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

Steady-state level and turnover rate of the tripeptide Tyr-Gly-Gly as indexes of striatal enkephalin release in vivo and their reduction during pentobarbital anesthesia

Tyr-Gly-Gly (YGG) was recently shown to be an extraneuronal metabolite of opioid peptides derived from proenkephalin A, formed in brain by the action of "enkephalinase" (membrane metalloendopeptidase, EC 3.4.24.11) and degraded by aminopeptidases. The dynamic state of YGG in mouse striatum was studied by evaluating the changes in its level elicited by inhibitors of these peptidases. Inhibition of YGG synthesis by Thiorphan or acetorphan reduced YGG levels with a t1/2 (mean +/- SEM) of 12 +/- 2 min, indicating an apparent turnover rate (mean +/- SEM) of 18 +/- 2 pmol/mg of protein per hr. An apparent turnover rate of 18 +/- 2 pmol/mg of protein per hr was derived from the rate of YGG accumulation elicited by the aminopeptidase inhibitor bestatin. In addition, accumulation of Tyr-Gly-Gly-Phe-Met (YGGFM) in an extrasynaptosomal fraction after blockade of its degradation by Thiorphan and bestatin occurred at a rate of 18 +/- 3 pmol/mg of protein per hr, which is likely to reflect the rate of enkephalin release in vivo. Hence, the three series of data suggest that striatal enkephalins rapidly turn over--e.g., with a t1/2 in the 1-hr range. Pentobarbital anesthesia reduced by about 60% the rate of YGG accumulation elicited by bestatin and the extrasynaptosomal YGGFM accumulation elicited by Thiorphan and bestatin. This suggests that the activity of striatal enkephalin neurons is depressed during anesthesia. Pentobarbital (and chloral hydrate) did not affect the steady-state level of YGGFM but rapidly reduced that of YGG. Hence, the steady-state levels of YGG seem a reliable index of changes in enkephalin release, and measuring levels of characteristic fragments might therefore provide a general means of evaluating neuropeptide release in vivo.

Neuromedin N: high affinity interaction with brain neurotensin receptors and rapid inactivation by brain synaptic peptidases

Neuromedin N (NN) is a novel neurotensin (NT)-like hexapeptide recently isolated from porcine spinal cord. NN competitively inhibited the binding of monoiodinated [Trp11]NT to rat brain synaptic membranes with a 19-fold lower potency than NT. In the presence of 1 mM 1,10-phenanthroline or 10 microM bestatin, the potency of NN relative to NT was increased about 5-fold. NN was readily degraded by rat brain synaptic membranes, and NN-(2-6) was the major degradation product. NN-(2-6) did not bind to NT receptors at concentrations up to 1 microM whether or not peptidase inhibitors were present in the binding assay. The rate of degradation by synaptic membranes was nearly 2.5 times higher for NN than for NT. NN degradation by membranes was totally prevented by 1,10-phenanthroline and markedly inhibited by bestatin. The presence of NN in the central nervous system, its high potency to interact with brain NT receptors and its rapid inactivation by brain synaptic peptidases make it a potential neurotransmitter candidate acting at the NT receptor.

Stabilization of methionine-enkephalin in human and rat blood

Methods of preventing the degradation of 3H-methionine-enkephalin (3H-ME) in human blood both at 37 degrees C and under conditions of immediate cooling were examined. We found that, contrary to previous suggestions, use of aprotinin (with or without immediate cooling) was ineffective in preventing the degradation of 3H-ME in blood. Thus, previous reports on the circulating levels of ME which relied on such procedures to stabilize the ME may have reported artifactually low values. However, we found that citric acid effectively prevents 3H-ME breakdown in both human and rat blood. Thus, we propose the use of citric acid, mixed with blood immediately upon collection, as an effective method for the stabilization of ME in blood.

The endogenous tripeptide Tyr-Gly-Gly as a possible metabolite of opioid peptides in rat brain: identification, regional distribution, effects of lesions and formation in depolarized slices

Using a sensitive radioimmunoassay, the tripeptide Tyr-Gly-Gly (YGG) which corresponds to the N-terminal sequence of opioid peptides was detected in rat brain and identified by HPLC. Its regional distribution paralleled that of (Met5)enkephalin (YGGFM), a marker of enkephalin neurons. Ablation of these neurons in the striato-pallidal pathway by intrastriatal kainate, induced a significant decrease in YGG levels in caudateputamen and globus pallidus (-49%), consistent with the hypothesis that YGG originates from enkephalin neurons. When pallidal slices were incubated under various conditions, YGG was mainly found in the incubation medium indicating a predominantly extracellular localization. Depolarization of these slices by a K+-stimulus elicited a release of YGGFM accompanied by a marked increase in YGG levels. Bestatin and amastatin further enhanced YGG levels, reflecting the participation of aminopeptidases in the metabolism of the tripeptide and its precursor. Captopril, an inhibitor of the angiotensin-converting enzyme (ACE) showed no effect on the recovery of YGGFM and YGG. In contrast, the formation of YGG was completely prevented by Thiorphan (IC50 value = 9 nM) and phosphoramidon, two inhibitors of "enkephalinase" (EC 3.4.24.11; membrane metallo-endopeptidase), thus identifying the latter as the YGG-forming enzyme. The K+-induced increase in YGG + YGGFM levels in medium containing bestatin exceeded by about 60% the amount of YGGFM released from tissues, suggesting that YGG was mainly formed by extracellular hydrolysis of the various opioid fragments of the proenkephalin molecule. In vivo, YGG levels of cerebral regions were also markedly reduced in rats treated with acetorphan, a parenterally active "enkephalinase" inhibitor. All data suggest that YGG levels constitute an index of opioid peptide release.

Metabolic profiling of opioid peptides in canine pituitary and selected brain regions using HPLC with a radioreceptor assay detector

A combination of gradient reversed-phase high performance liquid chromatography (RP/HPLC) with a radioreceptor assay detector that uses two ligands is used to obtain effectively the metabolic profile of endogenous receptoractive opioid peptides in the canine pituitary and in seven selected brain regions including the hypothalamus, caudate nucleus, mid-brain, amygdala, thalamus, pons-medulla, and the hippocampus. Gradient RP/HPLC separates a mixture of endogenous peptides over a wide range of hydrophobicities. A novel opioid preparation from canine limbic system synaptosomes is utilized in a radioreceptorassay screen; tritiated etorphine (ET) or D-2ala, D-5leuleucine enkephalin (DADL) is used as the competitively displaced ligand. This receptor-rich preparation contains several receptor types, and thus serves well as a screen with the required low level of specificity. Subsequent analysis with other detectors of high specificity (MS, RIA) will follow this screen in other studies. Etorphine interacts with several of the opioid peptide-preferring receptors, whereas DADL is more specific towards the delta receptor that preferentially binds the smaller pentapeptides of the enkephalin family. The highest amount of peptide receptor activity found in this study is in the pituitary tissue, a smaller amount in the hypothalamus and caudate nucleus, and still lower amounts in the other five brain tissue extracts. This variation in peptide concentration most probably reflects three separate factors that operate in this biologic system: differential tissue-specific processing patterns of the large peptide precursors; distribution of the three opioid peptide systems; and the receptor preparation and the radioligand used in the assay. The structures of the receptoractive compounds in each RP/HPLC peak await mass spectrometric confirmation.

Influences of bradykinin and substance P on the met-enkephalin-like peptide content in the rat incisor pulp

The present study was aimed to examine possible influences of bradykinin (BK) and substance P (SP) on met-enkephalin (ME)-like peptide content in the rat incisor pulp. Des-Arg9-[Leu8]-BK, a potent BK-antagonist, significantly reduced the increased content of ME-like peptides induced by noxious stimulation, while the effect of BK-antagonist was reversed in combination with BK. Morphine decreased the increased content of ME-like peptides. Ethylketocyclazocine, a kappa-agonist, also decreased the increased content of the peptides. From these results, it was suggested that BK might be a trigger in the increase of ME-like peptide content induced by noxious stimulation and, in contrast, ME-like peptides in the pulp might inhibit BK release from the pulp in a negative feedback mechanism. On the other hand, [D-Pro2,D-Trp7,9]-SP, a potent SP-antagonist, did not show any significant influence to ME-like peptide content in the pulp. Furthermore, the content was not changed following cutting of inferior alveolar nerve. From these results, it was suggested that ME-like peptides in the pulp cells might be independent on SP-containing nerves in the pulp.

Dissociated effects of inhibitors of enkephalin-metabolising peptidases or naloxone on various nociceptive responses

The antinociceptive effects of Thiorphan, an 'enkephalinase' inhibitor, or bestatin, an aminopeptidase inhibitor, as well as of their association and the pronociceptive effects of naloxone, an opiate receptor antagonist, were evaluated in various analgesic tests in mice. These tests could be classified into two groups: (i) those tests in which the two peptidase inhibitors display naloxone-sensitive antinociceptive activity, particularly when administered together, and in which naloxone displays pronociceptive activity (vocalisation, hot-plate jump, writhing), (ii) those tests in which the two peptidase inhibitors and naloxone are ineffective (tail withdrawal, hot-plate licking, tail-flick). In contrast to the above, either morphine or [Met5]enkephalin in subthreshold dosage administrated together with the peptidase inhibitors displayed antinociceptive activity in the two groups of tests. The threshold dosages of morphine were the lowest in tests of the first group. The dissociated and opposite effects of peptidase inhibitors and naloxone per se might reflect a variable participation of endogenous enkephalins (or other opioid peptides) in the control of various nociceptive responses.

Organophosphorus anticholinesterases do not mediate analgesia through inhibition of enkephalin degradation

The effect on enkephalin degradation of the four highly potent organophosphorus anticholinesterases, soman, sarin, tabun and DFP was studied in synaptosomal fractions of rat brain striata. None of the agents effected any of the enkephalin degrading enzymes, the puromycin sensitive aminopeptidase, the p-hydroxymercurybenzoate (p-HMB) sensitive dipeptidyl aminopeptidase or the phosphoramidon sensitive enkephalinase. Furthermore, no peptidase function of acetylcholinesterase was found, when Leu-enkephalin was used as substrate at low concentrations (27 nM). Supporting the in vitro data, no difference was obtained in the striatal levels of Met- and Leu-enkephalin between rats receiving a high single dose of soman and controls. The results show that the analgesic effect of anticholinesterases are more likely due to mechanisms other than inhibition of enkephalin degradation.

Substrate and inhibitor studies of thermolysin-like neutral metalloendopeptidase from kidney membrane fractions. Comparison with bacterial thermolysin

The inhibitory constants of a series of synthetic N-carboxymethyl peptide inhibitors and the kinetic parameters (Km, kcat, and kcat/Km) of a series of model synthetic substrates were determined for the membrane-bound kidney metalloendopeptidase isolated from rabbit kidney and compared with those of bacterial thermolysin. The two enzymes show striking similarities with respect to structural requirements for substrate binding to the hydrophobic pocket at the S1' subsite of the active site. Both enzymes showed the highest reaction rates with substrates having leucine residues in this position while phenylalanine residues gave the lowest Km. The two enzymes were also inhibited by the same N-carboxymethyl peptide inhibitors. Although the mammalian enzyme was more susceptible to inhibition than its bacterial counterpart, structural variations in the inhibitor molecules affected the inhibitory constants for both enzymes in a similar manner. The two enzymes differed significantly, however, with respect to the effect of structural changes in the P1 and P2' positions of the substrate on the kinetic parameters of the reaction. The mammalian enzyme showed the highest reaction rates and specificity constants with substrates having the sequence -Phe-Gly-Phe- or -Phe-Ala-Phe- in positions P2, P1, and P1', respectively, while the sequence -Ala-Phe-Phe- was the most favored by the bacterial enzyme. The sequence -Gly-Gly-Phe- as found in enkephalins was not favored by either of the enzymes. Of the substrates having an aminobenzoate group in the P2' position, the mammalian enzyme favored those with the carboxyl group in the meta position while the bacterial enzyme favored those with the carboxyl group in the para position.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of various enkephalin analogs and dipeptides on the enzymatic activity of enkephalinase B isolated from calf-brain striatum

The inhibitory potency of several enkephalin analogs and dipeptides on the calf-brain enkephalinase B activity was established with the aim to characterize its active site. Highest potency was measured for dipeptides with a large side chain on both amino acids. The nature of the distal amino acid is of minor importance, provided it is not a glycine. Free carboxylic function is required for good interaction, whereas the stereochemical configuration of the dipeptide is less so. Enkephalinase B has only little affinity for D-Ala2-Leu-enkephalin. The data are to be used for the design of new enkephalinase B inhibitors.

D-phenylalanine: a putative enkephalinase inhibitor studied in a primate acute pain model

D-Phenylalanine, along with morphine, acetylsalicylic acid and zomepirac sodium were evaluated for their antinociceptive actions in monkeys (M. fascicularis) trained to autoregulate nociceptive stimulation using a discrete-trials, aversive-threshold paradigm. Morphine sulfate produced dose-related increases in aversive threshold which were reversible after administration of naloxone (12.5 or 25 micrograms/kg i.m.). D-Phenylalanine (500 mg/kg p.o.) produced a small increase in aversive threshold which was not statistically significant and not naloxone reversible. Acetylsalicylic acid (200 mg/kg p.o.) but not zomepirac sodium (200 mg/kg p.o.) in combination with D-phenylalanine (500 mg/kg) produced a small statistically significant increase in aversive threshold. Our results argue against the hypothesis that D-phenylalanine is responsible for increasing aversive thresholds via opiate receptor mechanisms involving increased activity of enkephalins at synaptic loci. Previous studies by others in rats and mice showed that D-phenylalanine and acetylsalicylic acid produced increases in nociceptive thresholds which were naloxone reversible. Our failure to find opiate receptor mediated analgesia in a primate model with demonstrated opiate receptor selectivity and sensitivity is discussed in terms of previous basic and clinical research indicating an analgesic role for D-phenylalanine. Possible species difference in drug action is discussed in terms of inhibition by D-phenylalanine of carboxy-peptidase-like enkephalin processing enzymes as well as inhibition of carboxypeptidase-like enkephalin degrading enzymes.

Control mechanisms of peripheral enkephalin hydrolysis in mammalian plasma

The protection of the adrenal-released enkephalins from enzyme hydrolysis by endogenous plasma components was studied in laboratory animals and in man. The results indicate that mechanisms active in protecting leu-enkephalin from hydrolysis are present in the plasma of all species examined. The protection seems to be due to two groups of substances, possibly of peptidic nature. The amount of protection given by these substances seems to be sufficient to play a significant role in controlling the physiological levels of leu-enkephalin released into the bloodstream.

Pharmacological "enkephalinase" inhibition in man

"Enkephalinase", a peptidase capable of degradating enkephalins, has been recently characterized in man, in both plasma and cerebro-spinal fluid (CSF). This study was designed to evaluate the ability of putative "enkephalinase" inhibitors, D-phenylalanine, captopril and thiorphan to decrease "enkephalinase" activity (EKA) in plasma and CSF in human sufferers. All drugs studied decreased plasma EKA. Captopril and thiorphan also decreased CSF EKA. Of the three drugs tested thiorphan proved to be the most potent "enkephalinase" inhibitor in both plasma and CSF. These results show the usefulness of EKA assessment as a procedure for evaluating the potency and specificity of putative "enkephalinase" inhibitors in man.

Enkephalinase activity in both physiological and pathological conditions in man

Among all the enzymes involve din enkephalin degradation "enkephalinase", cleaving the Gly3-Phe4 amide bond, is considered for its specificity with endogenous enkephalins and their receptors. This enzyme, first identified in membrane bound form, has been recently characterized in a soluble one by a new sensitive fluorimetric method substituting the radiometric technique. The possibility to evaluate "enkephalinase" activity in human plasma, amniotic fluid and cerebro spinal fluid (CSF) allows us to investigate its behavior in various physiological and pathological conditions in which alterations of the endogenous opioid system are hypothesized. Our studies were focused on pregnancy, the first period of life, idiopathic headache and opioid addiction. In these conditions "enkephalinase" activity (EKA) generally results increased. In some cases the activity is proportional to the increased amount of substrate, in other cases no correlation seems apparent.

The importance of disulfide bridges in human endopeptidase (enkephalinase) after proteolytic cleavage

The neutral endopeptidase (NEP) is a membrane-bound enzyme, which is solubilized by treatment with the protease, papain. Papain did not affect the apparent catalytic activity or the molecular mass of the purified human enzyme in SDS-PAGE. When NEP was treated with a reducing agent after papain digestion, it dissociated into smaller, lower molecular mass fragments. Amino acid analysis and s-carboxymethylation of the half cystine residues indicated that NEP contains four S-S bridges. We concluded that, although covalent bonds appear to be cleaved in NEP by papain, its activity and structure are sustained by S-S bridges.

Study of endogenous Tyr-Gly-Gly, a putative enkephalin metabolite, in mouse brain: validation of a radioimmunoassay, localisation and effects of peptidase inhibitors

The tripeptide Tyr-Gly-Gly, a hydrolysis product of enkephalins and related opioid peptides obtained with 'enkephalinase', was identified and quantified in various regions of mouse brain by means of HPLC and a sensitive and specific radioimmunoassay. Similar levels i.e. about 8 pmol/brain were found after the animals were killed by various procedures, including microwave irradiation, suggesting its pre-mortem formation. The distribution of Tyr-Gly-Gly immunoreactivity among brain regions was highly heterogeneous and paralleled to a certain extent the [Met5]enkephalin distribution, molar levels of Tyr-Gly-Gly representing 10-30% of those of the enkephalin. Following gentle homogeneisation of striata in 0.32 M sucrose and centrifugation, 73% of Tyr-Gly-Gly immunoreactivity was recovered in the supernatant, a result consistent with its extracellular localisation in vivo. Administration of enkephalinase inhibitors rapidly elicited marked decrease in Tyr-Gly-Gly immunoreactivity whereas bestatin, an aminopeptidase inhibitor, elicited 100% increase and captopril, an ACE inhibitor, was without significant effect. These data indicate that the tripeptide is in a dynamic state in the brain and that its levels might reflect the release of endogenous enkephalins or related opioid peptides and their subsequent metabolism by enkephalinase.

Purification of human urinary prokallikrein. Identification of the site of activation by the metalloproteinase thermolysin

Human urinary active kallikrein and prokallikrein were separated on DEAE-cellulose and octyl-Sepharose columns and both purified to homogeneity by affinity chromatography, gel filtration and hydrophobic h.p.l.c. Prokallikrein was monitored during purification by trypsin activation followed by determination of both amidase and kininogenase activity. After trypsin activation, purified prokallikrein had a specific kininogenase activity of 39.4 micrograms of bradykinin equivalent/min per mg and amidase activity of 16.5 mumol/min per mg with D-Val-Leu-Arg-7-amino-4-trifluoromethylcoumarin. Purified active kallikrein had a specific activity of 47 micrograms of bradykinin/min per mg. The molecular mass of prokallikrein was 48 kDa on electrophoresis and 53 kDa on gel filtration whereas active kallikrein gave values of 46 kDa and 53 kDa respectively. Antisera to active and prokallikrein were obtained. In double immunodiffusion and immunoelectrophoresis, antiserum to active kallikrein reacted with active and pro-kallikrein. Antiserum to prokallikrein contained antibodies to determinants not found in active kallikrein, presumably due to the presence of the activation peptide in the proenzyme. Human prokallikrein can be activated by thermolysin, trypsin and human plasma kallikrein. Activation of 50% of the prokallikrein (1.35 microM) was achieved in 30 min with 25 nM-thermolysin, 78 nM-trypsin or 180 nM-human plasma kallikrein. Thus thermolysin was the most effective activator. Thermolysin activated prokallikrein by releasing active kallikrein with N-terminal Ile1-Val2. Thus human tissue (glandular) prokallikrein can be activated by two types of enzymes: serine proteinases, which cleave at the C-terminus of basic amino acids, and by a metalloproteinase that cleaves at the N-terminus of hydrophobic amino acids.

Neutral endopeptidase 24.11 in human neutrophils: cleavage of chemotactic peptide

Membrane metallo-endopeptidase (NEP; neutral endopeptidase, kidney-brush-border neutral proteinase, enkephalinase, EC 3.4.24.11) cleaves peptides at the amino side of hydrophobic amino acids. While the enzyme is known to be in organs such as kidney and brain, we found it in human neutrophils. These cells cleaved the NEP substrate glutaryl (Glut)-Ala-Ala-Phe-(4-methoxynaphthylamine) (Glut-Ala-Ala-Phe-MNA) at a rate of 9.5 nmol X hr-1 per 10(6) cells, and phosphoramidon (1 microM) inhibited the hydrolysis by 90%. Intact neutrophils from donors who smoked had NEP activities about twice that of nonsmokers. Subcellular fractionation and sucrose density gradient centrifugation of lysed neutrophils showed that most of the NEP activity was membrane bound. A washed membrane fraction from human neutrophils rapidly cleaved 0.5 mM Glut-Ala-Ala-Phe-MNA (96 nmol X min-1 X mg-1) and the hydrolysis was inhibited by phosphoramidon and by specific antiserum to human renal NEP. The washed membrane fraction also rapidly cleaved 0.1 mM bradykinin (34 nmol X min-1 mg-1) and 0.1 mM fMet-Leu-Phe (49 nmol X min-1 X mg-1). The membrane-bound enzyme cleaved the peptide substrates at the same site as the homogeneous human renal NEP, and phosphoramidon and thiorphan inhibited the hydrolysis. Kinetic studies with pure human renal NEP showed that the chemotactic peptide fMet-Leu-Phe was one of the best biologically active substrates (Km, 59 X 10(-6) M; kcat, 3654 min-1). Immunocytochemistry at the light microscopic level revealed a high concentration of NEP on the cell membrane of neutrophils. This was confirmed with electron microscopy using the immunogold technique on ultrathin cryosections. These studies indicate that NEP in neutrophils may have important functions in inflammation and chemotaxis.

Quantitative autoradiographic determination of angiotensin-converting enzyme binding in rat pituitary and adrenal glands with 125I-351A, a specific inhibitor

We describe a quantitative autoradiographic technique which allows measurement of angiotensin-I-converting enzyme [ACE] (kininase II, peptidyldipeptide hydrolase, EC 3.4.15.1) levels in discrete areas of pituitary and adrenal glands in individual animals. Tissue sections were incubated with 125I-351A, a specific ACE inhibitor, and results were obtained with computerized densitometry and comparison to 125I standards. There were high levels of ACE in both the anterior and posterior lobes of the pituitary, with no detectable binding in the intermediate lobe. The maximum binding capacity (Bmax) was 920 +/- 62 fmol/mg protein for the anterior pituitary and 1162 +/- 67 fmol/mg protein for posterior pituitary. The binding affinity constant (Ka) was 0.95 +/- 0.11 X 10(9) M-1 and 1.20 +/- 0.19 X 10(9) M-1 for the anterior and posterior lobes, respectively. In the adrenal gland, there were two distinct areas of specific binding, the adrenal medulla and the adrenal capsule-zona glomerulosa area. The Bmax for the adrenal medulla was 652 +/- 80 fmol/mg protein and 294 +/- 53 fmol/mg protein for the adrenal capsule-zona glomerulosa. The Ka for 351A was 1.04 +/- 0.19 X 10(9) M-1 and 1.74 +/- 0.40 X 10(9) M-1 for medulla and adrenal capsule-zona glomerulosa respectively. The results support the existence of local ANG systems active in both the pituitary and adrenal glands.

High enkephalyl peptide degradation, due to angiotensin-converting enzyme-like activity in human CSF

The metabolism of enkephalin peptides was studied in human cerebrospinal fluid. The degradation rates of (Leu)-enkephalin and (Leu)-enkephalin-Arg6 were compared and the latter was degraded at a 10-fold higher rate. The major enzyme activity was investigated by Mr determination and inhibition experiments, showing marked similarity with angiotensin-converting enzyme.

Mechanisms of leu-enkephalin hydrolysis in human plasma

The present work describes the kinetics of enkephalin hydrolysis by plasma enzymes and the fragmentation pattern of both the parent peptide and of the first hydrolysis by-products. The degradation kinetics were followed by positive identification of the hydrolysis fragments by chromatographic methods, by amino acid analysis and by scintillation counting of tritium-labeled enkephalin. In addition, the results presented confirm the role of the low molecular weight plasma components in the control of the hydrolysis of the peripherally-released enkephalins.

Peptidases that terminate the action of enkephalins. Consideration of physiological importance for amino-, carboxy-, endo-, and pseudoenkephalinase

The term "enkephalinase" has been frequently applied to enzyme activity in a variety of tissue preparations. In some cases there has been the implication that cleavage of a specific peptide bond in the enkephalin molecule results from the action of a single enzyme with the major responsibility of inactivating synaptic enkephalin. It is not known to what extent diverse enkephalin-degrading enzymes, with differing peptide bond specificities, may act in concert at any given synapse. There do exist, however, enzymes having known characteristic specificities with respect both to peptide substrates, including enkephalins, and to identifiable peptide bonds. Thus, at any given site of enkephalin release there probably resides a characteristic assembly of peptidases concerned with inactivation of this neuromediator. We propose that the term "enkephalinase" be used to encompass the entire family of enkephalin-degrading enzymes, and that "aminoenkephalinase", "carboxyenkephalinase", "endoenkephalinase" and "pseudoenkephalinase" should designate enzymes of known specificities with respect to both peptide substrates and particular peptide bonds.

Isolation and characterization of an enkephalin-hydrolyzing dipeptidyl-aminopeptidase from calf-brain striatum

Cytosolic dipeptidyl-aminopeptidase with a high affinity for Leu-enkephalin (Km = 5-7 microM) was partially purified from the 25,000 g supernatant of calf-brain striatum. The procedure included pH 4.5 denaturation, DEAE-cellulose chromatography and Blue Sepharose CL-6B chromatography and resulted in preparations that are free from other enkephalin-hydrolyzing enzymes. This enzyme, which is called enkephalinase B, has a positively charged group in its active site and presumably also a Zn atom since the loss in activity induced by EDTA treatment can be restored without loss of substrate affinity by low concentrations of ZnSO4.

High-performance liquid chromatographic analysis of pulmonary metabolites of Leu- and Met-enkephalins in isolated perfused rat lung

A high-performance liquid radiochromatographic analytical system has been developed which allows the determination of [3H]Leuenkephalin, [3H]Met-enkephalin and their potential metabolites [3H]TyrGlyGlyPhe, [3H]TyrGlyGly, [3H]TyrGly and [3H]tyrosine. Using this procedure, the biotransformation of each of the above enkephalins after 20 min of recirculating transit through isolated perfused rat lungs resulted in the formation of two major metabolites: tyrosine and TyrGlyGly in each case. The results indicate that significant metabolism of enkephalins may occur in the pulmonary circulation.

The production and characterization of a monoclonal antibody specific for the 94,000 dalton enkephalin-degrading peptidase from rabbit kidney brush border

We have prepared a monoclonal antibody specific for a major 94,000 dalton protein from the brush border membrane of rabbit kidney cortex. The monoclonal antibody was used for the immunoaffinity purification of this protein after solubilization of brush border membranes with octylglucoside. The 94,000 dalton protein is a peptidase capable of cleaving the Gly3-Phe4 bond of methionine-enkephalin. Identification of this peptidase as a previously described 94,000 dalton enkephalinase of kidney cortex was confirmed by its sensitivity to EDTA and inhibitors such as thiorphan and phosphoramidon.

Involvement of monoaminergic and peptidergic components in cathinone-induced analgesia

Evidence has been obtained suggesting that cathinone-induced analgesia depends upon stimulation of alpha-adrenoceptors, followed by release of opioid peptides and by activation of serotonergic pathways. This hypothesis is supported by the following. (1) Cathinone potentiated morphine analgesia and the whole effect was antagonized by naloxone whereas onto the cathinone potentiation was counteracted by phenoxybenzamine. (2) Bestatin potentiated cathinone-induced analgesia and this effect was sensitive to both naloxone and phenoxybenzamine blockade. (3) The analgesic effect of cathinone + bestatin was further potentiated by the serotonin uptake inhibitor citalopram.

Peptidase-containing neurons in rat striatum

The effects of surgical lesions on peptidase activity have been studied in the striatonigral system of the rat brain. Knife cuts separating the anterior part of the caudate putamen from the globus pallidus resulted in a decrease in the activity of angiotensin-converting enzyme and alanyl aminopeptidase in both the globus pallidus and substantia nigra. The activity of nigral prolyl endopeptidase and leucyl aminopeptidase was also decreased. An increase in dipeptidyl aminopeptidase and arginyl endopeptidase activity was observed in both the caudate putamen and globus pallidus. These results suggest that the striatal neurons containing angiotensin-converting enzyme or alanyl aminopeptidase project to both the globus pallidus and substantia nigra, and the neurons containing prolyl endopeptidase and/or leucyl aminopeptidase project to the substantia nigra. Dipeptidyl aminopeptidase and arginyl endopeptidase are probably associated with glial function.

Synthesis and analgesic properties of N-phosphorylated derivatives of Phe-Ala and Phe-Gly

N-Phosphoryl dipeptides derived from Phe-Ala, Phe-Ala-NH2 and Phe-Gly-NH2 were synthesized and their analgesic activity was evaluated in mice. Intracerebroventricular administration of P-Phe-Ala-NH2 produced a 100-fold increase in the analgesic potency of Phe-Ala and led to a potentiation and prolongation of the analgesic effect of the exogenously administered Met-enkephalin.

Comparison of dipeptidyl carboxypeptidase and endopeptidase activities in the three enkephalin-hydrolysing metallopeptidases: "angiotensin-converting enzyme", thermolysin and "enkephalinase"

Angiotensin-converting enzyme (ACE), thermolysin and "enkephalinase", three metallopeptidases cleaving the Gly3-Phe4 amide bond of enkephalins, were compared regarding substrate specificity and effects of butanedione, an arginyl-directed reagent. The hydrolysis of enkephalins and analogues was more affected by the nature of P1 and P2 residues in the case of thermolysin than in those of ACE or "enkephalinase"; amidation of the C-terminal carboxylate decreased drastically the hydrolysis by ACE but only marginally by thermolysin and the effect was intermediate for "enkephalinase". With adequate model substrates, the ratio of dipeptidylcarboxypeptidase to tripeptidylcaroxypeptidase (endopeptidase) activities were of 25 for ACE, 3 for "enkephalinase" and only 0.3 for thermolysin. Finally a butanedione treatment increased thermolysin activity, but abolished ACE activity; it reduced "enkephalinase" activity by 80% when measured with a free C-terminal carboxylate enkephalin analogue but only slightly with the corresponding amidated derivative. A critical role of an Arg residue in ACE and, to a lesser extent, in "enkephalinase" (but not in thermolysin) is suggested to be responsible for the preferential dipeptidylcarboxypeptidase activity of these two enzymes.

An HPLC/push-pull perfusion technique for investigating peptide metabolism

We have combined the push-pull perfusion of radiolabelled peptide, with reverse phase--HPLC procedures as a valuable method for monitoring peptide metabolism in the CNS. This model not only permits studies on metabolism of neuropeptides at specific sites in the CNS, but more importantly, allows (unlike in vitro methods) the study to be made under the appropriate physiological conditions. We have demonstrated unambiguously for the first time, the metabolism of CCK-8 in situ.

Enhancement of colonic motor response to feeding by central endogenous opiates in the dog

The role of endogenous opiates in the colonic motor response to feeding has been investigated in four dogs chronically fitted with two strain gages on the proximal and distal colon and a cannula in cerebral lateral ventricle. A daily meal stimulated the colonic motility during 8-10 hrs. The colonic motility index was significantly higher during this period when an enkephalinase inhibitor, tiorphan, was intracerebroventricularly (i.c.v.) administered at a dose of 0.1 mg/kg before the meal. This effect was blocked by a previous i.c.v. administration of naltrexone (0.1 mg/kg) and reproduced by (D-Ala2, Met5) enkephalinamide (DALAMIDE) at a dose of 50 mg/kg. I.c.v. administration of tiorphan or DALAMIDE did not modify the colonic motility in dogs fasted for 48 hrs. The postprandial motility index remained unchanged after intravenous administration of tiorphan or DALAMIDE at the same dosages. These results provide evidence for a central control of the colonic motor response to feeding by endogenous enkephalins in dogs.

Proctolin: a potent inhibitor of aminoenkephalinase

Proctolin is a potent selective inhibitor of aminoenkephalinase. The specificity of its inhibition of various aminopeptidases is similar to that of puromycin; it inhibits aminoenkephalinase, but not leucine aminopeptidase or aminopeptidase M. Enkephalin breakdown by synaptic plasma membrane, but not by brain slices, is sensitive to proctolin. The inhibition by proctolin is partially caused by its resistance to enzymatic breakdown. The inhibition is of mixed type and is concentration dependent, and the two amino acids at the N-terminal are important for its action. The minimal structure for inhibition is a dipeptide with a basic amino acid at the N-terminal and a basic or an aromatic amino acid at the C-terminal.

Characterization of membrane-bound aminopeptidases from rat brain: identification of the enkephalin-degrading aminopeptidase

Rat brain aminopeptidase activity was solubilized from membranes by incubation with thiols. This novel procedure resulted in the release of the same two aminopeptidases (MI and MII) previously shown to be solubilized by the nonionic detergent Triton X-100. The solubilized aminopeptidases MI and MII were resolved by ion-exchange chromatography and further purified by hydroxylapatite chromatography. Aminopeptidase MI was shown to hydrolyze only the beta-naphthylamides of arginine and lysine whereas aminopeptidase MII exhibited a broad specificity with respect to amino acid beta-naphthylamides. Only aminopeptidase MII hydrolyzed Leu-enkephalin at a significant rate, indicating that this enzyme can account for the membrane-bound enkephalin aminopeptidase activity. The enkephalin-degrading aminopeptidase is potently inhibited by opioid (alpha-neo-endorphin and dynorphin) as well as nonopioid (substance P, somatostatin, and angiotensin I) peptides in the range of 0.2-2.0 microM. The regional distribution of aminopeptidases MI and MII in rat brain are rather different, with aminopeptidase MII distribution more closely paralleling the distribution of opiate receptors.

Distribution of dipeptidyl peptidase II (Dpp II) in rat spinal cord

The histochemical localization of dipeptidyl peptidase II (Dpp II; E.C. 3.4.14.2) activity was demonstrated at the light microscope level in the rat spinal cord. Prominent staining was observed in motoneurons of the ventral horn and in medium to large neurons in the deep laminae of the dorsal horn, the intermediate gray, and in lamina X surrounding the spinal canal. Within neurons, Dpp II was localized largely in cell perikarya and large primary dendrites with no staining observed in cell nuclei. Neurons in the superficial dorsal horn lack Dpp II enzyme activity. Nonneuronal elements which also stained prominently were pericytes associated with blood vessels and ependymal cells lining the lumen of the spinal canal. A few oligodendrocytes and astrocytes were also stained, but they represented a minor component of the total amount of Dpp II activity. Following ventral root injury, Dpp-II-containing motoneurons degenerate; some glial cells in the region of degenerating neurons become Dpp II positive. The localized distribution of Dpp II in spinal cord neurons suggests that this proteolytic enzyme may play a role in the metabolism of an unidentified neuropeptide.

Assessment of the role of "enkephalinase" in cholecystokinin inactivation

Cholecystokinin octapeptide and the C-terminal tetrapeptide are hydrolysed by a highly purified preparation of "enkephalinase" (EC 3.4.24.11). In both cases the Asp-PheNH2 bond is hydrolysed and the Gly4-Trp5 bond of the octapeptide is also cleaved, though more slowly. Evaluated from the appearance of Phe-NH2, the Km for the hydrolysis of the octapeptide by the purified peptidase is 57 microM and that for the tetrapeptide 65 microM. The apparent affinities of these peptides for the enzyme in striatal membranes are similar. The importance of this hydrolysis in the inactivation of endogenous cholecystokinin was assessed by studying the fate of cholecystokinin immunoreactivity released from slices of rat cerebral cortex and striatum by depolarization with potassium. In the absence of any peptidase inhibitor only 16% of the peptide released from the tissue was recovered in immunoreactive form in the medium, indicating that endogenous cholecystokinin octapeptide is, like other neuropeptides, rapidly and extensively hydrolysed following release. Selective inhibition of "enkephalinase" by Thiorphan (DL-3-mercapto-2-benzylpropanoyl glycine) did not significantly alter the recovery from slices of cerebral cortex and had only a very slight effect in the case of striatal slices. This suggests that, while cholecystokinin octapeptide is a substrate for "enkephalinase", this enzyme plays a less important (if any) role in the inactivation of endogenous cholecystokinin than for the opioid peptides.

Enantiomers of [R,S]-thiorphan: dissociation of analgesia from enkephalinase A inhibition

The [R] and [S] enantiomers of the enkephalinase A inhibitor [R,S]-thiorphan have been prepared by asymmetric synthesis. The [S] isomer is principally responsible for the angiotensin converting enzyme inhibitory activity of [R,S]-thiorphan, whereas there were only small differences in the ability of the [R] and [S] isomers to inhibit enkephalinase both in vivo and in vitro. In contrast, the in vivo analgesic activity of [R,S]-thiorphan resided principally in the [R] isomer. These data indicate a surprising dissociation of enkephalinase inhibition from analgesic activity. The fact that the two enantiomers of [R,S]-thiorphan were effective inhibitors of enkephalinase, yet the [R] isomer had substantially greater analgesic activity, indicates that factors other than enkephalinase inhibition may be important for [R, S]-thiorphan's analgesic properties.

Rapid sequestration and degradation of somatostatin analogues by isolated brain microvessels

Somatostatin (SRIF) is a putative peptide neurotransmitter that may interact with brain capillaries following neurosecretion of the peptide. The present studies investigate the binding and metabolism of SRIF analogues in isolated bovine brain microvessels. 125I-[Tyr1]SRIF was rapidly degraded by capillary aminopeptidase with a half-time of approximately 3 min at 23 degrees C. The microvessel aminopeptidase had a low affinity and high capacity for the peptide, Km = 76 microM and Vmax = 74 nmol min-1 mgp-1. 125I-[Tyr11]SRIF was converted to free iodotyrosine at a much slower rate, presumably by a lower-activity endopeptidase. 125I-[Try11]SRIF was rapidly bound by microvessels, whereas another basic peptide, [Tyr8]bradykinin, or an acidic peptide, CCK8, or a neutral peptide, leucine enkephalin, were bound to a considerably less extent. The binding of 125I-[Tyr11]SRIF to the capillaries was nonsaturable up to a concentration of 1 microgram/ml of unlabeled peptide, and the binding reaction was extremely rapid, reaching equilibrium within 5 s at either 0 degrees C or 37 degrees C. Approximately 20% of the SRIF bound by the microvessels was resistant to acid wash and presumably represented internalized peptide. In addition, the 125I-[Tyr11]SRIF bound rapidly to the endothelial cytoskeleton remaining after a 1% Triton X-100 extraction of the microvessels. The peptide-cytoskeletal binding reaction was nonsaturable up to 1 microgram/ml of unlabeled [Tyr11]SRIF, but it was inhibited by 0.5% polylysine or 0.8 M KCl and was stimulated by 1 mM dithiothreiotol. These studies suggest that brain microvessels rapidly sequester and degrade SRIF analogues and that this may represent one mechanism for rapid inactivation of the neuropeptides subsequent to neurosecretion.

Peripheral enkephalin hydrolysis in different animal species: a comparative study

Using column and thin layer chromatography, plasma hydrolysis of leu-enkephalin has been studied in man and several laboratory animals. The hydrolysis kinetics determined in the various species examined are considerably different. In addition, also the enzyme forms evidentiated, their molecular weight distribution and relative ratios have been found to vary greatly in the animals under test. Our data suggest that the widely different hydrolysis kinetics reported by various authors are attributable to the differences between species, rather than to differences in the analytical techniques employed.