Co-identity of brain angiotensin converting enzyme with a membrane bound dipeptidyl carboxypeptidase inactivating Met - enkephalin

Inhibition of dynorphin-converting enzymes prolongs the antinociceptive effect of intrathecally administered dynorphin in the mouse formalin test

The effects of peptidase inhibitors on the antinociceptive induced by intrathecally (i.t.) administered by dynorphin A and dynorphin B in the mouse formalin test were examined. When administered i.t. 5 min before the injection of 0.5% formalin solution into the dorsal surface of a hindpaw, dynorphin A (0.5-2 nmol) and dynorphin B (2-8 nmol) produced a dose-dependent and significant reduction of the paw-licking response. Dynorphin A (2 nmol) and dynorphin B (8 nmol)-induced antinociception disappeared completely within 90 min and 60 min, respectively. p-Hydroxymercuribenzoate, a cysteine proteinase inhibitor, and phosphoramidon, and endopeptidase 24.11 inhibitor simultaneously administered with dynorphin A or dynorphin B. Significantly prolonged antinociception induced by both dynorphins. However, captopril, and angiotensin-converting enzyme inhibitor, bestatin (a general aminopeptidase inhibitor) and a serine proteinase inhibitor phenylmethanesulfonyl fluoride, were active. Dynorphin converting enzyme(s) transform dynorphin-related peptides to [Leu5]enkephalin and [Leu5]enkephalin-Arg6. Neither [Leu5]enkephalin nor [Leu5]enkephalin-Arg6, even at high dose (10 nmol), produced any antinociceptive effect. However, [Leu5[enkephalin-Arg6, but not [Leu5]enkephalin, produced a significant antinociceptive effect when co-administered with phosphoramidon. Therefore, the prolongation of the antinociception induced by both dynorphins in the presence of phosphoramidon, may be due to inhibition of [Leu5]enkephalin-Arg6 degradation. The present results indicate that dynorphin-converting enzyme(s) may be important enzyme(s) responsible for terminating dynorphin-A- and dynorphin-B-induced antinociception at the spinal cord level in mice.

Localization of angiotensin converting enzyme by in vitro autoradiography in the rabbit brain

The distribution of angiotensin converting enzyme was examined in the rabbit brain by in vitro autoradiography with the specific radiolabelled inhibitor 125I-351A. In the rabbit, the highest concentrations of radioligand binding were found in the choroid plexus, blood vessels, subfornical organ, vascular organ of the lamina terminalis, area postrema and inferior olive. High levels of binding were found throughout the basal ganglia, consistent with the results in all other species studied. In the midbrain the central gray and the superior colliculus displayed high levels of binding. In the medulla oblongata high levels of binding were associated with the nucleus of the solitary tract and dorsal motor nucleus of vagus, consistent with the pattern in other species. There was moderate labelling throughout both the cerebral and cerebellar cortices, which contrasts to the rat but is consistent with the situation in primates. Angiotensin converting enzyme (ACE) is more widely distributed in rabbit brain that in rat, human and Macaca fascicularis, and the results suggest ACE has a very general role in the metabolism of neuropeptides. Inhibitors of converting enzyme are very widely used in the treatment of hypertension and heart disease, and the rabbit should provide a useful model for examining the effects of these drugs in the brain.

Characterisation of [3H]ceranapril recognition sites in rat and human brain tissue

The present studies assessed the nature of the recognition site for [3H]ceranapril in tissue from rat and human brain. [3H]Ceranapril exhibited high affinity saturable specific (defined by 1 microM captopril) binding to homogenates of tissue from both rat and human brain (mean pKd values between 8.42 and 8.69). High binding densities were observed in rat striatum and homogenates of tissue from human caudate (Bmax values 3317 +/- 192 and 1900 +/- 110 fmol/mg protein respectively), with comparatively low densities in cortical tissues. In kinetic experiments, association of [3H]ceranapril to homogenates of rat and human cortex was found to be rapid and fully reversible (K+1 = 6 x 10(5) M-1 sec-1 and 2.4 x 10(6) M-1 sec-1, K-1 = 7.6 x 10(-3) sec-1 and 4.5 x 10(-3) sec-1 respectively). In competition studies, lisinopril, captopril, unlabelled ceranapril, epicaptopril and fosinopril, all competed to a similar extent and with similar rank order of potency for the binding of [3H]ceranapril to homogenates of both rat and human brain. In in vivo studies, pretreatment of rats with either captopril or lisinopril (15 micrograms/250 g) significantly reduced the content of tritium in brain, as measured 20 min after intravenous administration of [3H]ceranapril. From these experiments [3H]ceranpril appears to selectively label, with high affinity, the inhibitor binding site of angiotensin converting enzyme and this site appears to be similar in both species studied.

Angiotensin I converting enzyme and the changes in our concepts through the years. Lewis K. Dahl memorial lecture

Changes in our concepts of angiotensin I converting enzyme are reviewed briefly. The actions of this enzyme go beyond liberating angiotensin II from angiotensin I or inactivating bradykinin. Its very wide distribution in the body and its activity in vitro indicate involvement in the metabolism of other biologically active peptides. The recent molecular cloning of the human enzyme confirmed the existence of a hydrophobic C-terminal peptide that forms the short transmembrane domain of this plasma membrane-bound enzyme. The much longer external portion contains two homologous active site domains but probably only one functional active center. Finally, in spite of the great progress made in studying angiotensin converting enzyme, there are many challenging problems waiting to be solved.

Nootropic effects of ACE inhibitors in mice

The angiotensin converting enzyme (ACE) inhibitors captopril and enalapril and the nootropic piracetam reduced the amnesiogenic effects of cerebral electroshock treatment in mice. These compounds also directly improved passive-avoidance learning if administered before the learning trial. When given immediately after the learning trial, captopril and piracetam were active, but not enalapril. Captopril, but neither enalapril nor piracetam, facilitated memory retrieval after a 2-month retention interval. Unlike those of piracetam, the memory-improving effects of captopril and enalapril are not established by aldosterone-receptor blockade, suggesting that the two types of drug act via different mechanisms of action.

Effects of an angiotensin converting enzyme (ACE) inhibitor on plasma endorphin level

1. In addition to their antihypertensive effect, ACE inhibitors have been reported to increase general well-being, general health and vitality and work performance. The cause of these effects is not known. A possible mechanism may be release of beta-endorphins. 2. In the present study changes in plasma concentration of beta-endorphins on days with ACE inhibitor treatment (n = 12) and on non-treatment control days (n = 12) were compared in 6 patients. 3. Both on control and treatment days the beta-endorphin level fell, by 7.1 and 10.0%, respectively, from 8.00 a.m. to 8.00 p.m., reflecting the known diurnal rhythm of this opioid. This difference between the control and treatment days is not statistically significant. 4. The study should be extended to determine endorphin concentration in the cerebrospinal fluid, and other opioids should be looked for.

Isolation of human liver angiotensin-converting enzyme by chromatofocusing

Angiotensin-converting enzyme (EC 3.4.15.1) has been isolated from human liver by chromatofocusing. The isolation procedure permitted us to obtain a 9000-fold purified enzyme with a 22% yield. Specific activity of the angiotensin-converting enzyme was 10 units/mg of protein. The molecular mass of enzyme determined by polyacrylamide gel electrophoresis under denaturing conditions was 150,000. The isoelectric point (4.2-4.3) was also determined by chromatofocusing. The Km values of the enzyme for hippuryl-L-histidyl-L-leucine and N-benzyloxycarbonyl-L-phenylalanyl-L-histidyl-L-leucine are 5000 and 125 microM, respectively. The human liver angiotensin-converting enzyme is inhibited by bradykinin-potentiating factor SQ 20881 (IC50 = 18 nM).

Affinity chromatography and some properties of the angiotensin-converting enzyme from human heart

Human heart angiotensin-converting enzyme has been purified by affinity chromatography on immobilized N-[1(S)-carboxy-5-aminopentyl]-Gly-Gly. The isolation procedure permitted a 1650-fold-purified enzyme to be obtained. The specific activity of angiotensin-converting enzyme was 38 units per mg protein. The molecular weight of enzyme determined by polyacrylamide gel electrophoresis in denaturing conditions was 150,000. The isoelectric point (5.3) of the enzyme was determined by chromatofocusing. The Km values of the enzyme for Bz-Gly-His-Leu and angiotensin I are 1.2 mM and 10 microM, respectively. Substrate inhibition of heart angiotensin-converting enzyme with a K's of 14 mM has been shown. The human heart enzyme is inhibited by SQ 20881 (IC50 = 40 nM). It was shown that NaCl, CaCl2 as well as Na2SO4 in the absence of Cl- are activators of the heart angiotensin-converting enzyme, whereas CH3COONa and NaNO3 have no effect on a catalytic activity of the enzyme.

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.

Increase in vasoactive intestinal polypeptides (VIP) by the angiotensin converting enzyme (ACE) inhibitor lisinopril in congestive heart failure. Relation to haemodynamic and hormonal changes

1. The effects of the angiotensin-converting enzyme (ACE) inhibitor lisinopril on plasma vasoactive intestinal polypeptides (VIP) and plasma noradrenaline, adrenaline and dopamine were studied in 12 patients with congestive heart failure over two consecutive 48-hr periods. The first day in each period served as a treatment day and the second as a control day. 2. A parallel monitoring was made of various hormonal parameters related to the renin-angiotensin-aldosterone system, and a right-heart catheter was used to monitor haemodynamics at rest. 3. Potent inhibition of the renin-system (as demonstrated by decreases in angiotensin converting enzyme (ACE) activity, angiotensin II and plasma aldosterone) together with improved haemodynamics (decreases in mean right atrial pressure, mean pulmonary arterial pressure, mean pulmonary capillary wedge pressure and mean systemic arterial pressure) were recorded. 4. Plasma VIP was significantly increased by a mean of 20.3% (P less than 0.01) on the lisinopril treatment days compared with the control days, whereas circulating catecholamines showed no significant pattern of change. 5. It is postulated that the potent vasodilatory neuromodulator VIP is implicated in the ACE inhibitor effects. 6. The ACE is a non-specific peptidase that previously has been implicated in the potentiation of other vasoactive endogenous systems (kinins and enkephalins).

Sodium deprivation increases the antinociceptive activity of morphine

In rats maintained for 50 days on a low-sodium diet and with a compensatory hyperactivity of the renin-angiotensin system, the antinociceptive activity of morphine was significantly longer-lasting than in controls. It is suggested that the renin-angiotensin system modulates opioid system responsivity.

Intra-cerebroventricular captopril reduces plasma ACTH and vasopressin responses to hemorrhagic stress

The role of the brain renin-angiotensin system in mediating the peripheral hormone response to acute hemorrhagic stress (15 ml/kg over 10 min) was studied in 6 sheep during an intracerebroventricular infusion (2.8 micrograms/min) of the angiotensin-converting enzyme inhibitor, captopril. When compared with control experiments the plasma ACTH and vasopressin (AVP) response to hemorrhage was markedly reduced and delayed during icv captopril, which did not affect the response of plasma angiotensin II (AII). These results suggest that the normal and rapid response in ACTH and AVP secretion accompanying hemorrhagic stress is dependent on increased brain production of AII.

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.

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.

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.

The metabolism of neuropeptides. The hydrolysis of peptides, including enkephalins, tachykinins and their analogues, by endopeptidase-24.11

Endopeptidase-24.11 (EC 3.4.24.11), purified to homogeneity from pig kidney, was shown to hydrolyse a wide range of neuropeptides, including enkephalins, tachykinins, bradykinin, neurotensin, luliberin and cholecystokinin. The sites of hydrolysis of peptides were identified, indicating that the primary specificity is consistent with hydrolysis occurring at bonds involving the amino group of hydrophobic amino acid residues. Of the substrates tested, the amidated peptide substance P is hydrolysed the most efficiently (Km = 31.9 microM; kcat. = 5062 min-1). A free alpha-carboxy group at the C-terminus of a peptide substrate is therefore not essential for efficient hydrolysis by the endopeptidase. A large variation in kcat./Km values was observed among the peptide substrates studied, a finding that reflects a significant influence of amino acid residues, remote from the scissile bond, on the efficiency of hydrolysis. These subsite interactions between peptide substrate and enzyme thus confer some degree of functional specificity on the endopeptidase. The inhibition of endopeptidase-24.11 by several compounds was compared with that of pig kidney peptidyldipeptidase A (EC 3.4.15.1). Of the inhibitors examined, only N-[1(R,S)-carboxy-2-phenylethyl]-Phe-p-aminobenzoate inhibited endopeptidase-24.11 but not peptidyldipeptidase. Captopril (D-3-mercapto-2-methylpropanoyl-L-proline), Teprotide (pGlu-Trp-Pro-Arg-Pro-Gln-Ile-Pro-Pro) and MK422 [N-[(S)-1-carboxy-3-phenylpropyl]-L-Ala-L-Pro] were highly selective as inhibitors of peptidyldipeptidase. Although not wholly specific, phosphoramidon was a more potent inhibitor of endopeptidase-24.11 than were any of the synthetic compounds tested.

Acute and chronic hemodynamic effects of enalapril (MK-421) in congestive heart failure

Enalapril, a new long-acting angiotensin-converting enzyme inhibitor, was administered orally to 12 patients with stable congestive cardiac failure, NYHA function class III-IV. Acute and chronic hemodynamic effects were evaluated in addition to clinical response. The results of this open pilot study indicated marked reduction of pulmonary capillary wedge pressure from 21.8 +/- 5.9 mm Hg (mean +/- 1 SD) to 13.3 +/- 4.5 mm Hg (P less than 0.01) and peripheral resistance from 1837 +/- 860 dynes X sec-1 X cm-5 to 1063 +/- 584 dynes X sec-1 X cm-5 at 6 hr (P less than 0.01). Well-tolerated hypotension with mean arterial pressure from 88.0 +/- 11.6 mm Hg to 73.1 +/- 11.7 mm Hg at 6 hr (P less than 0.01) was recorded. No significant increase in cardiac output was observed. Angiotensin-converting enzyme activity was powerfully inhibited at the time of peak hemodynamic effect from 25.3 +/- 9.8 U/ml to 4.9 +/- 3.4 U/ml (P less than 0.01) and sustained, but attenuated reduction at 24 hr (8.7 +/- 4.7 U/ml) was observed. All patients reported subjective improvement and this clinical improvement has been sustained during follow-up from 19 to 21 months although baseline hemodynamic parameters at chronic re-catheterization did not demonstrate significant improvement. The pharmacodynamics and toxicity of enalapril as compared to captopril are discussed. The long half-life, low toxicity and gradual onset of action are seen as representing a clinical advantage with regard to patient therapy.

The metabolism of neuropeptides. Both phosphoramidon-sensitive and captopril-sensitive metallopeptidases are present in the electric organ of Torpedo marmorata

A membrane fraction from the electric organ of Torpedo marmorata hydrolyses the Gly3-Phe4 bond of [D-Ala2, Leu5]enkephalin as well as the Gly-His bond of benzoyl-Gly-His-Leu. The hydrolysis of benzoyl-Gly-His-Leu is completely inhibitable by Captopril (I50 = 19nM), consistent with peptidyl dipeptidase activity, but enkephalin hydrolysis is inhibited to a maximum of only 70%. The residual activity hydrolysing enkephalin is inhibited by phosphoramidon (I50 = 15nM) and therefore resembles endopeptidase-24.11, a mammalian plasma-membrane enzyme implicated in the metabolism of neuropeptides. Both enkephalin-hydrolysing activities in Torpedo electric organ are inhibited by 1,10-phenanthroline, like their mammalian counterparts. The peptidases may function in the hydrolysis of endogenous peptides or in neurotransmitter exocytosis in the electric organ.

Fluorogenic substrates for the enkephalin-degrading neutral endopeptidase (Enkephalinase)

Rat brain neutral endopeptidase ("Enkephalinase") was shown to hydrolyze a series of fluorogenic substrates of the general structure 2-aminobenzoyl-(amino acid)n- leucylalanylglycine -4- nitrobenzylamide . The hydrolysis of these substrates was competitively inhibited by Leu5-enkephalin, demonstrating that these are indeed substrates for the rat brain neutral endopeptidase. Cleavage of the fluorogenic substrates yielded leucylalanylglycine -4- nitrobenzylamide as a common product. In addition, a series of inhibitors previously shown to inhibit thermolysin-like enzymes inhibited the hydrolysis of both Leu5-enkephalin and the synthetic substrates. The results of this study (a) demonstrate that the enkephalin-degrading endopeptidase is similar in specificity to thermolysin, (b) provide a continuous sensitive assay system for the enzyme, and (c) point out the potential use of this substrate class for probing the specificity of 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 degrading enzymes are present in the electric organ of Torpedo californica

Two proteolytic activities that degrade [Leu5]enkephalin were found in Torpedo californica electric organ. One is a soluble aminopeptidase that degrades enkephalin at the Tyr1-Gly2 peptide bond, and the second is an endopeptidase that degrades enkephalin at the Gly3-Phe4 peptide bond. The aminopeptidase is inhibited by low concentrations of puromycin and bestatin. More than 60% of the endopeptidase is associated with the particulate fraction and is almost completely inhibited by low concentrations of captopril (SQ 14225) or SQ 20881 (potent inhibitors of angiotensin converting enzyme). Thiorphan and phosphoramidon (potent enkephalinase inhibitors) are much less effective. The pattern of cleavage and inhibition of the particulate endopeptidase thus resembles that of angiotensin converting enzyme.

Angiotensin-converting enzyme, enkephalinase A and aminopeptidases in the breakdown of enkephalin--studies in cell cultures

Neuroblastoma cells (N1E-115) and glial cells possess aminopeptidases, dipeptidyl carboxypeptidases and dipeptidyl aminopeptidases, which are located in the plasma membranes. Neuronal cells possess angiotensin-converting enzyme (ACE), which splits the Gly-Phe bond and generates Tyr-Gly-Gly from enkephalin. They have no enkephalinase A. In contrast, glial cells possess enkephalinase A but no ACE. Not only the dipeptidyl carboxypeptidases on neuronal and glial cells are different. These cells have aminopeptidases which have to be distinguished as well. This differentiation is made by using bestatin as an aminopeptidase inhibitor and reveals peptidases of high and poor bestatin-sensitivity.

Captopril potentiates the vasodepressor action of Met-enkephalin in the anaesthetized rat

The transient vasodepressor action of Met-enkephalin (10-80 micrograms kg-1, i.v.) in anaesthetized rats was significantly potentiated by the angiotensin-converting enzyme inhibitor, captopril (2 mg kg-1, i.v.); at this dose, it failed to modify the transient vasodepressor action of the non-specific vasodilator, nitroprusside (2.5, 5.0, 10 micrograms kg-1, i.v.). Captopril (2 mg kg-1, i.v.) caused a slow, progressive fall in the blood pressure of anaesthetized spontaneously hypertensive (SH) rats when compared to vehicle-treated controls. Pretreatment with naloxone (1.5 mg kg-1, i.v.) 30 min earlier failed to alter significantly the hypotensive action of captopril in anaesthetized SH rats. It was concluded that although captopril potentiated the vasodepressor action of Met-enkephalin in anaesthetized normotensive rats, potentiation of endogenous opioids does not appear to be involved in the hypotensive action of captopril in anaesthetized SH rats.

Mood changes during captopril therapy for hypertension. A double-blind pilot study

A small double-blind pilot study was carried out to assess whether captopril treatment in hypertension has a euphoriant effect. Eight patients were maintained on constant therapy of atenolol and bendrofluazide for at least 4 weeks before and throughout the study. Captopril 25 mg three times daily or matching placebo was administered double-blind for 6 weeks, with crossover to placebo or captopril from Weeks 7 to 12. Psychiatric assessment was made at Weeks 3, 6, 9, and 12. During the captopril phase, blood pressure was reduced, plasma angiotensin II lowered, and plasma renin raised. Mood was slightly, but significantly, lower during captopril administration; thus, there was no evidence of an euphoriant effect of captopril. This pilot trial also indicates the feasibility of the approach, and such studies of hypertensives under therapy should be usefully extended and refined.

Selective increase of angiotensin-converting enzyme activity in discrete extrahypothalamic areas of Brattleboro rats

We studied the activity of angiotensin I converting enzyme (ACE, kininase II, E.C. 3.4.15.1) in discrete areas of the brainstem and limbic system, and in circumventricular organs, pineal gland and choroid plexus of homozygous Brattleboro rats (DI) which are characterized by vasopressin deficiency and diabetes insipidus, with or without vasopressin replacement. We also determined ACE activity in heterozygous Brattleboro (HZ) and Long-Evans (LE) control rats. We found changes in ACE activity in several brain areas and the pineal gland of Brattleboro rats. ACE activity was increased in DI rats with respect to HZ and LE controls in the A1 area of the brainstem, locus coeruleus, and triangular nucleus of the septum. ACE activity in the A2 area of the brainstem, the nucleus tractus spinalis nervi trigeminii and the pineal gland was enhanced in both HZ and DI rats with respect to that of LE controls, but was not different between HZ and DI rats. ACE activity did not change in the other extrahypothalamic areas studied. The elevated ACE activity in extrahypothalamic areas of DI rats was not reversed by vasopressin replacement. These results suggest that a relationship between central vasopressin and angiotensin or bradykinin systems may exist in selective extrahypothalamic areas of the rat brain, and that peripherally administered vasopressin cannot influence this relationship.

Purification and inhibition by neuropeptides of angiotensin-converting enzyme from rat brain

Angiotensin-converting enzyme was solubilized with papain from a particulate fraction of rat brain and purified to apparent homogeneity by a procedure including DEAE-cellulose, hydroxylapatite, Sephadex G-200, Cys(Bzl)-Pro-Sepharose, and ricin-Sepharose chromatography. Bradykinin potentiators, SQ 14,225, and Arg-Pro-Pro strongly inhibited the activity of the purified enzyme, whereas Phe-Ala, phosphoramidon, and pentobarbital exerted little inhibitory effect on the activity. Among neuropeptides investigated, substance P, bradykinin, and Leu-enkephalin (Arg6) exerted strong inhibitory actions on the enzyme. Furthermore, the latter two peptides were shown to be good substrates for the enzyme. Thus, angiotensin-converting enzyme of rat brain is distinct from endogenous enkephalinase and may interact with various neuropeptides located in the brain.

Complete differentiation between enkephalinase and angiotensin-converting enzyme inhibition by retro-thiorphan

Thiorphan, N-[(R,S)-3-mercapto-2-benzylpropanoyl]glycine is a highly potent inhibitor (Ki = 3.5 nM) of "enkephalinase," a metalloendopeptidase cleaving the Gly-Phe bond (positions 3 and 4) of enkephalins in brain tissue. In accordance with this property, thiorphan displays antinociceptive activity after systemic administration. However, thiorphan also inhibits to a lesser extent (Ki = 140 nM) the widely distributed angiotensin-converting enzyme, a carboxydipeptidase implicated in blood pressure regulation. Therefore, in view of an eventual clinical use of enkephalinase inhibitors, it was very important to develop fully specific compounds. Such derivatives were obtained taking into account that N-methylation of the ultimate amide bond of dipeptides strongly decreases enkephalinase affinity without affecting angiotension-converting enzyme recognition, whereas retro-inversion of the amide bond leads to the inverse effect. Thus, the retro-inverso dipeptide (R)-H2N-CH(CH2 phi)-NHCO-CH2-CO2H exhibits an inhibitory potency on enkephalinase (IC50 approximately equal to 12 muM) close to that of the natural dipeptide L-Phe-Gly (IC50 approximately equal to 3 muM). This result shows the topological analogy between the crucial components involved in enkephalinase recognition both in active dipeptides and structurally related retro-inverso isomers. Taking into account these observations, retro-thiorphan, (R,S)-HS-CH2-CH-(CH2 phi)-NHCO-CH2-COOH, was prepared. As compared to thiorphan, the retro isomer is 50% as potent (Ki = 6 nM) on enkephalinase but displays a drastic loss of potency on angiotension-converting enzyme (IC50 greater than 10,000 nM). This specificity was interpreted as a consequence of differences in the stereochemical constraints involving enzyme-inhibitor hydrogen bonding. This hypothesis is supported by reported crystallographic studies on related enzymes such as thermolysin and carboxypeptidase A. As expected, retro-thiorphan exhibits about the same analgesic potency as thiorphan on the hot plate and writhing tests in mice. Therefore, the topological concept of retro-inverso isomers could be extended to other enkephalinase inhibitors, allowing the design of potent and highly selective compounds occurring as new classes of analgesic and psychoactive agents.

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.

Angiotensin-converting enzyme (kinase II) in pituitary gland of spontaneously hypertensive rats

Angiotensin-converting enzyme (ACE) (kinase II; dipeptidyl carboxypeptidase, EC 3.4.15.1) activity was measured in pituitary gland of young (4-week-old) and adult (18-week-old) male, spontaneously hypertensive rats (SHR) and in age-matched normotensive male Wistar-Kyoto (WKY) control rats. In the three lobes of the pituitary gland ACE activity was significantly higher in young than in adult animals, in both SH and WKY rats. In the anterior lobe, ACE activity was lower in SHR when compared to age-matched Wistar-Kyoto controls. In contrast, ACE activity in the intermediate lobe of the pituitary gland was higher in SHR, and in particular in young animals. The observed differences between young WKY and SH rats in both the intermediate and anterior lobes did not appear to be due to a modified affinity of ACE for the substrate hippuryl-His-Leu, but to alterations in ACE maximal velocity or number of available molecules. No differences in ACE activity were detected between SHR and WKY rats in the posterior lobe. Total protein content was higher in the intermediate lobe and lower in the posterior lobe of young SHR when compared to normotensive controls. The present results suggest the possibility for a role of pituitary ACE in spontaneous (genetic) hypertension in rats.

Enkephalin-degrading activity in Xenopus laevis tadpoles and adults blood

The enkephalin-degrading enzyme (alpha-aminoacyl-peptide hydrolase,EC 3.4.11.11) responsible for the rapid inactivation of enkephalins in human blood was purified and compared to the enkephalin-degrading system of Xenopus laevis tadpoles and adult blood. The specificity and the kinetic constants of the Xenopus enzyme(s) were determined after partial purification. Even if remarkable similarities between the Xenopus and the human enzyme exist, still they show differences in specificity towards peptides whose N-terminal is Phe. Amphibians are able to manufacture enkephalins and the present work shows that they are endowed with an enkephalin-degrading system comparable to the soluble one of human blood.

Differential recognition of "enkephalinase" and angiotensin-converting enzyme by new carboxyalkyl inhibitors

New carboxylalkyl compounds derived from Phe-Leu and corresponding to the general formula C6H5-CH2-CH(R)CO-L.Leu with R = -COOH, 3, R = -CH2-COOH, 4, R = -NH-CH2-COOH, 5, R = -NH-(CH2)2-COOH, 6, have been found to inhibit the breakdown of the Gly3-Phe4 bond of [3H] Leu-enkephalin or [3H]D.Ala2-Leu-enkephalin resulting from the action of the mouse striatal metallopeptidases: "enkephalinase" or angiotensin-converting enzyme (A.C.E.). The carboxyl coordinating ability of the Zn atom seems to be significantly higher in ACE than in "enkephalinase". Moreover, IC50 values against "enkephalinase" were found in the same range whatever the length of the chain bearing the carboxyl group whereas a well-defined position of this group with respect to the Zn atom is required for strong ACE inhibition. These features suggest a larger degree of freedom of the carboxyalkyl moieties within the active site of "enkephalinase". Therefore the differential recognition of active sites of both peptidases leads to: i) N-(carboxymethyl)-L-Phe-L-Leu, 5, a competitive inhibitor of "enkephalinase" (KI = 0.7 microM) and ACE (KI = 1.2 microM) which could be used as mixed inhibitor for both enzymes; ii) N-[(R,S)-2-carboxy, 3-benzylpropanoyl]-L-Leucine, 3, a full competitive inhibitor of "enkephalinase" (KI = 0.34 microM) which does not interact with ACE (IC50 greater than 10,000 microM). This compound can be considered as the first example of a new series of highly potent and specific "enkephalinase" inhibitors.

Adrenal involvement in captopril-induced potentiation of morphine analgesia

The involvement of the angiotensin-adrenal system as a possible mechanism in the potentiation of morphine analgesia by the angiotensin-converting enzyme inhibitor, captopril (SQ 14225), was studied in rats. Captopril pretreatment sensitized the animals to the analgesic effects of morphine while angiotensin II exerted an attenuating influence. These effects, however, were not demonstrable in adrenalectomized animals. Although captopril could inhibit the plasma angiotensin-converting enzyme activity, it appeared to have no significant effect on the brain enzyme. It has been suggested that the effects of captopril and angiotensin II on morphine analgesia are mediated indirectly through their effects on adrenal function.

A highly specific aminotripeptidase of rat brain cytosol. Substrate specificity and effects of inhibitors

An aminopeptidase preferentially hydrolyzing Leu- or Ala-Gly-Gly was purified from rat brain cytosol and detailed studies have been performed on its substrate specificity and the effects of inhibitors. The enzyme was devoid of di- and oligopeptidase contamination. Biologically active tripeptides such as Met-Leu-Tyr (chemotactic factor), Gly-His-Lys (liver growth factor) and Thr-Val-leu central nervous system tripeptide) were hydrolyzed at rates 0.05-0.15-times that of Leu-Gly-Gly. Melanostatin (Pro-Leu-GlyNH2) did not serve as a substrate. Substrates bearing N-terminal charged groups, or ones with proline in positions 2 or 3, or those with D-amino acid in positions 1 or 2, or with C-terminal CONH2, were poorly hydrolyzed or did not act as substrates, providing information on subsites involved in enzyme catalysis. The enzyme was inhibited competitively by bestatin (Ki 10-7 M) and by Captopril (2.5.10-7 M, D-3-thio-2-methylpropanyl proline) and by low concentrations of Zn2+ or PCMB, and at higher concentrations by TPCK and PMSF. Inhibition was observed for the chemotactic factor (I50 13 microM) and for the central nervous system tripeptide (195 microM). The enhanced action of Captopril was attributed to the presence of the -SH and -CH3 groups, since inhibition was shared by di- and tripeptides with proline in positions 2 and 3. The specificity pattern of brain enzyme was different from that reported for kidney and intestine.

Degradation of enkephalins: the search for an enkephalinase

The opioid peptides methionine-enkephalin and leucine-enkephalin appear to exert their biological effects through a receptor mediated mechanism. There appears to be three potential mechanisms for enkephalin degradation which could serve to control enkephalin levels in the vicinity of enkephalin receptors. These are, 1) cleavage of the tyrosyl-glycine bond by aminopeptidases, 2) cleavage of the glycl-glycine bond by a dipeptidyl aminopeptidase, and 3) cleavage of the glycyl-phenylalanine bond by a dipeptidyl carboxypeptidase. In this review the biochemical properties of these potential enkephalinases are described, and the evidence for each acting as an enkephalinase is reviewed.

Angiotensin-converting enzyme in discrete areas of the rat forebrain and pituitary gland

With the use of a sensitive radioisotopic method we have examined the activity of the angiotensin-converting enzyme (ACE, E.C. 3.4.15.1) in specific nuclei of the rat forebrain and in the anterior, intermediate and posterior lobes of the pituitary gland of the rat. We reported that ACE activity is heterogeneously distributed in the rat forebrain, with a 200-fold difference between the lowest and the highest values. Highest enzyme activities were found in the subfornical organ and in the posterior lobe of the pituitary gland. High ACE activity was also detected in the intermediate and anterior lobes of the pituitary gland, the caudate nucleus, and the medial habenular nucleus. Substantial activity also existed in the globus pallidus, the median eminence, the supraoptic and paraventricular nuclei, the lateral habenular nucleus and the organon vasculosum laminae terminalis. Our results demonstrate that one of the components of the renin-angiotensin system, the angiotensin-converting enzyme, is highly localized to a few discrete brain structures and the pituitary gland. These findings suggest that angiotensin II could be formed locally in some of these structures, supporting previous immunohistochemical data.

Kidney neutral endopeptidase and the hydrolysis of enkephalin by synaptic membranes show similar sensitivity to inhibitors

Neutral endopeptidase (EC 3.4.24.11) from pig kidney hydrolyses [125I]iodo-insulin B-chain and leucine-enkephalin. Both activities were equally sensitive to inhibition by phosphoramidon [N-(alpha-L-rhamnopyranosyloxyhydroxyphosphinyl)-L-leucyl-L-tryptophan] and thiorphan [N-(DL-2-benzyl-3-mercaptopropionyl)glycine]. Thermolysin hydrolysis of insulin B-chain was also sensitive to both inhibitors. The hydrolysis of the Gly3-Phe4 bond of Leu-enkephalin by synaptic membranes prepared from pig brain was partially inhibited by phosphoramidon and thiorphan. Synaptic membranes appear to contain another endopeptidase activity that is insensitive to these reagents. These observations suggest that enzymes similar to the kidney endopeptidase may play a general role in neuropeptide metabolism.

Hemodynamic and hormonal responses during captopril therapy for heart failure: acute, chronic and withdrawal studies

The hemodynamic and hormonal responses to acute and chronic captopril therapy and to its temporary withdrawal were studied in seven patients with congestive heart failure. Maximal hemodynamic and hormonal effects were reached with 25 to 50 mg doses of captopril. Since plasma angiotensin II levels were significantly higher 6 1/2 hours than 1 hour after administration of captopril, the drug should be given not less often than three times daily. No evidence of hormonal "escape" during long-term (mean 4 1/2 months) captopril therapy was observed, and initial hemodynamic responses were well maintained. Cessation of captopril administration resulted in abrupt increases in circulating angiotensin II levels, in arterial pressure, and in both pulse rate and plasma norepinephrine, but no decrease in cardiac function in the short-term was detected.

Distribution of angiotensin-converting enzyme activity in specific areas of the rat brain stem

Angiotensin-converting enzyme (ACE) activity was measured by a radiochemical assay in 30 specific areas of the rat brain stem. ACE activity is unevenly distributed, with a 60-fold difference between the lowest and the highest activity. The area postrema exhibits the highest activity. The substantia nigra (pars reticulata), the locus coeruleus, the areas A1 and A2, the nuclei commissuralis, and tractus solitarii have a substantial ACE activity, whereas the lowest activity is found in the raphe nuclei and the nuclei of the reticular formation.