Enkephalin catabolism in vitro and in vivo

Characterization of carboxypeptidase A6, an extracellular matrix peptidase

Carboxypeptidase A6 (CPA6) is a member of the M14 metallocarboxypeptidase family that is highly expressed in the adult mouse olfactory bulb and broadly expressed in embryonic brain and other tissues. A disruption in the human CPA6 gene is linked to Duane syndrome, a defect in the abducens nerve/lateral rectus muscle connection. In this study the cellular distribution, processing, and substrate specificity of human CPA6 were investigated. The 50-kDa pro-CPA6 is routed through the constitutive secretory pathway, processed by furin or a furin-like enzyme into the 37-kDa active form, and secreted into the extracellular matrix. CPA6 cleaves the C-terminal residue from a range of substrates, including small synthetic substrates, larger peptides, and proteins. CPA6 has a preference for large hydrophobic C-terminal amino acids as well as histidine. Peptides with a penultimate glycine or proline are very poorly cleaved. Several neuropeptides were found to be processed by CPA6, including Met- and Leu-enkephalin, angiotensin I, and neurotensin. Whereas CPA6 converts enkephalin and neurotensin into forms known to be inactive toward their receptors, CPA6 converts inactive angiotensin I into the biologically active angiotensin II. Taken together, these data suggest a role for CPA6 in the regulation of neuropeptides in the extracellular environment within the olfactory bulb and other parts of the brain.

Gradient liquid chromatography of leucine-enkephalin peptide and its metabolites with electrochemical detection using highly boron-doped diamond electrode

Boron-doped diamond thin film (BDD) electrodes have been used to study the oxidation reactions and to detect leucine-enkephalinamide (LEA) and its metabolites, tyrosine (T), tyrosyl-alanine (TA), tyrosyl-alanine-glycine (TAG) and leucine-enkephalin (LE) using cyclic voltammetry (CV), flow-injection analysis (FIA), and gradient liquid chromatography (LC) with amperometric detection. At diamond electrodes, well-defined and highly reproducible cyclic voltammograms were obtained with signal-to-background (S/B) ratios 5-10 times higher than those observed for glassy carbon (GC) electrodes. The analytical peaks of LC for LEA and its metabolites were well resolved. No deactivation of BDD electrodes was found after several experiments with standard as well as plasma samples, indicating high stability of the electrode. Calibration curves were linear over a wide range from 0.06 to 30 microM with regression coefficients of 0.999 for all compounds. The limits of detection obtained based on a signal-to-noise ratio of 3:1 were 3, 2.2, 2.7, 20 and 11 nM for T, TA, TAG, LE and LEA, respectively. These values were at least one order lower than those obtained at GC electrodes, which has given limits of detection of 22.88, 20.64, 89.57, 116.04 and 75.67 for T, TA, TAG, LE and LEA, respectively. Application of this method to real samples was demonstrated and validated using rabbit serum samples. This work shows the promising use of conducting diamond as an amperometric detector in gradient LC, especially for the analysis of enkephalinamide and its metabolites.

Subcellular analysis of Tyr-aminopeptidase activities in the developing rat cerebellum

The endogenous opioid system seems to play important roles in the developing cerebellum. The first opioid peptide isolated, Met-enkephalin, is expressed transiently in this brain area. In the present study, several enzyme activities capable of hydrolyzing enkephalins are measured during the first month of cerebellar development, using Tyr-beta-naphthylamyde as substrate and puromycin as inhibitor of one of the membrane-bound aminopeptidases. Puromycin-sensitive soluble and membrane-bound aminopeptidase activities decrease in the synaptosomal and mitochondrial fractions at the end of the first month of life, just when enkephalin-like immunoreactivity decreases in the cerebellum. Membrane-bound enzyme also decreases in the myelinic fraction. Synaptosomal activity increases after birth, coinciding with decreases in the activity in the microsomal fraction. Puromycin-insensitive and membrane-bound aminopeptidase shows less significant developmental changes and they occur mainly in the first week of life, coinciding with the axonal and dendrite growth. These results could suggest a possible role of these enzymes, together with the rest of the opioid system, in cerebellar development.

Localization of aminopeptidase N and dipeptidyl peptidase IV in pig striatum and in neuronal and glial cell cultures

The subcellular distribution of the plasma membrane ectoenzymes, aminopeptidase N (aminopeptidase M) and dipeptidyl peptidase IV, has been examined by fractionating homogenates of porcine striata by a discontinuous Percoll gradient centrifugation procedure which distinguishes fractions containing pre- and post-synaptic elements. The two enzymes showed different distributions--dipeptidyl peptidase IV did not show a significant pre-synaptic location, whereas aminopeptidase N was present on both pre- and post-synaptic fractions. Immunofluorescent staining on mixed and neuron-enriched primary cultures of pig striatal tissue using affinity purified antibodies to the aminopeptidase and to the dipeptidyl peptidase revealed the ectoenzymes on distinct populations of cells. The astrocytic identity of the aminopeptidase N-staining cells was established by correlation with immunostaining for glial fibrillary acidic protein and for vimentin by confocal microscopy. Ultracryosections of striatum immunostained with gold-labelled immunoglobulins of differing diameters demonstrated aminopeptidase N on pericytes and confirmed its location on endothelial and astrocytic glial cells. Thus, several independent approaches indicated that aminopeptidase N, in addition to being present on endothelial and synaptic membranes, is found on astrocytes and pericytes in the perivascular neuropil, whereas dipeptidyl peptidase IV is less widely distributed on microvessels and appears not to have a synaptosomal location.

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.

Novel inhibitors of enkephalin-degrading enzymes. IV: Structure-activity relationships within the penicillins as enkephalinase inhibitors

A range of penicillins have been examined as competitive reversible inhibitors of enkephalinase (neutral endopeptidase; EC 3.4.24.11). Carfecillin (Ki = 0.18 microM) was the most potent inhibitor in the series, whereas cloxacillin (27.5 microM), ampicillin (41.0 microM), nafcillin (58.7 microM) and carbenicillin (158 microM) had moderate potency and benzyl penicillin (885 microM), mezlocillin (473 microM) and azlocillin (556 microM) were weak inhibitors. Structure-activity relationships within the series have been rationalised from a consideration of molecular graphics analysis of the match between receptor binding groups with thiorphan as well as log P values.

Enkephalinase-inhibitors modulate immune responses

Exogenous opioid peptides, and enkephalins in particular, modulate a variety of immune performances in vivo and in vitro. In this study, the immunomodulatory role of endogenous opioids was investigated by means of central and peripheral administration of four peptidase inhibitors in the rat. Animals sensitized with sheep red blood cells (SRBC) were daily treated intraperitoneally (i.p.) with 0.2 mg/kg or 1 mg/kg of each inhibitor, or intracerebroventricularly (i.c.v.) with 0.02 mg/kg and 0.2 mg/kg of bestatin, des-tyrosine-methionine-enkephalin, (Des-Tyr1)Met-Enk; and actinonin, and 0.005 and 0.5 mg/kg of N-Carboxymethyl-phenilalanine-leucine,(N-C)Phe-Leu. Controls were injected i.p. and i.c.v. with saline. The results revealed that in animals treated i.p. with 0.2 mg/kg of bestatin and (N-C)Phe-Leu potentiated the plaque-forming cell (PFC) response and hemagglutinin production. In contrast, these immune responses were suppressed by 1 mg/kg. On the other hand, i.p. doses of 0.2 and 1 mg/kg of actinonin and (Des-Tyr1)Met-Enk potentiated humoral immune responses. When given i.c.v., all of the inhibitors used exerted clear dose-dependent immunomodulatory effects, i.e. increase in the PFC response and hemagglutinin production when given at lower doses (0.005-0.02 mg/kg), and decrease when injected with higher doses (0.2-0.5 mg/kg). These effects of enkephalin-related peptidase inhibitors, applied i.p. and i.c.v., suggest the involvement of endogenous enkephalins in immune mechanisms.

Novel inhibitors of enkephalin-degrading enzymes. III: 4-Carboxymethylamino-4-oxo-3 (phenylamino) butanoic acids as enkephalinase inhibitors

4-Carboxymethylamino-4-oxo-3-(4'-aminophenylamino) butanoic acid (25), its ethyl ester (26) and the corresponding unsubstituted-aryl analogues (17) and (16) are fairly potent inhibitors of enkephalinase (neutral endopeptidase; EC 3.4.24.11), Ki = 0.14-0.39 microM, with weak inhibitory potency, Ki = 15-75 microM, towards aminopeptidase MII. In the mouse abdominal constriction test, the esters (26) and (16) showed systemic inhibitory (antinociceptive) activity with ED50 values 62 +/- 3.05 and 81 +/- 1.74 mg/kg respectively. In the mouse tail immersion test, both (26) and (16) exhibited antinociceptive activity when administered intracerebroventricularly and (26) also exhibited a systemic effect which was only partially reversed by naltrexone. The antinociceptive effect seen with (26) reflects its ranking in vitro as an inhibitor of enkephalinase (Ki = 0.14 microM) but it is possible that this effect is not totally opioid-mediated. Compounds (26) and (16) represent the first combined inhibitors of enkephalinase and aminopeptidase MII.

Oral absorption of peptides: influence of pH and inhibitors on the intestinal hydrolysis of leu-enkephalin and analogues

Leu-enkephalin (YGGFL) and several analogues were chosen as model peptides for the study of peptide absorption and hydrolysis in the rat jejunum. An HPLC assay was adapted to detect YGGFL or the analogues and metabolites. Peptide hydrolysis was studied in the rat jejunum using a single-pass perfusion method. Extensive hydrolysis of YGGFL was observed in the rat jejunum and approaches to reduce its metabolism were studied. The brush border enzymes are a major site of enkephalin hydrolysis. Lumenal peptidases were secondary to the brush border enzymes in hydrolyzing the enkephalins in this system. In the in situ perfusion system, YGGFL is hydrolyzed primarily to Tyr and GGFL by the brush border aminopeptidase and to YGG and FL by brush border endopeptidase. Lowering the jejunal pH below 5.0 significantly reduces aminopeptidase activity and, to a lesser extent, endopeptidase activity. An aminopeptidase inhibitor, amastatin, produced more pronounced inhibitory effects at higher pH and the endopeptidase inhibitors, tripeptides YGG and GGF, are effective even below pH 5.0. Coperfusion of YGGFL with a combination of aminopeptidase and endopeptidase inhibitors, e.g., amastatin and YGG, is more effective in inhibiting hydrolysis since both metabolic pathways are inhibited. Leu-D(Ala)2-enkephalin, while showing enhanced stability against aminopeptidase hydrolysis, is hydrolyzed at the Gly-Phe bond by the endopeptidase. Its hydrolysis is not affected by pH changes or amastatin but is decreased by YGG. The YGGFL wall permeability was estimated and is not a limiting factor for oral absorption.

D-Ala-2-Me-Phe-4-Met-(O)-ol-enkephalin in the nucleus tractus solitarius of the rat produces cardiorespiratory depression

1. The synthetic Met-enkephalin, D-Ala-2-Me-Phe-4-Met-(O)-ol-enkephalin (FK 33-824). 1 or 2 micrograms, after its injection into the nucleus tractus solitarius (NTS) of Wistar rats, anesthetized with pentobarbital and breathing spontaneously, produced a transient increase in blood pressure followed by sustained and significant (P less than 0.05) hypotension and bradycardia. This occurred in a dose dependent manner. 2. FK 33-824 in the NTS, 1 or 2 micrograms, also produced a marked respiratory depression. 3. In anesthetized rats, in which hypoventilation was prevented by mechanical ventilation, there was a definite reduction in blood pressure and heart rate that was considerably and significantly (P less than 0.05) less than that observed in spontaneously breathing rats. 4. Blood pressure fluctuations occurred after NTS injection that were more marked in spontaneously breathing animals but still occurred in animals that were ventilated mechanically. 5. FK 33-824, 1 and 2 micrograms in the NTS was fatal within 100 min for all animals but was prevented by mechanical ventilation. Higher doses of FK 33-824, 10 micrograms in the NTS, however, induced fatal ventricular arrhythmias even in the mechanically ventilated rat. 6. Thus, FK 33-824 in the NTS decreases blood pressure and heat rate in spontaneously breathing as well as mechanically ventilated rats, but much of the effect on blood pressure and heart rate is due to the profound respiratory depression in the spontaneously breathing rat.

Studies on the subsite specificity of the rat brain puromycin-sensitive aminopeptidase

The specificity of the puromycin-sensitive aminopeptidase from rat brain was examined. Using L-alanyl-beta-naphthylamide as substrate Vmax of the reaction was shown to be pH independent over the range of 5.5-9.0, while Km exhibited a pKa of 7.7. This latter value corresponds to the pKa of the amino group of the substrate. Using X-Ala and X-Leu to examine the specificity of the P1 site it was found that Arg and Lys exhibit the highest affinity, followed by Met, Val, Leu, Trp, and Phe, which bind congruent to 5- to 20-fold less well. Although Km varied more than 20-fold within this series, Vmax showed considerably less variation. Significantly weaker binding was observed with a P1 Gly, Ala, Ser, or Pro with no binding detectable with a P1 Glu. The presence of a P'1 Leu compared to P'1 Ala results in an approximate 10-fold decrease in Km with little change in Vmax. The effect of varying P'1 residues was examined with the series Leu-X. In this case basic and hydrophobic amino acids, with the exception of Val, all exhibit nearly the same Km. The binding of Arg-Arg and Lys-Lys showed the same Km as obtained for Arg-Leu or Lys-Leu, respectively. When Leu-Ser-Phe was compared to Leu-Ser the P'2 residue led to a 100-fold decrease in Km and slightly less than a 5-fold increase in Vmax. In contrast the addition of a P'2 Met to Leu-Trp results in only a 3-fold decrease in Km and a 3-fold increase in Vmax. The results indicate a preference for a basic or hydrophobic residue in the P1 and P'1 sites and indicate subsite-subsite interactions which primarily affect binding.

N-terminal tetrapeptide of dermorphin and D-Arg-substituted tetrapeptides: inactivation process of the antinociceptive activity by peptidase

Degradation products of the N-terminal tetrapeptide of dermorphin, H-Tyr-D-Ala-Phe-Gly-OH (ALPG) and D-Arg2-substituted tetrapeptide analogs of dermorphin, H-Tyr-D-Arg-Phe-Gly-OH (ARPG), H-Tyr-D-Arg-Phe-Gly-NH2 (TDAPG-NH2) and H-Tyr-D-Arg-Phe-beta-Ala-OH (TDAPA) by enkephalin degrading enzymes were studied by using reversed-phase high-performance liquid chromatography. After 5 and 25 hr incubations of the peptides with solubilized enzymes of mouse brain or spinal cord, liberation of the appreciable Tyr1 residue was observed in ALPG but not in ARPG, TDAPG-NH2 and TDAPA. When ARPG and TDAPG-NH2 were incubated with enzymes for 25 hr, a main degradation product was the N-terminal tripeptide produced from the hydrolysis of Phe3-Gly4 bond. Conversely, TDAPA did not produce the N-terminal tripeptide after 25 hr incubation with enzymes. In the enzyme assay, Tyr1-D-Arg2 bond of ARPG, TDAPG-NH2 and TDAPA was more stable than that of ALPG to the cleavage by aminopeptidase M (AP-M). Phe3-Gly4 bond of ALPG, ARPG and TDAPG-NH2 were easily hydrolyzed by carboxypeptidase Y (CP-Y) within 3 hr incubation, whereas the hydrolysis of Phe3-beta-Ala4 bond of TDAPA by CP-Y was not observed after 3 hr incubation. The present results and previous behavioural data suggest that a potent and prolonged antinociceptive activity of the D-Arg-substituted tetrapeptides is mainly attributed to the stability of Tyr1-D-Arg2 bond against aminopeptidase of peptidases.

Analgesic effects of D-amino acids in four inbred strains of mice

1. Prominent strain differences of mice were found in analgesic effects of D-amino acids. 2. In C57BL/6CrSlc and C3H/HeSlc mice, pain threshold, which was determined by using a hot-plate method, increased to 140-175% of the control after the systemic treatment of all three D-amino acids employed, such as D-phenylalanine, -leucine and -methionine, whereas in DBA/2CrSlc or BALB/cCrSlc mice, out of three only one D-amino acid, D-phenylalanine or -leucine, produced significant increase of pain threshold. 3. This lack of ability to perceive analgesic effects of specific amino acids observed in the latter two strains suggests that there probably exist different analgesia-inducing mechanisms for each of three D-amino acids in mice and the latter two strains lack two of them.

Measurement of methionine enkephalin and leucine enkephalin in rat brain regions by high-performance liquid chromatography with coulometric electrochemical detection

A method is described for the determination of two pentapeptides, methionine enkephalin (H-Tyr-Gly-Gly-Phe-Met-OH) (ME) and leucine enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH) (LE) in discrete rat brain regions. Separation and quantitation were performed by reversed-phase high-performance liquid chromatography with coulometric detection. Perchloric acid extracts of the tissue after enzyme inactivation by heat treatment were passed through a normal-phase solid-phase extraction diol (COHCOH) column, and endogenous ME and LE were subsequently eluted with methanol. The mobile phase was 1-propanol-phosphate buffer (pH 5.5) (9:91). Eluted samples were detected electrochemically using dual coulometric electrodes operated in screen mode. Each of these enkephalins gave a linear response over the range 40-160 ng/ml cerebellar homogenate (0.8-3.2 ng absolute amount on column). Analytical recoveries of synthetic ME and LE, added to the homogenates, were 70 +/- 3 and 70 +/- 10%, respectively, when compared with enkephalins dissolved in water. The mean between-assay coefficients of variation for synthetic ME and LE were lower than 10.7 and 7.4%, respectively, over the concentration range studied. The within-assay coefficients of variation for synthetic ME and LE were 11.4 and 9.5%, respectively, at the lowest concentration. The present method has been applied to a study determining the levels of endogenous ME and LE in discrete rat brain regions.

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.

Simultaneous analysis of methionine- and leucine-enkephalin from rat brain: quantification by liquid chromatography-electrochemistry

This study focuses on the application of liquid chromatography with electrochemical detection (LC-ED) for the analysis of methionine-enkephalin (ME) and leucine-enkephalin (LE) extracted from rat brain regions. The high applied potentials necessary for enkephalin detection required the development of an efficient sample processing protocol. Brain extracts were processed using chromatographic mode sequencing (CMS). The decrease in electroactive interfering substances by CMS improved the chromatographic resolution of ME and LE and the electrode performance. Other qualitative and analytical methods were used to evaluate the enkephalin data obtained by LC-ED for rat brain regions. This study demonstrates that LC-ED provides both the sensitivity and specificity necessary for the analysis of enkephalins from rat brain regions.

[D-Ala2, (F5) Phe4]-dynorphin 1-13-NH2 (DAFPHEDYN): a potent analog of dynorphin 1-13

Intracerebroventricular administration of the dynorphin analog, [D-Ala2,(F5)Phe4]-dynorphin 1-13-NH2 (DAFPHEDYN) in rats produced diuresis and profound analgesia. Both effects were antagonized by central administration of naltrexone or naloxone. Intravenous administration of 10, 25, and 50 mg/kg of DAFPHEDYN failed to induce diuresis. The increased potency of DAFPHEDYN was apparent from the failure of an equal dose of the parent compound (dynorphin 1-13) to produce diuresis and the failure of [D-Ala2]-dynorphin 1-13-NH2 to produce analgesia. Radioligand binding studies indicated the DAFPHEDYN retains the same degree of kappa selectivity as the parent compound (dynorphin 1-13) though a drop in affinity occurred. DAFPHEDYN may be of significant interest because it retains the essential pharmacology of the parent compound and exhibits marked in vivo potency.

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.

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.

The degradation of dynorphin A in brain tissue in vivo and in vitro

The demonstration of analgesia following in vivo administration of dynorphin A (Dyn A) has been difficult. In contrast, a number of electrophysiological and behavioral effects reported with in vivo injection of Dyn A can be produced by des-tyrosine dynorphin A (Dyn A 2-17). This suggested the extremely rapid amino terminal degradation of dynorphin A. To test this hypothesis, we examined the degradation of dynorphin A following in vivo injection into the periaqueductal gray (PAG) as well as in vitro using rat brain membranes under receptor binding conditions. In vivo, we observed the rapid amino terminal cleavage of tyrosine to yield the relatively more stable destyrosine dynorphin A. This same cleavage after tyrosine was observed in vitro. Inhibition of this aminopeptidase activity in vitro was observed by the addition of dynorphin A 2-17 or dynorphin A 7-17 but not after the addition of dynorphin A 1-13, dynorphin A 1-8, dynorphin B or alpha-neo-endorphin suggesting a specific enzyme may be responsible. The detection of the behaviorally active des-tyrosine dynorphin A following in vivo injection of dynorphin A suggests that this peptide may play an important physiological role.

Calcium-dependent release of tyrosine in brain elicited by stimulation of neuropeptide receptors

We sought to establish whether the endogenous opiate-receptor agonist Met-enkephalin (m-ENK) selectively modulates the release of endogenous tyrosine (Tyr) from brain slices prepared from the corpus striatum (CS). Amino acids (AAs) released from slices of CS and, for comparison, cerebral cortex (Cx) were measured by HPLC. Incubation of slices with m-ENK (1-10 microM) increased the basal release of Tyr (up to 293% of control) from CS, but not Cx, whereas other nonneurotransmitter AAs, phenylalanine (Phe) and valine (Val), were unchanged. The release of the putative neurotransmitter AAs glutamate (Glu), taurine (Tau), and glycine (Gly) were similarly increased by 50-150% with m-ENK in slices of CS, but not Cx. The enhanced release of AAs by m-ENK was prevented by removal of extracellular Ca2+ or by preincubation with the opiate receptor antagonist naloxone. Neuronal depolarization by potassium (5-55 mM) in the presence of Ca2+ did not affect the release of Tyr, whereas release of neurotransmitter AAs such as gamma-aminobutyric acid (GABA) were markedly increased. The increase in basal Tyr release by m-ENK was not the result of a decreased uptake of Tyr. Relative to slices, the basal release of Tyr, Phe, and Val from a synaptosomal (P2) preparation of CS was small (8-51%) compared to that of GABA, Gly, Glu, and Tau (49-123%). Nonetheless, m-ENK (10 microM) markedly increased the release of Tyr (to 833%), but not Glu, Gly, and Tau from the P2 fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Autoradiographic localization of mu- and delta-opiate receptors in the forebrain of the rat

The autoradiographic distributions of mu opiate receptors, labeled in vitro by [125I]D-Ala2-MePhe4-Met(o)5-ol-enkephalin (FK), and delta-opiate receptors, labeled by [3H]D-Ala2-D-Leu5-enkephalin (DADLE) in the presence of oxymorphone to block high affinity binding to the mu site, were examined and compared in the forebrain of the rat. The mu- and delta-receptors were differentially distributed in most structures. mu Binding sites were found in nearly all gray matter structures and showed heterogeneous patterns of density that were correlated with cytoarchitecture and neuronal connections. Laminar density profiles were seen in laminated structures such as olfactory bulb, cerebral cortex and hippocampus. Highest mu binding densities were in striatal patches and the habenular streak. delta Sites had distinct laminar patterns in the main olfactory bulb and cortex which differed from the mu patterns. The external plexiform layer of the main olfactory bulb had the greatest density of delta binding sites; cortex and striatum were also densely labeled. The septum, globus pallidus, preoptic area and hypothalamus were lightly labeled by both ligands. The magnocellular hypothalamic nuclei had negligible mu and delta labeling. The thalamus had dense mu but sparse delta sites. mu And delta binding sites were both present in the amygdala but had different distributions. Two fiber tracts--optic tract and fasciculus retroflexus--had FK labeling. In contrast, a portion of the corpus callosum was labeled by DADLE and not by FK. The results suggest an association of mu-opiate receptors with sensory, especially olfactory, and limbic projections in the forebrain, and delta-opiate receptors with intrinsic and commissural forebrain pathways.

Stress-induced changes in the analgesic and thermic effects of opioid peptides in the rat

Stress (e.g. restraint) potentiates analgesia and alters changes in body temperature induced by morphine administered either systemically or intracerebroventricularly (i.c.v.) in rats. In order to extend the generality of this phenomenon to opioid peptides, we determined whether the analgesic and thermic effects of i.c.v. D-Ala2-D-Leu5-enkephalin (DADLE) or D-Ala2-N-MePhe4-Gly5(ol)-enkephalin (DAGO), agonists selective for delta- and mu-opioid receptors, respectively, were affected by restraint stress. Analgesia was measured in the tail-flick test and core body temperature by rectal probe. The unstressed rats exhibited a dose-dependent increase in tail-flick latencies after administration of either DAGO or DADLE. Restrained rats treated with DAGO or DADLE had a greater analgesic response to each dose of peptide than did unstressed rats; both the magnitude and duration of the drug effect were increased. The unstressed group of rats responded to all doses of DAGO and DADLE with an increase of core temperature. In contrast, restrained rats showed a decrease of core temperature following injection with either DAGO or DADLE. Thus, restraint stress can significantly modify the effects of DAGO and DADLE on analgesia and body temperature in a manner that is qualitatively and quantitatively similar to that observed previously for morphine administered by the i.c.v. route.

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.

High-performance liquid chromatography of tyrosine-containing peptides by pre-column derivatization involving formylation followed by fluorescence reaction with 1,2-diamino-4,5-dimethoxybenzene

A pre-column fluorescence derivatization method is described for the high-performance liquid chromatographic determination of tyrosine-containing peptides. A tyrosyl residue in the peptide is first formylated in an alkaline medium in the presence of chloroform, and the resulting aldehyde is then converted into a fluorescent derivative by reaction with 1,2-diamino-4,5-dimethoxybenzene. The derivative is separated on a reversed-phase column (LiChrosorb RP-18) by isocratic elution with an aqueous acetonitrile-containing potassium chloride-hydrochloric acid buffer (pH 2.2) and sodium 1-hexanesulphonate. The method is selective and fairly sensitive; the lower limits of detection for the tyrosine-containing peptides tested are in the range 3.4-26.2 pmol in a 100-microliter injection volume.

Separation and detection of closely related endorphins by liquid chromatography-electrochemistry

The specific aim of this study was to investigate the selectivity of liquid chromatography--electrochemistry for minor variations in the structure of small endorphins. Using isocratic mobile phases, chromatographic conditions were established for the separation of a series of closely related endorphins. Hydrodynamic voltammetry showed that each peptide exhibited a characteristic oxidative behavior that was also reflected in peak current ratios. Changes in a small moiety altered both the chromatographic behavior and electroactivity of these neuropeptides.

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.

Enkephalin degradation by human erythrocytes and hemolysates studied using 1H NMR spectroscopy

High resolution (400 MHz) 1H spin-echo NMR spectroscopy was used to monitor the degradation of leucine-enkephalin, and peptide fragments of it, by human erythrocytes and hemolysates. We showed that leucine-enkephalin is rapidly degraded by the cytosolic peptidases of the human erythrocyte, and we have elucidated the most probable pathway of degradation. Computer simulations of the proposed pathway, using a model incorporating the experimentally derived steady-state kinetic parameters obtained for the individual enzyme steps, showed close agreement with the experimental results. From a methodological perspective, the work demonstrates the value of 1H spin-echo NMR spectroscopy for rapidly elucidating, both qualitatively and quantitatively, an entire peptide-degradation pathway as it operates in situ.

Antinociceptive effect of some carboxypeptidase A inhibitors in comparison with D-phenylalanine

It had previously been shown that D-phenylalanine and hydrocinnamic acid, two in vitro inhibitors of carboxypeptidase A, possess an analgesic action when injected i.p. in mice. We have studied the in vivo effects of indole-3-acetic acid, another carboxypeptidase A inhibitor, and of the following analogs of D-phenylalanine substituted in position 4: D-tyrosine, p-fluoro-D-phenylalanine and trifluoroacetyl-p-fluoro-D-phenylalanine. Whereas indole-3-acetic acid caused a higher and shorter analgesia in comparison with D-phenylalanine, p-fluoro-D-phenylalanine and its N-trifluoroacetyl derivative yielded both a greater and a much longer lasting analgesic effect. Since the latter compound showed only slight inhibitory activity on carboxypeptidase A in vitro, we suggest that inhibition of this enzyme and analgesia might not be directly correlated.

The metabolism of neuropeptides. Phase separation of synaptic membrane preparations with Triton X-114 reveals the presence of aminopeptidase N

The property of solutions of Triton X-114 to separate into detergent-rich and detergent-poor phases at 30 degrees C has been exploited to investigate the identities of the aminopeptidases in synaptic membrane preparations from pig striatum. When titrated with an antiserum to aminopeptidase N (EC 3.4.11.2), synaptic membranes solubilized with Triton X-100 revealed that this enzyme apparently comprises no more than 5% of the activity releasing tyrosine from [Leu]enkephalin. When assayed in the presence of puromycin, this proportion increased to 20%. Three integral membrane proteins were fractionated by phase separation in Triton X-114. Aminopeptidase activity, endopeptidase-24.11 and peptidyl dipeptidase A partitioned predominantly into the detergent-rich phase when kidney microvillar membranes were so treated. However, only 5.5% of synaptic membrane aminopeptidase activity partitioned into this phase, although the other peptidases behaved predictably. About half of the aminopeptidase activity in the detergent-rich phase could now be titrated with the antiserum, showing that aminopeptidase N is an integral membrane protein of this preparation. Three aminopeptidase inhibitors were investigated for their ability to discriminate between the different activities revealed by these experiments. Although amastatin was the most potent (IC50 = 5 X 10(-7) M) it failed to discriminate between pure kidney aminopeptidase N, the total activity of solubilized synaptic membranes and that in the Triton X-114-rich phase. Bestatin was slightly more potent for total activity (IC50 = 6.3 X 10(-6) M) than for the other two forms (IC50 = 1.6 X 10(-5) M). Puromycin was a weak inhibitor, but was more selective. The activity of solubilized membranes was more sensitive (IC50 = 1.6 X 10(-5) M) than that of the pure enzyme or the Triton X-114-rich phase (IC50 = 4 X 10(-4) M). We suggest that the puromycin-sensitive aminopeptidase activity that predominates in crude synaptic membrane preparations may be a cytosolic contaminant or peripheral membrane protein rather than an integral membrane component. Aminopeptidase N may contribute to the extracellular metabolism of enkephalin and other susceptible neuropeptides in the brain.

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.

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.

A study of intracerebroventricular self-administration of leucine or methionine enkephalin by rats in response to intermittent electric shocks

Rats were taught to self-administer Leu-E (10, 25 and 100 micrograms/microliters) or Met-E (0.5, 10 and 100 micrograms/microliters) through a cannula implanted in the lateral cerebral ventricle (i.c.v.). Their self-injection behaviour was studied before, during and after nociceptive stimulation. In the course of the control period of the experiment, the rats rapidly learned lever pressing for self-injection of enkephalin but they did not increase their self-administration of Leu-E or Met-E during the nociceptive electrical stimulation period. Also studied were the acute effect of i.c.v. enkephalin and morphine on tail-flick latency (sec) and electrical threshold vocalization (mA). The analgesic effect of Leu-E and Met-E was of short duration (less than 2-6 min). The mean rise (i.e., analgesia) of the tail-flick threshold showed a significant difference after i.c.v. Met-E only. The acute i.c.v. effect of 20 or 30 micrograms of morphine induced a long-lasting analgesia, greater than 40 min. These results show that Leu-E and Met-E are not rewarding during a nociceptive stimulus. This may be due to the short and inconstant analgesic action of i.c.v. enkephalins.

Two pools of beta-endorphin-like immunoreactivity in blood: plasma and erythrocytes

While attempting to delineate the reason for the reported extreme variability of beta-endorphin-like immunoreactivity (beta-ir) in human plasma (eg., nondetectable to 1 ng/ml) by standard radioimmunoassay, we noted that a substantial portion of circulating beta-ir was associated with erythrocytes. That erythrocyte associated beta-ir is authentic beta-endorphin (beta-EP) was confirmed by high performance liquid chromatography (HPLC). Analysis of blood samples from rabbits, rats and mice revealed the presence of beta-ir in erythrocytes from these species as well. These results suggest that there are two pools of beta-endorphin-like immunoreactivity in blood: plasma and erythrocytes.

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.

Analgesic properties of N-terminal substituted phenylalanyl-alanine

Phenylalanylalanine, an in vitro inhibitor of enkephalinase, and some of its N alpha-derivatives are shown to possess an analgesic action when injected i.p. and i.c.v. into mice in the presence or absence of Leu5-enkephalin. In the second case a synergistic response is observed. The intensity of the analgesic response depends markedly on the nature of the N-terminal substituent which affects the hydrophobic character of the resulting dipeptide, its subsequent transport and probably its rate of biotransformation by cleaving enzymes.

Analysis of enkephalins, beta-endorphins and small peptides in their sequences by highly sensitive high-performance liquid chromatography with electrochemical detection: implications in opioid peptide metabolism

Sensitivity in the 10-100 pg range for enkephalins, beta-endorphin, tyrosine (T), 12 tyrosylglycine (T-G) and tyrosylglycylglycine (T-G-G) was attained by using a high-performance liquid chromatographic (HPLC) method with electrochemical detection which is at least 100 times more sensitive than HPLC with UV detection. The chromatographic conditions on a reversed-phase C18 silica column were 50 mM sodium phosphate buffer (pH 2.1) (A) in acetonitrile-methanol (1:1) (B), isocratic mixture, flow-rate 0.6-1 ml/min, UV detection at 205 nm, electrochemical oxidation potential + 1.25 V. The separation of T, T-G and T-G-G was obtained by using 10% B while the separation of the pentapeptide, enkephalins required 40% B. Separation of enkephalins from beta-endorphin was attained at a shorter retention times did not exceed 15 min. This method can be used to determine tissue levels and pharmacodynamics of enkephalins and beta-endorphin. A highly specific measurement of the different enzymes involved in the metabolism of enkephalin has been achieved.

Protection of enkephalins from enzymatic degradation utilizing selective metal-chelating inhibitors

Metal ion-chelating agents inhibited enkephalin degradation by a rat striatal membrane-associated endopeptidase termed 'enkephalinase'. The combination of a hydrophobic dipeptidyl moiety and a transition metal-chelating moiety in the same molecule resulted in very efficient and selective inhibitors of enkephalinase. The mercaptoacetyl dipeptides (2-mercaptoacetyl-Leu-Phe and 2-mercaptoacetyl-Phe-Leu) and the N-phosphorylated dipeptides (phosphoryl-Leu-Phe and phosphoramidon) inhibited enkephalinase with IC50 values of 15, 70, 0.3 and 1 nM respectively, but were much less potent against the aminopeptidase and angiotensin converting enzyme, two other metalloenzymes implicated in the degradation of the enkephalins in brain. The inhibition of enkephalinase, using phosphoryl-Leu-Phe as a selective inhibitor, resulted in a 4 fold increase in the amount of enkephalin recovered following K+ depolarization of rat striatal slices.

Minireview. Peptides and the blood-brain barrier

Most neuropeptides are known to occur both in the central nervous system and in blood. This, as well as the occurrence of central nervous peptide effects after peripheral administration, show the importance of studying the relationships between the peptides in the two compartments. For many peptides, such as the enkephalins, TRH, somatostatin and MIF-1, poor penetration of the blood-brain barrier was shown. In other cases, including beta-endorphin and angiotensin, peptides are rapidly degraded during or just after their entry into brain or cerebrospinal fluid. Some peptides, such as insulin, delta-sleep-inducing peptide, and the lipotropin-derived peptides, enter the cerebrospinal fluid to a slight or moderate extent in the intact form. Many peptide hormones, such as insulin, calcitonin and angiotensin, act directly on receptors in the circumventricular organs, where the blood-brain barrier is absent. Oxytocin, vasopressin, MSH, and an MSH-analog alter the properties of the blood-brain barrier, which may result in altered nutritient supply to the brain. In conclusion, the diffusion of most peptides across the brain vascular endothelium seems to be severely restricted. There are, however, several alternative routes for peripheral peptides to act on the central nervous system. The blood-brain barrier is a major obstacle for the development of pharmaceutically useful peptides, as in the case of synthetic enkephalin-analogs.