Separation and characterization of a dipeptidyl aminopeptidase that degrades enkephalins from monkey brain

Ets-1/Elk-1 is a critical mediator of dipeptidyl-peptidase III transcription in human glioblastoma cells

Dipetidyl-peptidase III is a metallopeptidase involved in a number of physiological processes and its expression has been reported to increase with the histological aggressiveness of human ovarian primary carcinomas. Because no information regarding the regulation of its expression was available, experiments were designed to clone, define and characterize the promoter region of the human dipeptidyl-peptidase III (DPP-III) gene. In this study, we cloned a 1038 bp 5'-flanking DNA fragment of the human DPP-III gene for the first time and demonstrated strong promoter activity in this region. Deletion analysis revealed that as few as 45 nucleotides proximal to the transcription start site retained approximately 40% of the activity of the full-length promoter. This promoter lacked the TATA box but contained multiple GC boxes and a single CAAT box. Similarly, two Ets-1/Elk-1-binding motifs are present in the first 25 nucleotides from the transcription start site. Binding of Ets-1/Elk-1 proteins to these motifs was visualized by electrophoretic mobility shift and chromatin immunoprecipitation assays. Mutations of these binding sites abolished not only binding of the Ets protein, but also the intrinsic promoter activity. Increased DNA-binding activity of Ets-1/Elk-1 by v-Ha-ras also augmented the mRNA level and promoter activity of this gene. Similarly, co-transfection of DPP-III promoter-reporter constructs with Ets-1 expression vector led to a significant increase in promoter activity. From these results, we conclude that Ets-1/Elk-1 plays a critical role in transcription of the human DPP-III gene.

Binding profile of the endogenous novel heptapeptide Met-enkephalin-Gly-tyr in zebrafish and rat brain

Zebrafish is considered a model organism, not only for the study of the biological functions of vertebrates but also as a tool to analyze the effects of some drugs or toxic agents. Five opioid precursor genes homologous to the mammalian opioid propeptide genes have recently been identified; one of these, the zebrafish proenkephalin, codes a novel heptapeptide, the Met-enkephalin-Gly-Tyr (MEGY). To analyze the pharmacological properties of this novel ligand, we have labeled it with tritium ([(3)H]MEGY). In addition, we have also synthesized two analogs: (d-Ala(2))-MEGY (Y-d-Ala-GFMGY) and (d-Ala(2), Val(5))-MEGY (Y-d-Ala-GFVGY). The binding profile of these three agents has been studied in zebrafish and rat brain membranes. [(3)H]MEGY presents one binding site in zebrafish, as well as in rat brain membranes, although it shows a slight higher affinity in zebrafish brain. The observed saturable binding is displaced by naloxone, thus confirming the opioid nature of the binding sites. Competition binding assays indicate that the methionine residue is essential for high-affinity binding of MEGY and probably of other peptidic agonists in zebrafish, whereas the change of a Gly for a d-Ala does not dramatically affect the ligand affinity. Our results show that the percentage of [(3)H]MEGY displaced by all the ligands studied is higher than 100%, thus inferring that naloxone (used to determine nonspecific binding) does not bind to all the sites labeled by [(3)H]MEGY. Therefore, we can deduct that some of the MEGY binding sites should not be considered classical opioid sites.

Identification of dipeptidyl peptidase III in human neutrophils

We have found activity of dipeptidyl peptidase (DPP) III, one of the most important enkephalin-degrading enzymes in the central nervous system, in human neutrophils. HPLC analysis of the peptide fragments produced by treatment of leucine-enkephalin with isolated neutrophils in the presence of inhibitors of other enkephalin-degrading enzymes revealed that the enzyme in human neutrophils cleaved dipeptides from the NH(2) terminus of leucine-enkephalin, suggesting the presence of DPPIII activity in human neutrophils. Using a specific synthesized substrate and proteinase inhibitors, it was found that the neutrophils have 19.2 +/- 3.6 microM/h/5 x 10(6) cells of beta-naphthylamine for the enzyme. It was also confirmed that spinorphin and tynorphin, both reported to inhibit the activities of enkephalin-degrading enzymes, had potent inhibitory activities (IC(50): 4.0 and 0.029 microg/ml, respectively) against the enzyme. The presence of DPPIII activity in human neutrophils suggests that the biologically active peptides which are associated with enkephalin play a physiological role in regulating enkephalin or inflammatory mechanisms in peripheral tissues.

Dipeptidyl peptidase III is a zinc metallo-exopeptidase. Molecular cloning and expression

We have purified dipeptidyl peptidase III (EC 3.4.14.4) from human placenta. It had a pH optimum of 8.8 and readily hydrolysed Arg-Arg-beta-naphthylamide. Monoamino acid-, Gly-Phe-, Gly-Pro- and Bz-Arg-beta-naphthylamides were not hydrolysed at all. The enzyme was inhibited by p-chloromercuriphenylsulphonic acid, metal chelators and 3,4-dichloroisocoumarin and contained 1 mol of zinc per mol of enzyme. The zinc dissociation constant was 250 fM at pH 7. 4 as determined by the zinc binding study. We isolated, by immunological screening of a Uni-ZAP XR cDNA library constructed from rat liver mRNA species, a cDNA clone with 2633 bp encoding the rat enzyme. The longest open reading frame encodes a 827-residue protein with a theoretical molecular mass of 92790 Da. Escherichia coli SOLR cells were infected with the pBluescript phagemid containing the cloned cDNA and established the overexpression of a protein that hydrolysed Arg-Arg-beta-naphthylamide. The recombinant protein was purified and the amino acid sequence of the protein was confirmed. We presumed that the putative zinc-binding domain involved in catalysis was present in the recombinant enzyme. It was a novel zinc-binding motif in that one amino acid residue was inserted into the conserved HEXXH motif characteristic of the metalloproteinases.

Inhibitors of enkephalin-degrading enzymes as potential therapeutic agents

A limited number of enzymes such as membrane metalloendopeptidase (enkephalinase) and angiotensin converting enzyme appear to be involved in deactivation and modulation of circulatory regulatory peptides. Peptides such as the enkephalins are also involved in a large number of physiological processes. This multiplicity of physiological roles has made it difficult to establish the therapeutic role of enkephalin-degrading enzyme inhibitors. Other factors such as difficulty in quantification and thus measurement of processes involved in pain and mental illness have also hindered the process of establishing any therapeutic role of enkephalin-degrading enzyme inhibitors in these conditions. However, they have proved to be useful pharmacological 'tools'. The most likely therapeutic role at present appears to be in the treatment of cardiovascular disorders. As a 'profile' of pharmacological actions of enkephalin-degrading enzymes emerges, it is becoming apparent that bioavailability rather than a high degree of specificity or inhibitory potency may be the most important factor. This may be used to an advantage in future developments by the use of less specific or combined inhibitors in the form of prodrugs, designed to be active at specific sites such as the central nervous system.

Dipeptidyl peptidase I from goat brain: purification, characterization and its action on Leu-enkephalin

Brain dipeptidyl peptidase (DPP) I has been purified 2990-fold to apparent homogeneity shown by a single protein band in electrophoreses at pH 4.5, 8.4 and in SDS-PAGE at pH 7.2. The purification techniques included homogenization of brain acetone powder, autolysis at pH 4.2, 30-80% (NH4)2SO4 fractionation. Sephadex G-100 column chromatography, heat treatment at 65 C. organomercurial affinity chromatography. CM-Sephadex cation-exchange chromatographies at pH 5.6 and 5.0 and anion-exchange chromatography on DEAE-Sephadex at pH 6.8. The enzyme hydrolysed synthetic substrate Gly-Arg-4-methoxy-beta-naphthyl-amide maximally at pH 6.0. The Km values for Gly-Arg-beta-naphthylamide and Gly-Arg-4-methoxy-beta-naphthylamide substrates were 0.10 mM and 0.14 mM respectively. The enzyme was inhibited by thiol inhibitors like p-chloromercuribenzoic acid, iodoacetic acid, iodoacetamide and microbial inhibitors leupeptin and antipain. Molecular weight estimations on a calibrated Sephadex G-200 column afforded a value of 180,000 Da while in denaturing conditions on sodium dodecyl sulphate polyacrylamide gel electrophoresis, the subunit molecular weight was 22,000 Da. The subunit structure of the native enzyme was unfolded in presence of different concentrations of urea. In 8 M urea, the enzyme dissociated completely into monomers of 25,000 Da but 6, 5 and 4 M urea concentrations revealed the existence of dimers, tetramers and hexamers. Leu-enkephalin. Tyr-Gly-Gly-Phe-Leu was degraded by DPPI into Tyr-Gly and Gly-Phe-Leu with no further degradation of the newly generated tripeptide.

Dipeptidase activities in rat brain synaptosomes can be distinguished on the basis of inhibition by bestatin and amastatin: identification of a kyotorphin (Tyr-Arg)-degrading enzyme

The neuropeptide kyotorphin (Tyr-Arg) was degraded by rat brain synaptosomes via a synaptic membrane-bound peptidase which was inhibited by bestatin but not by amastatin. The Km for kyotorphin was 8 x 10(-6) M and the Ki for bestatin was 1 x 10(-7) M. The kyotorphin-degrading enzyme was distinguished from at least one other dipeptide-hydrolyzing activity in synaptosomes which was inhibited by both bestatin and amastatin. Gel permeation chromatography of detergent-extracted synaptosomes resulted in the separation of the dipeptide-hydrolyzing activities. A single kyotorphin-degrading enzyme peak was observed which had a M(r) = 52,000. The activity peak could degrade other dipeptides including Phe-Arg, a synaptic membrane-generated metabolic of bradykinin. The kyotorphin-degrading enzyme appears to be novel and can be distinguished from other known dipeptidases on the basis of substrate specificity, subcellular localization, and inhibition profile.

Human placental dipeptidyl aminopeptidase III: hydrolysis of enkephalins and its stimulation by cobaltous ion

The degradation of enkephalin and related peptides by highly purified dipeptidyl aminopeptidase III (EC 3.4.14.4) was studied. The enzyme releases the N-terminal dipeptide units from substrates greater in length than the tetrapeptide. The enzyme exhibits an optimum of pH 7.5, Km of 81 microM and Vmax of 0.043 mumole/min for Leu-enkephalin. Its activity was markedly stimulated by Co2+, with both the Km and Vmax being increased. Among the enkephalin-related peptides examined, des-Tyr1-Leu-enkephalin was the most rapidly hydrolyzed with Co2+, but only slight stimulation was observed with Co2+.

Dipeptidyl peptidase III and alanyl aminopeptidase in the human seminal plasma: origin and biochemical properties

Human seminal plasma contained two distinct enzyme activities hydrolysing ArgArgNA. The enzymes were separated by anion exchange chromatography and further purified by gel filtration and/or hydrophobic interaction chromatography. The enzyme eluting at the lower NaCl concentration (0.26 mol/l) displayed an optimum at pH 5.7-6.0 (enzyme A), while the other enzyme eluted at 0.32 mol/l NaCl and showed an optimum at pH 8.5-9.0 (enzyme B). Enzyme A was found to coelute with an aminopeptidase which hydrolysed various amino acid derivatives as well as dipeptide naphthylamides sequentially. Both enzymes were sensitive to heavy metal ions (Cd, Cu, Hg, Pb) and chelating agents (EDTA, o-phenanthroline) and moderately sensitive to di-isopropylfluorophosphonate (DFP) or phenylmethylsulfonylfluoride (PMSF). After EDTA suppression both activities were partially reactivated by divalent metal ions, particularly by Co2+. Enzyme A was highly sensitive to amastatin, bestatin and puromycin, while enzyme B was not markedly influenced. With different substrates the modifier characteristics of enzyme A were equal. High concentrations of some substrates suppressed the hydrolysis rates of both enzymes. Enzyme B was much more sensitive to the thermal treatment than enzyme A. Tentative molecular masses of 110 kD and 80 kD were obtained for enzymes A and B, respectively. Enzyme B was found in all male reproductive tissues (testis, epididymis, vas deferens, ampulla, seminal vesicles, prostate), while enzyme A was only detected in the prostatic homogenate. Thus, ArgArgNA in the human seminal plasma is hydrolysed by dipeptidyl peptidase III, which may originate from different reproductive organs, while the prostate is responsible for the secretion of an aminopeptidase with a wide substrate spectrum including dipeptidyl derivatives.

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.

A new aminopeptidase in monkey cerebral membrane fraction: hydrolysis of enkephalin

A new aminopeptidase, which cleaves the Tyr1-Gly2 bond of enkephalin, was partially purified from the monkey brain membrane fraction. The molecular weight of the enzyme was estimated to be about 53,000, and the optimum pH was in the neutral region (pH 6.5). The enzyme hydrolyzed Leu-enkephalin with a Km value of 238 microM. It strongly hydrolyzed L-tyrosine and L-leucine beta-naphthylamide, but showed only weak affinity for L-arginine or L-alanine beta-naphthylamide. The enzyme was much more potently inhibited by bestatin (IC50: 2 x 10(-8) M) than the other specific aminopeptidase inhibitors examined, while it showed low sensitivity to puromycin and actinonin, inhibitors of cerebral enkephalin-degrading aminopeptidase and aminopeptidase M, respectively. These results indicate that the new enkephalin-degrading aminopeptidase is clearly distinct from aminopeptidase M, which has been reported to be a key enzyme in enkephalin inactivation.

Angiotensin III: a potent inhibitor of enkephalin-degrading enzymes and an analgesic agent

Various angiotensins, bradykinins, and related peptides were examined for their inhibitory activity against several enkephalin-degrading enzymes, including an aminopeptidase and a dipeptidyl aminopeptidase, purified from a membrane-bound fraction of monkey brain, and an endopeptidase, purified from the rabbit kidney membrane fraction. Angiotensin derivatives having a basic or neutral amino acid at the N-terminus showed strong inhibition of the aminopeptidase. Dipeptidyl aminopeptidase was inhibited by angiotensins II and III and their derivatives, whereas the endopeptidase was inhibited by angiotensin I and its derivatives. The most potent inhibitor of aminopeptidase and dipeptidyl aminopeptidase was angiotensin III, which completely inhibited the degradation of enkephalin by enzymes in monkey brain or human CSF. The Ki values for angiotensin III against aminopeptidase, dipeptidyl aminopeptidase, endopeptidase, and angiotensin-converting enzyme, which degraded enkephalin, were 0.66 X 10(-6), 1.03 X 10(-6), 2.3 X 10(-4), and 1.65 X 10(-6) M, respectively. Angiotensin III potentiated the analgesic activity of Met-enkephalin after intracerebroventricular coadministration to mice in the hot plate test. Angiotensin III itself also displayed analgesic activity in that test. These actions were blocked by the specific opiate antagonist naloxone.

Purification and characterization of enkephalinase B from rat brain membrane

Enkephalinase B from rat brain membrane which hydrolyzes enkephalin at the Gly-Gly bond was purified about 9400-fold to apparent electrophoretic homogeneity. The enzyme, which has a molecular weight of 82,000, consists of a single polypeptide chain. The enzyme has a pH optimum of 6.0-6.5 and is stable in the neutral pH region. The Km values of Met-enkephalin and Leu-enkephalin for this enzyme were 5.3 X 10(-5) M and 5.0 X 10(-5) M, respectively. The enzyme was inactivated by metal chelators, EDTA and o-phenanthroline and restored by the addition of divalent metal ions, Zn2+, Mn2+ or Fe2+, but was not inhibited by bestatin, amastatin, phosphoramidon or captopril. The enzyme hydrolyzed Met-enkephalin and Leu-enkephalin effectively. Although the enzyme belongs to the dipeptidyl aminopeptidase class, enkephalin-related peptides such as Leu-enkephalin-Arg, dynorphin (1-13) or alpha-endorphin and other biologically active peptides examined were hardly, or not at all, hydrolyzed. It was assumed that enkephalinase B functions mainly in enkephalin degradation in vivo.

Hydrolysis of neo-kyotorphin (Thr-Ser-Lys-Tyr-Arg) and [Met]enkephalin-Arg6-Phe7 by angiotensin-converting enzyme from monkey brain

Angiotensin-converting enzyme (ACE; dipeptidyl carboxypeptidase, EC 3.4.15.1) from monkey brain was partially purified 274-fold with 4.5% yield. The optimum pH of the enzyme was 8.2, and its Km was 3.3 mM, with hippuryl-His-Leu as the substrate in 300 mM NaCl. Its molecular weight (Mr) was estimated to be approximately 260,000 by gel filtration on Sephadex G-200. On high-performance liquid chromatographic analysis, ACE hydrolyzed neo-kyotorphin Thr-Ser-Lys-Tyr-Arg) with liberation of kyotorphin (Tyr-Arg), the [Met]enkephalin releaser. ACE also converted [Met]enkephalin-Arg6-Phe7 to [Met]enkephalin; then the enzyme slowly hydrolyzed the resulting [Met]enkephalin. The Km values of the enzyme for neo-kyotorphin and [Met]enkephalin-Arg6-Phe7 were 0.58 and 0.30 mM respectively. Thus, brain ACE may have a role in the formation of kyotorphin and [Met]enkephalin from their precursors but has little part in [Met]enkephalin degrading processes.

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.

Separation of enkephalin-degrading enzymes from longitudinal muscle layer of bovine small intestine. Enzyme inhibition by arphamenine A

The enkephalin-degrading enzymes, such as aminopeptidase, dipeptidyl aminopeptidase, dipeptidyl carboxypeptidase and carboxypeptidase, were purified partially by DEAE-cellulose column chromatography, using longitudinal muscle from bovine small intestine. These enzyme were inhibited by EDTA and o-phenanthroline. Several protease inhibitors of microbial origin, and synthetic compounds, were tested for their abilities to inhibit these enkephalin-degrading enzymes. Among them, arphamenine A, a new potent inhibitor for aminopeptidase B, was shown to be a useful compound in inhibiting all of the enkephalin-degrading enzymes in small intestine.

Enkephalin-degrading dipeptidyl carboxypeptidase in guinea pig serum: its properties and action on bioactive peptides

A dipeptidyl carboxypeptidase, which cleaved the Gly3-Phe4 bond of enkephalins, was purified from guinea pig serum 420-fold. The optimum pH of the enzyme was in the neutral range (pH 7.25), and the molecular weight was estimated to be approx. 280,000. The enzyme hydrolyzed Met- and Leu-enkephalin with Km values of 0.30 and 0.50 mM, respectively. The enzyme was inhibited by metal chelators and p-chloro-mercuribenzoate. Captopril showed high inhibitory potency, while phosphoramidon and Phe-Ala showed no effect on the enzyme activity. Therefore, the obtained enzyme can be classified as an angiotensin-converting enzyme (EC 3.4.15.1). Among the bioactive peptides examined, bradykinin and angiotensin I were hydrolyzed by the enzyme. Angiotensin III showed a stronger inhibitory effect than that of angiotensin II. Substance P, gastrin I, and secretin were also inhibitory toward the enzyme activity. On high-performance liquid chromatography analysis, Met-enkephalin-Arg6-Phe7 and Leu-enkephalin-Arg6 were cleaved sequentially at the second peptide bond of the C terminus. Thus, the dipeptidyl carboxypeptidase in guinea pig serum may play a role not only in the angiotensin-bradykinin system but also in the metabolism of circulating enkephalins and other bioactive peptides.

Degradation of kyotorphin by a purified membrane-bound-aminopeptidase from monkey brain: potentiation of kyotorphin-induced analgesia by a highly effective inhibitor, bestatin

Kyotorphin (Tyr-Arg) was rapidly degraded in rat brain homogenates and the Vmax and Km were 29.4 nmol/mg protein/min and 16.6 microM, respectively. This degradation was effectively inhibited by bestatin (IC 50; 0.08 microM) and p-chloromercuribenzoate (IC 50; 0.70 microM). Kyotorphin was also degraded by a membrane-bound aminopeptidase from monkey brains. The Vmax and Km of kyotorphin degradation by the aminopeptidase were 20.0 nmol/mg protein/min and 29.2 microM, respectively. The degradation of kyotorphin was also inhibited effectively by bestatin (KI; 0.4 microM). Co-administration with bestatin 50 micrograms (i.cist.) potentiated the analgesic effects of kyotorphin (i.cist.) by 4.8 times, and these effects were abolished by pretreatment with naloxone 0.5 mg/kg s.c. These results suggest that potentiation of analgesia by bestatin may be due to the protection against the degradation of kyotorphin and released enkephalin by a membrane-bound aminopeptidase.

How is kyotorphin (Tyr-Arg) generated in the brain?

Kyotorphin (Tyr-Arg) was rapidly degraded in the brain homogenates and purified membrane-bound aminopeptidase from monkey brains. The degradation of kyotorphin by these preparations was effectively inhibited by bestatin. When brain homogenates or slices were incubated with bestatin, kyotorphin was accumulated time-dependently in a rate of 1.0 or 2.1 pmol/mg protein/hr, respectively. The bestatin-induced kyotorphin accumulation was inhibited by leupeptin, p-chloromercuribenzoate, but not phenylmethylsulfonylfluoride or diisopropylphosphate. The kyotorphin accumulation was concentrated in the P2 (crude mitochondrial) fraction, particularly in the particulate or synaptosomal fraction. These findings suggest that kyotorphin may be generated in vitro from precursor proteins by membrane-bound, leupeptin-sensitive "kyotorphin converting enzymes" in close vicinity to membrane-bound aminopeptidase which rapidly degrades kyotorphin generated.

Inhibition of a membrane-bound enkephalin-degrading aminopeptidase by bestatin analogs

A variety of bestatin analogs were examined as potent inhibitors of a membrane-bound enkephalin-degrading aminopeptidase that was purified from monkey brain. Bestatinyl amino acid derivatives showed strong inhibition of this enzyme. The most effective was bestatin-L-Arg X AcOH, with a Ki value of 0.21 X 10(-8) M with Leu-enkephalin as substrate. It exhibited competitive kinetics and was about 100-fold more potent than bestatin. This compound seems to be useful for pharmacological and other studies.

Bidentate peptides: highly potent new inhibitors of enkephalin degrading enzymes

Three series of bidentates bearing an hydroxamic or an N-Acyl-N-hydroxy amino group on structures related to Phe-Gly or Phe-Ala exhibit strong inhibitory potency against purified enkephalinase with IC50 values in the 4 to 15 nM range. As with thiol-containing inhibitors, such as thiorphan, the most active compounds are those in which a methylene spacer separates the benzyl P1' moiety from the Zn coordinating residue. Formation of a bidentate complex with the metal enzyme is clearly demonstrated by a loss of potency of three order of magnitude following the removal of one component of the bidentate group. All the compounds studied are unable to interact with angiotensin converting enzyme (IC50 greater than 10,000 nM). Moreover, compounds of the general formula HONHCO-CH2-CH(CH2 phi)-CONH-CH(R)-COOH belonging to the most active series of enkephalinase blockers (IC50 approximately 4 nM) behave also as highly potent and competitive inhibitors (IC50 approximately 10 nM) of a Tyr-Gly releasing dipeptidylaminopeptidase purified from rat brain. The pure steroisomer [(R)-3-(N-hydroxy)carboxamido-2-benzylpropanoyl]-L-alanine designated kelatorphan, exhibits also a relatively good inhibitory potency against aminopeptidases (IC50 approximately 10 microM) and can be considered as the first virtually complete inhibitor of enkephalin metabolism. This very interesting property of inhibiting all three enzymes of enkephalin metabolism could enhance the required selectivity for a possible clinical use of these inhibitors as new analgesic and psychoactive drugs.

Analgesic effects of kelatorphan, a new highly potent inhibitor of multiple enkephalin degrading enzymes

Kelatorphan, [(R)-3-(N-hydroxy)-carboxamido-2-benzylpropanoyl]-L-alanine, represents the first virtually complete inhibitor of enkephalins metabolism with KI = 1.4 nM against enkephalinase, KI = 2 nM against the Gly2 -Gly3 cleaving dipeptidylaminopeptidase and KI = 7 microM on aminopeptidase activity. The analgesic effect of [Met5]enkephalin was potentiated 50000 times (ED50 approximately 10 ng) by intracerebroventricular co-administration in mice of kelatorphan (50 micrograms). This effect was significantly higher than that produced by bestatin (50 micrograms) + thiorphan (50 micrograms). Kelatorphan alone was at least two-fold more potent as analgesic than the above mixture of inhibitors.

Purification and characterization of an enkephalin-degrading aminopeptidase from guinea pig serum

An enkephalin-degrading aminopeptidase using Leu-enkephalin as a substrate was purified about 4100-fold from guinea pig serum. The purified preparation was apparently homogenous, showing one band on polyacrylamide gel electrophoresis. The molecular weight of the enzyme was approx. 92 000. The aminopeptidase had a pH optimum of 7.0 with Km values of 0.12 mM and 0.18 mM for Leu- and Met-enkephalin, respectively. The enzyme hydrolyzed neutral, basic and aromatic amino acid beta-naphthylamides, but did not the acidic one. The enzyme was inhibited strongly by metal-chelating agents, bestatin and amastatin and weakly by puromycin. Among several biologically active peptides, angiotensin III and substance P strongly inhibited the enzyme.

Differences in the structural requirements for selective interaction with neutral metalloendopeptidase (enkephalinase) or angiotensin-converting enzyme. Molecular investigation by use of new thiol inhibitors

Despite the similarities in their mechanism of action, the structural requirements for selective interaction with angiotensin-converting enzyme or enkephalinase are different. Inhibitory potency of a series of new mercaptoalkanoyl amino acids were determined on pure angiotensin-converting enzyme (EC 3.4.15.1) from porcine plasma and on neutral metalloendopeptidase (EC 3.4.24.11) purified from rat brain. This latter enzyme, first designated as enkephalinase, seems to be synaptically involved in the degradation of enkephalins. All tested compounds, whose design was based on the classical active-site model of metallopeptidases, are reversible and competitive inhibitors of both enzymes. Owing to the remarkable similarity in the general topology of metallopeptidases, the differences in optimal binding requirements to enkephalinase and angiotensin-converting enzyme were interpreted from crystallographic studies on related enzymes such as thermolysin and carboxypeptidase A. The large size of the S'1 subsite of enkephalinase allows efficient binding (Ki approximately equal to 2-30 nM) of aromatic and bulky hydrophobic residues such as a cyclohexyl ring. In contrast, a methyl group in position P'1 favors inhibitory potency against angiotensin-converting enzyme while a cyclohexyl ring leads to a complete loss of activity. This feature could mean that optimal binding of the Zn atom present in the catalytic site is a more stringent requirement in angiotensin-converting enzyme than in enkephalinase. An increase in the size of the P'2 component of thiol inhibitors potentiates the affinity for angiotensin-converting enzyme without a significant change on enkephalinase. Finally, methylation of the ultimate amide bond of inhibitors produces a 30-fold decrease in potency towards enkephalinase but does not affect the binding of angiotensin-converting enzyme. These findings allow a rational design of selective inhibitors of enkephalinase, an essential prerequisite for their possible clinical use as new analgesic and psycho-active agents.

Enkephalin degradation by enkephalinergic neuroblastoma cells. Involvement of angiotensin-converting-enzyme

Degradation of tritiated [Leu5]enkephalin was studied in cultures of neuroblastoma cells (clone N1E-115). Incubation of cells in suspension revealed Tyr as the main tritiated metabolite; however, Tyr-Gly-Gly and Tyr-Gly were detectable as well. In a crude membrane preparation of the neuroblastoma cells the level of Tyr is reduced to 13% and that of Tyr-Gly to 10% of the initial value, whereas Tyr-Gly-Gly is increased to about 5 times the initial value. Of the degraded enkephalin, 66% was accounted for by the formation of Tyr, 30% by the formation of Tyr-Gly-Gly and 4% by the formation of Tyr-Gly. The production of Tyr was inhibited by bestatin, an inhibitor of aminopeptidases, and that of Tyr-Gly-Gly by captopril, an inhibitor of angiotensin-converting-enzyme. The results prove the ability of neuroblastoma cells (N1E-115) to degrade enkephalin by aminopeptidase and the membrane-bound angiotensin-converting-enzyme.

Processing and degradation of met-enkephalin by peptidases associated with rat brain cortical synaptosomes

The generation of met-enkephalin (Tyr1-Gly2-Gly3-Phe4-Met5) from met-enkephalin-Arg6-Phe7 and subsequent degradation of the liberated peptides to the free amino acids by rat brain cortical synaptosomes in vitro was demonstrated by HPLC and amino acid analyses. Kinetic measurements of the individual steps of met-enkephalin processing and degradation upon incubation with synaptosomes revealed the following sequence of cleavage: 1. Hydrolysis of the Met5-Arg6 peptide bond, generating met-enkephalin and the dipeptide Arg-Phe. Captopril and EDTA inhibit this reaction. 2. Hydrolysis of the Tyr1-Gly2 peptide bond, generating Tyr and a tetrapeptide. Puromycin (ID50 = 5 X 10(-5) M) and parahydroxymercuribenzoate (ID50 = 5 X 10(-4) M) inhibit this reaction. 3. Hydrolysis of the Gly3-Phe4 peptide bond. Parahydroxymercuribenzoate (ID50 = 5 X 10(-4) M) inhibits this reaction completely. 1 mmol liter-1 Puromycin does not inhibit this reaction. 4. Hydrolysis of the Phe4-Met5 peptide bond. 5. Hydrolysis of the Gly2-Gly3 peptide bond. The pH optimum of all cleavage reactions was found to be around 7.8.

Identification of cell surface dipeptidylpeptidase IV in human fibroblasts

An antigen with dipeptidylpeptidase IV activity was identified at the surface of normal human fibroblasts. Hydrophobic interaction electrophoresis in phenyl-Sepharose revealed that the enzyme contained a hydrophobic domain, while lactoperoxidase-catalysed iodination with 125I of living cells indicated that the protein was located at the cell surface. Crossed immunoelectrophoresis with specific antibodies of acid-extracted or papain-treated cells showed a shift of the dipeptidylpeptidase IV peak to a faster mobility. The molecular properties of the fibroblast enzyme were clearly different from those described for dipeptidylpeptidase IV from other tissues and species. Fibroblast dipeptidylpeptidase IV contained two different disulphide-linked subunits, of apparent Mr values 125000 and 135000 (denatured and reduced). In gel filtration, an Mr of about 400000 was observed for the unreduced molecule. The enzymic properties of fibroblast dipeptidylpeptidase IV were very similar to those of the well-characterized pig kidney enzyme. Activity towards glycyl-L-prolyl-beta-naphthylamide was inhibited 50% by 0.023 mM-di-isopropylphosphorofluoridate. L-Alanyl-L-alanyl-beta-naphthylamide was hydrolysed ten times more slowly than glycyl-L-prolyl-beta-naphthylamide.

Purification and characterization of enkephalin-degradating enzymes from calf-brain striatum

Enkephalinase A and B are extracted from Triton-X 100 washed calf-brain particles and purified by DEAE-cellulose chromatography. Both enzymes have identical Km values in their membrane-bound and soluble form. Enkephalinase A has a pH optimum at 6.9 and a Km for Leu-enkephalin of 20-25 microM, which hardly depends on the pH. Thiorphan and phosphate are purely competitive inhibitors of Enkephalinase A with Ki values of 3 nM and 1.5 mM respectively (pH = 6.85). Enkephalinase B is not affected by phosphate or thiorphan. It has a Km for Leu-enkephalin of 10 microM, a pH optimum of 7.0 and is inhibited by low concentrations of apolar dipeptides.

Enkephalin degrading enzymes in cerebrospinal fluid

Enkephalins were rapidly degraded by specific enzyme systems in vivo. In cerebrospinal fluid (CSF), however, it has been undefined whether these enzyme systems existed. Our experiments showed enkephalins were hydrolyzed by the enzymatic activity in both CSF of human and monkey. The results by the thin layer chromatography and the high performance liquid chromatography revealed the reaction products of CSF and enkephalin were tyrosine, tyrosyl-glycine and tyrosyl-glycyl-glycine. Therefore, the enzymes in CSF were considered to be an aminopeptidase, a dipeptidyl aminopeptidase and a dipeptidyl carboxypeptidase. Our results suggest that in the assay of enkephalin in CSF, the effects of these enzymes should be considered.

Determination of Leu-enkephalin degradation by a soluble enzyme preparation from calf-brain striatum using reversed-phase high-performance liquid chromatography

A rapid procedure for the determination of Leu-enkephalin (Tyr-Gly-Gly-Phe-Leu) and its main metabolic degradation products (Tyr, Tyr-Gly-Gly and Tyr-Gly) by reversed-phase high-performance liquid chromatography was developed. The method has good precision, the coefficient of variation determined in the range 6-20 pmole being 1.5-3% (n = 8), and a very low detection limit of ca. 10 fmole for each metabolite. An unexpectedly high percentage of Tyr-Gly production is observed after enzymatic degradation of Leu-enkephalin by a solubilized enzyme preparation of calf-brain striatum.

Localization of the thiorphan-sensitive endopeptidase, termed enkephalinase A, on glial cells

Degradation of tritiated Leu-enkephalin was studied in cultures of primary astrocytes from rat brain. The incubation experiments with a cell suspension revealed Tyr as the main tritiated metabolite; however, Tyr-Gly-Gly and Tyr-Gly were detectable as well. Using a crude membrane preparation of the astrocytes, we found about equal amounts of Tyr and Tyr-Gly-Gly but only trace quantities of Tyr-Gly. The production of Tyr was completely inhibited by bestatin, an inhibitor of aminopeptidases, that of Tyr-Gly-Gly by thiorphan, a specific inhibitor of enkephalinase A. The results prove the ability of glial cells to degrade enkephalin by aminopeptidase and a membrane-bound enkephalinase A.

Inhibitors of calf-brain enkephalinase A and B

Enkephalinase A and B isolated from calf-brain striatum have comparable substrate specificity (Km for Leu-enkephalin = 1-3.10(-5) microM) but a quite different affinity for certain inhibitors: phosphate, Secobarbital and Thiorphan are effective inhibitors for Enkephalinase A (IC50 of 2.5 mM, 30 microM and 4 nM respectively), while Enkephalinase B does not react with any of these compounds. Both enzymes are inhibited by 1 mM EDTA and o-phenanthroline indicating the presence of a metal atom in or near their active sites. Although with different abilities, both enzymes recognize dipeptides having at least one hydrophobic amino-acid side chain. The potency of such dipeptides can be used for a description of the active site.