opioid binding sites during physical dependence on methionine enkephalin in the rat

Post-synaptic receptor modulation is thought to be one important mechanism involved in the adaptation of a neuronal system during chronic exposure to a drug. However, initial studies of opioid receptor regulation following chronic in vivo administration of narcotic agonists, such as morphine, reported no down-regulation in the number of opioid receptors in the brain. Subsequent studies, employing in vitro preparations, have reported evidence of opioid receptor down-regulation under specific conditions. It remains to be determined whether the in vitro phenomena of opioid receptor plasticity is relevant to the intact mammalian central nervous system. The data in this report shows that chronic in vivo administration the opioid peptide methionine enkephalin, results in a significant, regionally specific down-regulation of delta opioid receptors in rat brain: 30% decrease in receptor density in the striatum; no change in hypothalamus.

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.

Physiologic role of the peripheral enkephalinergic system in regulating cardiovascular homeostasis: evidence of interactions with the renin-angiotensin and kallikrein-kinin systems

On isolated heart preparation, it was found that Leu5-Enkephalin (Leu5-ENK) did not influence the cardiac function. On the other hand, Leu5-ENK induced a specific dose-related inhibition, in the cardiac perfusate, of the activities of kininase II (KII) and angiotensin converting enzyme (ACE) (but not of kininase I-KI). Instead no detectable alterations of the above enzymatic activities with the used concentrations of Leu5-ENK were observed in vitro. This opioid also increased specifically the effects induced by some of the autacoids, related to both renin-angiotensin and kallikrein-kinin systems, on the KII and ACE activities. A specific correlation between these Leu5-ENK-induced modifications and the functional responses of the heart to the same autacoids was observed. Naloxone (NAL) and more significantly ICI 174864 (ICI) opposed or reversed the inhibitory effect of the used opioid whereas they had neither inhibitory nor synergic effect on both KII and ACE activity by themselves. The possible physiologic role of the enkephalins in regulating cardiovascular function by acting peripherally on some humoral systems through modulatory mechanism was discussed.

Neutral endopeptidase inhibitors potentiate substance P- and capsaicin-induced cough in awake guinea pigs

To study the roles of substance P and endogenous neutral endopeptidase in mediating cough, we measured cough responses in awake guinea pigs in response to exogenous substance P and capsaicin aerosols in the presence and absence of the neutral endopeptidase inhibitors leucine-thiorphan and phosphoramidon. Substance P stimulated cough in very low concentrations (10(-17)-10(-16) M). In a second study where the investigator did not know whether substance P or diluent alone was aerosolized, substance P (10(-16) M) caused cough. Leucine-thiorphan (10(-5) M) and phosphoramidon (10(-5) M) potentiated substance P-induced cough; NEP inhibitors also potentiated capsaicin-induced cough significantly. These findings suggest that substance P is a potent stimulator of cough responses, that capsaicin-induced cough is mediated by substance P or another similar neuropeptide, and that cough responses are modulated by endogenous neutral endopeptidase.

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.

Neuropeptide-degrading endopeptidase activity of locust (Schistocerca gregaria) synaptic membranes

Locust adipokinetic hormone (AKH, pGlu-Leu-Asn-Phe-Thr-Pro-Asn-Trp-Gly-Thr-NH2) was used as the substrate to measure neuropeptide-degrading endopeptidase activity in neutral membranes from ganglia of the locust Schistocerca gregaria. Initial hydrolysis of AKH at neural pH by peptidases of washed neural membranes generated pGlu-Leu-Asn and Phe-Thr-Pro-Asn-Trp-Gly-Thr-NH2 as primary metabolites, demonstrating that degradation was initiated by cleavage of the Asn-Phe bond. Amastatin protected the C-terminal fragment from further metabolism by aminopeptidase activity without inhibiting AKH degradation. The same fragments were generated on incubation of AKH with purified pig kidney endopeptidase 24.11, and enzyme known to cleave peptide bonds that involve the amino group of hydrophobic amino acids. Phosphoramidon (10 microM), a selective inhibitor of mammalian endopeptidase 24.11, partially inhibited the endopeptidase activity of locust neural membranes. This phosphoramidon-sensitive activity was shown to enriched in a synaptic membrane preparation with around 80% of the activity being inhibited by 10 microM-phosphoramidon (IC50 = 0.2 microM). The synaptic endopeptidase was also inhibited by 1 mM-EDTA, 1 mM-1,10-phenanthroline and 1 microM-thiorphan, and the activity was maximal between pH 7.3 and 8.0. Localization of the phosphoramidon-sensitive enzyme in synaptic membranes is consistent with a physiological role for this endopeptidase in the metabolism of insect peptides at the synapse.

Studies on the tissue distribution of the puromycin-sensitive enkephalin-degrading aminopeptidases

An antiserum generated to the soluble form of the rat brain puromycin-sensitive enkephalin-degrading aminopeptidase was used to determine the tissue distribution of the soluble and membrane-associated forms of this enzyme. All tissues examined contained significant levels of the soluble enzyme form, with this enzyme accounting for greater than 90% of the arylamidase activity in brain, heart, and skeletal muscle. Native gel electrophoresis coupled with activity staining as well as inhibition studies were used to confirm the presence of this enzyme in various tissues. Serum was found not to contain this particular aminopeptidase. In contrast to the results obtained with the soluble enzyme form, brain was the only tissue found to contain the membrane-associated enzyme form. Although all tissues contained membrane-associated aminopeptidase activity only the brain enzyme could be maintained in solution in the absence of detergent. In addition, the brain membrane-associated enzyme could be distinguished from the membrane-associated aminopeptidase activity in other tissues on the basis of its sensitivity to inhibition by puromycin.

Isolation of endopeptidase-24.11 (EC 3.4.24.11, "enkephalinase") from the pig stomach. Hydrolysis of substance P, gastrin-releasing peptide 10, [Leu5] enkephalin, and [Met5] enkephalin

The purpose of this investigation was to isolate the cell-surface enzyme endopeptidase-24.11 from the stomach wall of the pig and to examine the hydrolysis of the gastric neuropeptides. Endopeptidase-24.11 was isolated from gastric membranes by immunoadsorbent chromatography using a monoclonal antibody to porcine kidney endopeptidase-24.11. The enzyme was purified with a yield of 1.2 micrograms/g wet wt of fundic muscle. A single polypeptide chain of apparent subunit molecular weight of 90,000 was identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Gastric endopeptidase-24.11 hydrolyzed substance P, gastrin-releasing peptide 10, [Leu5] enkephalin, and [Met5] enkephalin by cleavage of peptide bonds on the N-terminal side of hydrophobic amino acids. The enzymatic activity was inhibited completely by phosphoramidon (10(-6) M) and strongly by 1,10-phenanthroline (10(-3) M), but was unaffected by captopril (10(-5) M).

Inhibition of enkephalin degradation by phelorphan: effects on striatal [Met5]enkephalin levels and jump latency in mouse hot plate test

The effect of phelorphan (mercaptoacetyl-Phe-Phe), an inhibitor of various enkephalin-degrading enzymes, was tested in the mouse hot plate test and its efficacy to prevent endogenous enkephalin degradation in mouse striatum was determined. The i.c.v. injection of phelorphan (50 micrograms) increased the [Met5]enkephalin immunoreactivity by 41% in the extrasynaptosomal fraction of mouse striatum compared to a 43% increase induced by i.c.v. administration of thiorphan (50 micrograms) + bestatin (75 micrograms). Jump latency in the mouse hot plate test was significantly prolonged by phelorphan (25 micrograms i.c.v.). The analgesic effect of phelorphan was the same as that of thiorphan (25 micrograms i.c.v.) + bestatin (25 micrograms i.c.v.) and was antagonized by pretreatment with naloxone. These results suggest that phelorphan induces naloxone-reversible analgesia by elevation of [Met5]enkephalin levels in the synaptic cleft.

Assay for enkephalin-degrading peptidases in rat brain tissues by high-performance liquid chromatography with on-line post-column fluorescence detection

The activities of enkephalin-degrading peptidases such as enkephalinases A and B in rat brain tissues were simultaneously assayed by a high-performance liquid chromatographic method with fluorimetric detection with an automatic reaction system. Tyrosine and tyrosine-containing peptides produced enzymatically from the substrate, methionine-enkephaline, were separated by gradient elution on a reversed-phase column (TSK gel ODS-120T), and then converted into fluorescent derivatives for detection by reaction with hydroxylamine, cobalt(II) and borate reagents. The method permits the simple and sensitive detection of N-terminal tyrosine-containing fragments of the enkephalin peptide. The limits of detection are 5-20 pmol per assay tube for the N-terminal tyrosine-containing fragments. The enzyme activities in the regionally separated tissues were 54-191 pmol/min.mg protein for enkephalinase A and 79-153 pmol/min.mg protein for enkephalinase B, which were calculated from the formation of Tyr-Gly-Gly and Tyr-Gly, respectively, during the enzyme reaction.

Hydrolysis of alpha-human atrial natriuretic peptide in vitro by human kidney membranes and purified endopeptidase-24.11. Evidence for a novel cleavage site

alpha-Human atrial natriuretic peptide (hANP) is secreted by the heart and acts on the kidney to promote a strong diuresis and natriuresis. In vivo it has been shown to be catabolized partly by the kidney. Crude microvillar membranes of human kidney degrade 125I-ANP at several internal bonds generating metabolites among which the C-terminal fragments were identified. Formation of the C-terminal tripeptide was blocked by phosphoramidon, indicating the involvement of endopeptidase-24.11 in this cleavage. Subsequent cleavages by aminopeptidase(s) yielded the C-terminal dipeptide and free tyrosine. Using purified endopeptidase 24.11, we identified seven sites of hydrolysis in unlabelled alpha-hANP: the bonds Arg-4-Ser-5, Cys-7-Phe-8, Arg-11-Met-12, Arg-14-Ile-15, Gly-16-Ala-17, Gly-20-Leu-21 and Ser-25-Phe-26. However, the bonds Gly-16-Ala-17 and Arg-4-Ser-5 did not fulfil the known specificity requirements of the enzyme. Cleavage at the Gly-16-Ala-17 bond was previously observed by Stephenson & Kenny [(1987) Biochem. J. 243, 183-187], but this is the first report of an Arg-Ser bond cleavage by this enzyme. Initial attack of alpha-hANP by endopeptidase-24.11 took place at a bond within the disulphide-linked loop and produced a peptide having the same amino acid composition as intact ANP. The bond cleaved in this metabolite was determined as the Cys-7-Phe-8 bond. Determination of all the bonds cleaved in alpha-hANP by endopeptidase-24.11 should prove useful for the design of more stable analogues, which could have therapeutic uses in hypertension.

Enkephalin binding systems in human plasma. III: Comparative protection of different peptides

We have investigated the hydrolysis and protection from hydrolysis of several peptides by plasma enzymes and by the plasma components previously described as inhibitors of enkephalins' hydrolysis. The results shown indicate that all the peptides actually hydrolyzed are also partially protected from hydrolysis by the enkephalin-protecting substances. Protection is fairly uniform for all the peptides tested, but considerably higher in the case of leu- and met-enkephalin, suggesting a partial specificity of the protecting substances towards opioid peptides.

In vitro and in vivo degradation of human gastrin by endopeptidase 24.11

The degradation of human unsulfated heptadecapeptide gastrin (G-17) by human kidney endopeptidase 24.11 has been studied in vitro, and some of the products of degradation have been identified in plasma after in vivo infusion of G-17. The enzyme cleaved G-17 at four peptide bonds: Trp4Leu5, Ala11Tyr12, Gly13Trp14, and Asp16Phe17. The cleavage at Gly-Trp was rapid and 1-13 G-17 was an important intermediate. All the products of cleavage of synthetic 1-13 G-17 were also found after degradation of intact G-17. When normal human volunteers received infusions of G-17, there appeared in their blood peptides with the properties of 1-11, 1-13, 1-16, and 5-17 G-17 on the basis of immunochemical and high-performance liquid chromatographic properties. These observations provide evidence that endopeptidase 24.11 is involved in gastrin metabolism in humans, and may be responsible for the generation of G-17 fragments in the peripheral circulation.

Molecular cloning and amino acid sequence of human enkephalinase (neutral endopeptidase)

We have isolated a cDNA clone encoding human enkephalinase (neutral endopeptidase, EC 3.4.24.11) in a lambda gt10 library from human placenta, and present the complete 742 amino acid sequence of human enkephalinase. The human enzyme displays a high homology with rat and rabbit enkephalinase. Like the rat and rabbit enzyme, human enkephalinase contains a single N-terminal transmembrane region and is likely to be inserted through cell membranes with the majority of protein, including its carboxy-terminus, located extracellularly.

Enkephalin activity on antigen-induced proliferation of human peripheral blood mononucleate cells

The effect of the two opioid pentapeptides met- and leu-enkephalin and of the tetrapeptide TYR-GLY-GLY-PHE on the stimulated proliferation of human peripheral blood mononucleate cells (PBMC) is described. Cell populations obtained from different donors were induced to proliferate with a Candida antigen. In the presence of the antigen, met-enkephalin and TYR-GLY-GLY-PHE had a concentration-dependent diphasic effect. The proliferation of PBMC was stimulated at high peptide concentration, while it was inhibited at the lower concentrations used. The effect of leu-enkephalin appears to be rather ambiguous, and directed towards the inhibition rather than the stimulation of PBMC proliferation.

Characterization of a dipeptidyl aminopeptidase from bovine adrenal medulla

A dipeptidyl aminopeptidase was partially purified from a supernatant fraction of bovine adrenal medulla by gel filtration and anion-exchange chromatography. From gel filtration, the apparent molecular weight of the enzyme was 68,100 and its pH optimum was 9.5. Its Km for hydrolysis of the synthetic substrate arginylarginine-beta-naphthylamide was 5.5 X 10(-6) M. The enzyme was inhibited by metal ion chelating agents and thiol blocking agents, suggesting the requirement for both a metal ion and an active cysteine residue for its activity. Several peptides were cleaved by the dipeptidyl aminopeptidase involving the sequential removal of dipeptides from the N-terminus. Biologically active peptides, such as leucine-enkephalin, methionine-enkephalin, and angiotensin II, were hydrolyzed by the dipeptidyl aminopeptidase although opioid peptides with a length greater than five amino acid residues were not susceptible to hydrolysis. Other peptides with a blocked N-terminus (neurotensin, bombesin) or a proline residue adjacent to a potential cleavage site (substance P) were not hydrolyzed. The ability of this dipeptidyl aminopeptidase to degrade certain neuropeptides suggests that it could be involved in neuropeptide degradation.

On-line post-column fluorescence detection for N-terminal tyrosine-containing peptides in high-performance liquid chromatography

A detection system based on on-line post-column fluorescence derivatization is described for the determination of N-terminal tyrosine-containing peptides by reversed-phase high-performance liquid chromatography. The peptides are automatically converted into fluorescent derivatives by reaction with hydroxylamine, cobalt (II) and borate after peptide separation on a reversed-phase column (TSKgel ODS-120T) followed by passage through an ultraviolet absorbance detector. The reaction system permits the fluorescence detection at 435 nm (emission) with excitation at 335 nm for N-terminal tyrosine-containing synthetic peptides in as little as picomole amounts. The facile fluorescence detection of N-terminal tyrosine-containing fragments produced from methionine-enkephalin by enzymatic degradation using a rat brain homogenate was achieved by comparison with the ultraviolet absorption detection at 215 nm.

The effect of D-Ala-2-Me-Phe-4-Met-(0)-Ol enkephalin on blood pressure, heart rate and digoxin-induced arrhythmias in the guinea pig

The cardiovascular effects of D-Ala-2-Me-Phe-4-Met-(0)-Ol enkephalin were investigated after its administration into the fourth cerebroventricle of the guinea pig. This enkephalin is a synthetic Met-enkephalin analog that is more resistant to degradation and has a high affinity for opioid receptors. It produced a significant increase in blood pressure and decline in heart rate. At high concentrations, 100 micrograms/kg plus 100 micrograms/kg/hr in the fourth ventricle, it produced bradyarrhythmias that were sometimes fatal. At doses that did not alter survival or produce arrhythmia, namely 30 micrograms/kg plus 25 micrograms/kg/hr in the fourth ventricle, the response to digitalis was assessed. Significant leftward shifts in the relationships between digoxin and arrhythmia occurrence and development of fatal arrhythmias were observed. Thus, D-Ala-2-Me-Phe-4-Met-(0)-Ol enkephalin has definite cardiovascular effects that include potentiation of digoxin arrhythmias.

Kelatorphan potentiates the effect of [Met5]enkephalin in the substantia gelatinosa of the cat spinal cord

In anaesthetized spinal cats, kelatorphan, an inhibitor of enkephalin degradation, was administered microelectrophoretically while recording the excitation of lumbar dorsal horn neurones by noxious and innocuous peripheral stimuli. When administered near the cell bodies of laminae IV and V neurons, kelatorphan neither altered evoked responses nor potentiated the inhibition by [Met5]enkephalin of these cells. When ejected in the substantia gelatinosa, however, kelatorphan reduced the nociceptive responses of some laminae IV and V neurones, an effect blocked by electrophoretic naloxone. The selective inhibition of nociceptive responses by [Met5]enkephalin administered in the substantia gelatinosa was markedly potentiated by co-administration of kelatorphan, and this effect was also blocked by electrophoretic naloxone. Neurones inhibited by administration of kelatorphan alone in the substantia gelatinosa were excited by administration of naloxone alone at the same site. The results suggest that some dorsal horn neurones are tonically inhibited by an action of opioid peptides in the substantia gelatinosa, and indicate that enzymic degradation limits the action of both exogenous and endogenous enkephalin in this spinal region.

Role of endogenous opioids in soman (pinacolyl methylphosphonofluoridate)-induced antinociception

The effect of soman poisoning on the levels of methionine enkephalin and beta-endorphin in mice and rats were determined. Soman poisoning produced no significant effect on methionine enkephalin levels in the striatum of rats or mice or beta-endorphin levels in the pituitary gland of mice. In rats beta-endorphin levels were significantly reduced 24 hr post soman poisoning, but returned to control levels by 48 hr. In vitro, the hydrolysis of leucine enkephalin by aminopeptidase was virtually complete by 30 min and found to be the major route of degradation. The release of TYR-GLY-GLY in the presence or absence of puromycin (10 microM) was found to be low (less than or equal to 2.0%). A minor effect on TYR release in the presence of GLY-GLY-PHE-MET (50 microM) was insignificant. Preincubation of mouse striatum homogenates with soman (1 or 10 microM) did not inhibit the hydrolysis of leucine enkephalin. These results suggest that the long term antinociception following soman exposure is not due to either altered concentration of endogenous opioid-like substances or inhibition of the enzymes responsible for their degradation.

Proctolin degradation by membrane peptidases from nervous tissues of the desert locust (Schistocerca gregaria)

The hydrolysis of the insect neuropeptide proctolin (Arg-Tyr-Leu-Pro-Thr) by enzyme preparations from the nervous tissue of the desert locust (Schistocerca gregaria) was investigated. Neural homogenate degraded proctolin (100 microM) at neutral pH by cleavage of the Arg-Tyr and Tyr-Leu bonds to yield Tyr-Leu-Pro-Thr, Arg-Tyr and free tyrosine. Arg-Tyr was detected as a major metabolite when the aminopeptidase inhibitors amastatin and bestatin were present to prevent Arg-Tyr breakdown. Around 50% of the proctolin-degrading activity was isolated in a 30,000 g membrane fraction and was shown to be almost entirely due to aminopeptidase activity. The aminopeptidase had an apparent Km of 23 microM, a pH optimum of 7.0 and was inhibited by 1 mM-EDTA and amastatin [IC50 = 0.3 microM], but was relatively insensitive to bestatin, actinonin and puromycin. Phenylmethanesulphonyl fluoride (1 mM) and p-chloromercuriphenylsulphonic acid (1 mM) had no effect on this enzyme activity. Although the bulk of the Tyr-Leu hydrolytic activity was located in the 30,000 g supernatant, some weak activity was detected in a washed membrane preparation. This peptidase displayed a high affinity for proctolin (Km = 0.35 microM) and optimal activity at around pH 7.0. Synaptosome- and mitochondria-rich fractions were prepared from crude neural membranes. The aminopeptidase activity was concentrated in the synaptic-membrane preparation, whereas activity giving rise to Arg-Tyr was predominantly localized in the mitochondrial fraction. The subcellular localization of the membrane aminopeptidase is consistent with a possible physiological role for this enzyme in the inactivation of synaptically released proctolin.

Major localization of aminopeptidase M in rat brain microvessels

The localization of two enkephalin-hydrolysing aminopeptidases i.e. aminopeptidase M (aminopeptidase N, EC 3.4.11.2) relatively insensitive to puromycin (Ki = 78 microM), and a puromycin-sensitive aminopeptidase (Ki = 1 microM) was studied in rat brain. The two aminopeptidases were differentially identified and/or localized using polyclonal anti-aminopeptidase M antibodies displaying anticatalytic activity and the inhibitors puromycin, bestatin and amastatin. Microvessels represent a major localization of cerebral aminopeptidase M as shown by the intense immunostaining of their walls in sections from various regions as well as in a fraction isolated from cerebral cortex homogenates by a sieving procedure. As compared to the starting homogenate, aminopeptidase M activity was enriched about twenty fold in this microvascular fraction. Aminopeptidase M was identified in this fraction by comparing the inhibitory potencies of antibodies and peptidase inhibitors towards the hydrolysis of [tyrosyl-3,5-3H, Met5]enkephalin to those found for the purified enzyme. A rather high aminopeptidase M activity was also localized in choroid plexuses. Following differential and gradient centrifugation analysis of cerebral cortex homogenates, aminopeptidase M activity was also enriched (by five to six fold) in fractions containing synaptic membranes. No significant soluble aminopeptidase M activity could be detected. These data suggest a dual localization of cerebral aminopeptidase M in microvessels and synaptic membranes consistent with its roles in preventing the access of circulating peptides to brain as well as in inactivating neuropeptides released from cerebral neurones. In comparison, puromycin-sensitive aminopeptidase activity, which is about 100 fold higher than aminopeptidase M activity in brain, was relatively low in microvessels and non-detectable in fractions enriched in synaptic membranes, being almost entirely restricted to soluble fractions.

Phelorphan, an inhibitor of enzymes involved in the biodegradation of enkephalins, affected the withdrawal symptoms in chronic morphine-dependent rats

Intracerebroventricular administration of phelorphan (158 nmol/2 microliters), a blocker of dipeptidylaminopeptidase (enkephalinase B) and other enzymes involved in the enkephalin biodegradation, inhibited in chronic morphine-dependent rats, the occurrence of some of the naloxone-precipitated withdrawal symptoms. This effect of phelorphan was compared with an equimolar dose of the dipeptidyl-carboxypeptidase inhibitor (enkephalinase A), thiorphan. The results indicate that both drugs decrease some of the naloxone-precipitated withdrawal symptoms (writhing, digging, head hiding, chewing, diarrhoea and Straub tail), while others were potentiated (penile licking) or unaltered (wet dog shakes, grooming and rearing). In addition, phelorphan compared with the controls or thiorphan, pretreated animals, increased the frequency of paw tremor, head shakes, scratching, erection and ejaculation, but other symptoms were decreased (stretching) or unaltered (teeth chattering). The results are discussed in light of the differences in permeability and specificity of the two enkephalinase inhibitors. Furthermore, these data support the hypothesis that the use of enkephalinase inhibitors might be a promising way for the attenuation of the severity of the withdrawal syndrome.

Enkephalinase (EC 3.4.24.11) is highly localized to a striatonigral pathway in rat brain

Autoradiographic visualization of enkephalinase (membrane metalloendopeptidase, EC 3.4.24.11) in sagittal sections of rat brain using a 125I-labelled monoclonal antibody showed the presence of a dense immunoreactivity in a tract joining the striatum to the substantia nigra. Unilateral kainate injections into the striatum elicited a strong ipsilateral decrease in enkephalinase activity and immunoreactivity in both the injected area and substantia nigra, particularly its pars compacta. This demonstrates the presence of enkephalinase all along fibers of a striatonigral pathway.

Neutral endopeptidase-like enzyme controls the contractile activity of substance P in guinea pig lung

The responsiveness of isolated guinea pig lung parenchymal strips to substance P was enhanced by at least 100-fold in the presence of the endopeptidase inhibitors phosphoramidon (1 microM) or thiorphan (1 microM), but not with the converting enzyme inhibitor, captopril, or an inhibitor of serum carboxypeptidase N (both 1 microM). Responses of guinea pig tracheal rings to substance P were also markedly potentiated by phosphoramidon. The increase in tissue responsiveness by these inhibitors was relatively specific for substance P among several other spasmogenic peptides, including formyl-methionyl-leucyl-phenylalanine and the complement peptides C3a and C5a. The enhanced responses appear to result from a decrease in the rate of substance P degradation in the presence of neutral endopeptidase inhibitors. Specific binding of substance P to its receptor on bronchial membranes was increased by three- to fourfold in the presence of phosphoramidon. These data demonstrate an enhanced potential for substance P to contract lung tissues when degradation by a neutral endopeptidase-like enzyme is blocked.

Immunohistochemical localization of aminopeptidase M in rat brain and periphery: relationship of enzyme localization and enkephalin metabolism

An antiserum specific for rat aminopeptidase M has been used for the immunohistochemical localization of the enzyme in rat brain and peripheral tissues. The enzyme in brain is localized exclusively on blood vessels. Within the pituitary the enzyme was associated with the vasculature in the posterior lobe, on the surface of the intermediate lobe and on the surface of some cells in the anterior lobe. In the liver, fine cell staining was observed between parenchymal cells, in the ileum the entire lumenal surface was stained, while in the kidney both proximal tubular and a central tubular staining was detected. In each tissue aminopeptidase M is localized such that it can limit diffusion across specific barriers. Aminopeptidase M activity in brain has been proposed to function in the degradation of synaptically released enkephalins. Its localization on blood vessels requires that enkephalins diffuse prior to degradation, a concept not in concert with current hypotheses. Based on these studies it is proposed that diffusion away from enkephalinergic synapses plays a key role in terminating enkephalin action.

Molecular cloning and amino acid sequence of rat enkephalinase

cDNA clones encoding rat enkephalinase (neutral endopeptidase, EC 3.4.24.11) have been isolated in lambda gt10 libraries from both brain and kidney mRNAs and the complete 742 amino acid sequence of rat enkephalinase is presented. The enzyme possesses a single transmembrane spanning domain near the N-terminal of the molecule but lacks a signal sequence. Because enkephalinase has it active site located extracellularly and is thus an ectopeptidase, we suggest that the N-terminal transmembrane region of the enzyme anchors the protein in membranes and that the majority of the protein, including the carboxy terminus, is extracellular. Enkephalinase, a zinc-containing metallo enzyme, displays homology with other zinc metallo enzymes such as carboxypeptidase A, B and E, suggesting enzymatic similarities in these enzymes.

Synthesis of enkephalinase B inhibitors, and their activity on isolated enkephalin-degrading enzymes

Compounds in which a dipeptide moiety is linked to a metal chelating mercapto group were synthesized to obtain effective enkephalinase B inhibitors. Inhibitors containing two hydrophobic amino acid side-chains decrease enkephalinase B activity with a potency depending on the length of the spacer connecting the mercapto group and the dipeptide (IC50 values vary between 0.35 and 14 microM) and they also inhibit enkephalinase A and aminopeptidase activity. Compounds lacking the carboxy terminal side-chain are not recognized by enkephalinase B or aminopeptidase but are potent inhibitors of enkephalinase A. Our most potent enkephalinase B inhibitor is mercaptoacetyl-Phe-Phe (designated phelorphan), having an IC50 value of 0.35 microM for enkephalinase B. This compound also effectively inhibits enkephalinase A (IC50 = 0.02 microM) and aminopeptidase activity (IC50 = 13 microM). Phelorphan can therefore be considered as a complete inhibitor of enkephalin biodegradation.

New substrates for enkephalinase (neutral endopeptidase) based on fluorescence energy transfer

Novel fluorescent substrates for enkephalinase (neutral endopeptidase; EC 3.4.24.11) have been developed. These new assays are based on the disappearance of energy transfer between a tryptophan or a tyrosine residue and the 5-dimethylaminonaphthalene-1-sulfonyl group (dansyl) in the substrates dansyl-Gly-Trp-Gly or dansyl-Gly-Tyr-Gly upon hydrolysis of their Gly-Trp or Gly-Tyr amide bond by enkephalinase. No significant difference in Km or kcat values were found for dansyl-Gly-Trp-Gly and dansyl-Gly-Tyr-Gly, indicating that, in contrast to thermolysin, the active site of enkephalinase easily accommodates tryptophan residues. Both tryptophan and tyrosine-containing substrates can be used for continuous recording of enkephalinase activity and should prove useful for detailed study of the substrate specificity of this enzyme.

A novel proenkephalin processing carboxypeptidase and its activation by cyclic AMP dependent protein kinase

Carboxypeptidase B-like enzymes cleaving Met-enkephalin-Arg from synaptosomes of the rat striatum purified using a DEAE-cellulose column and Met-Arg-CH-Sepharose 4B affinity column proved to be different from enkephalin-convertase, lysosomal carboxypeptidase B-like enzyme, pancreas carboxypeptidase B and carboxypeptidase Y, in effects of inhibitors and activators, pH optimum (7.5-8.5) and molecular size (50,000). This enzyme, named "Processin CP-E" was activated by cAMP dependent protein kinase, and the Vmax was increased from 4.3 to 13.3 microM/min/mg protein, while the Km (28.2 microM) was unchanged.

Metabolism of opioid peptides by cerebral microvascular aminopeptidase M

Aminopeptidase M (EC 3.4.11.2), which can degrade low molecular weight opioid peptides, has been reported in both peripheral vasculature and in the CNS. Thus, we have studied the metabolism of opioid peptides by membrane-bound aminopeptidase M derived from cerebral microvessels of hog and rabbit. Both hog and rabbit microvessels were found to contain membrane-bound aminopeptidase M. At neutral pH, microvessels preferentially degraded low molecular weight opioid peptides by hydrolysis of the N-terminal Tyr1-Gly2 bond. Degradation was inhibited by amastatin (I50 = 0.2 microM) and bestatin (10 microM), but not by a number of other peptidase inhibitors including captopril and phosphoramidon. Rates of degradation were highest for the shorter peptides (Met5- and Leu5-enkephalin) whereas beta-endorphin was nearly completely resistant to N-terminal hydrolysis. Km values for the microvascular aminopeptidase also decreased significantly with increasing peptide length (Km = 91.3 +/- 4.9 and 28.9 +/- 3.5 microM for Met5-enkephalin and Met5-enkephalin-Arg6-Phe7, respectively). Peptides known to be present within or in close proximity to cerebral vessels (e.g., neurotensin and substance P) competitively inhibited enkephalin degradation (Ki = 20.4 +/- 2.5 and 7.9 +/- 1.6 microM, respectively). These data suggest that cerebral microvascular aminopeptidase M may play a role in vivo in modulating peptide-mediated local cerebral blood flow, and in preventing circulating enkephalins from crossing the blood-brain barrier.

Angiotensin-converting enzyme associated with Torpedo california electric organ membranes

Torpedo electric organ contains high concentrations of angiotensin converting enzyme (ACE) like activity, cleaving [Leu5]enkephalin at the Gly3-Phe4 peptide bond. Most of the activity cosediments with the cell membranes. The enzymatic preparation from membranes is inhibited by low concentrations of the ACE inhibitors, SQ 14225 and SQ 20881 (IC50 of 0.6 and 15 nM, respectively), and is weakly inhibited by the neutral endopeptidase inhibitors, phosphoramidon and thiorphan (IC50 of 30 microM and ca. 70 nM, respectively). The enzyme degrades hippuryl-His-Leu and is activated by NaCl. Hippuryl-His-Leu and [Leu5]enkephalin are degraded with Km of 93 and 41 microM, and Vmax of 21 and 10 nmol/mg protein/min, respectively. The specific activity of the ACE-like activity in homogenates of Torpedo electric organ is relatively high (6.3 nmol hippuryl-His-Leu/mg protein/min); this value is similar to that obtained for rat lung and rat striatum.

Mechanisms protecting plasma peptides from enzyme hydrolysis: a comparative study

1. The role of the enkephalin-protecting plasma substances in the protection of non-opioid peptides from enzyme hydrolysis has been studied in laboratory animals and in man. 2. The results obtained indicate that all the peptides hydrolyzed by the plasma enzymes are also protected from the hydrolysis by the enkephalin-protecting substances. 3. The protection is fairly uniform in all the species and for all the peptides examined. However, in the human species the protection of leucine enkephalin is considerably higher than the average. These results are discussed in terms of a possible differential inhibition of the different plasma aminopeptidases.

Synaptosomal membrane-bound form of endopeptidase-24.15 generates Leu-enkephalin from dynorphin1-8, alpha- and beta-neoendorphin, and Met-enkephalin from Met-enkephalin-Arg6-Gly7-Leu8

Brain contains a membrane-bound form of endopeptidase-24.15, a metalloendopeptidase predominantly associated with the soluble protein fraction of brain homogenates. Subcellular fractionation of the enzyme in rat brain showed that 20-25% of the total activity is associated with membrane fractions including synaptosomes. Solubilization of the enzyme from synaptosomal membranes required the use of detergents or treatment with trypsin. The specific activity of the enzyme in synaptosomal membranes measured with tertiary-butoxycarbonyl-Phe-Ala-Ala-Phe-p-aminobenzoate as substrate was higher than that of endopeptidase-24.11 ("enkephalinase"), a membrane-bound zinc-metalloendopeptidase believed to function in brain neuropeptide metabolism. Purified synaptosomal membranes converted efficiently dynorphin1-8, alpha- and beta-neoendorphin into leucine enkephalin and methionine-enkephalin-Arg6-Gly7-Leu8 into methionine enkephalin in the presence of captopril, bestatin, and N-[1-(R,S)-carboxy-2-phenylethyl]-Phe-p-aminobenzoate, inhibitors of angiotensin converting enzyme (EC 3.4.15.1), aminopeptidase (EC 3.4.11.2), and membrane-bound metalloendopeptidase (EC 3.4.24.11), respectively. The conversion of enkephalin-containing peptides into enkephalins was virtually completely inhibited by N-[1-(R,S)-carboxy-2-phenylethyl]-Ala-Ala-Phe-p-aminobenzoate, a specific active-site-directed inhibitor of endopeptidase-24.15, indicating that this enzyme was responsible for the observed interconversions. The data indicate that synaptosomal membranes contain enzymes that can potentially generate and degrade both leucine- and methionine-enkephalin.

Naloxone-insensitive potentiation of neurotensin hypothermic effect by the enkephalinase inhibitor thiorphan

The hypothermic effect of neurotensin (0.1 microgram) injected i.c.v. in mice is potentiated by the enkephalinase inhibitor thiorphan (10 micrograms, i.c.v.). This potentiation is not reversed by systemic naloxone. The hypothermic effect of neurotensin is not modified by the amino-peptidase inhibitor bestatin (50 micrograms, i.c.v.) nor by the angiotensin-converting enzyme inhibitor captopril (50 micrograms, i.c.v.). These data indicate the involvement of enkephalinase in the inactivation of neurotensin, at least when it is injected i.c.v.

Biological activity of Leu-enkephalin containing hydrophobic moieties

A series of enkephalin analogues (Tyr-Gly-Gly-Phe-Leu-C-NH-R;R=C6,C8,C10,C12 and C14; C=CH2 groups) with increasing hydrophobic character have been tested behaviourally in the analgesic hot plate and tail flick tests. The analogues were administered intraperitoneally and were found to possess anti-nociceptive activity. The results indicate that apparently there is not a direct relationship between increasing hydrophobicity and activity of the enkephalin-alkyl-amide analogues. The enkephalin analogues were found to behave in the same way as the enkephalins in producing tolerance.

Neuropeptide-specific peptidases: does brain contain a specific TRH-degrading enzyme?

The particulate fraction of brain homogenates contains an enzyme that cleaves the pyroglutamyl-histidyl bond of thyrotropin-releasing hormone (TRH) but is clearly distinct from the more widely distributed pyroglutamyl peptidase (EC 3.4.19.3). This particulate enzyme is highly localized to brain where it is found on synaptosomal membranes. It exhibits an unusual degree of substrate specificity. For example, it does not cleave the pyroglutamyl-histidyl bond of luteinizing hormone-releasing hormone (LHRH) or the pyroglutamyl histidyl bond of the chromogenic substrate pyroglutamyl-histidyl-2-naphthylamide. Evidence is reviewed supporting the possibility that this enzyme, first detected in serum and originally referred to as "thyroliberinase", may be the first neuropeptide-specific peptidase to be characterized.

Degradation of dynorphin-(1-13) and dynorphin-(1-17) by the neuroblastoma cell membrane. Evidence for the involvement of a cysteine protease

The membrane of mouse neuroblastoma N-18 cells degraded dynorphin-(1-13), dynorphin-(1-17), and Leu-enkephalin. The degradation of the former two peptides was inhibited strongly by N-ethylmaleimide, moderately by diisopropylphosphorofluoridate and phosphoramidon, and slightly by bestatin. When Leu-enkephalin was the substrate, however, the effects of phosphoramidon and bestatin were marked and those of N-ethylmaleimide and diisopropylphosphorofluoridate were negligibly small. Captopril did not affect the degradation of the two dynorphins and Leu-enkephalin, but inhibited the further cleavage of N-terminal fragments generated from dynorphin-(1-13) by the N-ethylmaleimide-sensitive protease. Thus, a cysteine protease and, probably, a serine protease are responsible to the initial fragmentation of the dynorphins.