The use of a monoclonal antibody for the rapid purification of kidney neutral endopeptidase ("enkephalinase") solubilized in octyl glucoside

Molecular and cellular targets of the MRI contrast agent P947 for atherosclerosis imaging

P947 (DOTA-Gd-peptide) was recently identified as an MRI contrast agent for the detection and characterization of the matrix metalloproteinases (MMP)-rich atherosclerotic plaques. Because this product displays a broad spectrum affinity for the MMP family, we hypothesized that it may also recognize other metalloproteinases overactivated in vulnerable atherosclerotic plaques. Therefore, this study aimed at describing, at the molecular and cellular level, the interactions between P947 and proteases of atherosclerotic plaques. Fluorimetric assays were used to measure the in vitro affinity of P947 toward recombinant and purified MMPs, angiotensin-converting enzyme (ACE), endothelin-converting enzyme (ECE-1), neutral endopeptidase (NEP), and both aminopeptidases A and N (APA and APN). Using similar fluorimetric assays associated with specific substrates, enzymatic activities were measured in vulnerable and stable plaques collected from human atherosclerotic carotid arteries. Ex vivo affinity of P947 for metalloproteinases in vulnerable lesions was subsequently determined. Interaction between P947 and major cell types present in atherosclerotic plaques was also investigated in different cell lines: PMA-1-differentiated THP-1 (macrophage), Ox-LDL-treated THP-1 (foam cell), Jurkat cell line (lymphocyte), and human umbilical vein endothelial cell (HUVEC, endothelial cell). Molecular targeting of P947 was confirmed by fluorimetry, ICP-MS, and in vitro MRI approaches. Potential application of P947 for detecting atherosclerotic plaques by in vivo MRI was tested in a rabbit model of atherosclerosis. In vitro, P947 displayed affinities for purified MMPs, ACE, ECE-1, NEP, APA, and APN in the micromolar range. Interestingly, MMPs, ACE, and APN exhibited higher activities in vulnerable plaques from human atherosclerotic carotid samples, as compared to stable plaques. ECE-1, NEP, and APA had either no activity or the same low activity in both vulnerable and stable plaques. P947 showed micromolar affinities for MMPs, ACE, and APN secreted by plaque samples. Moreover, P947 bound to THP-1 macrophages and THP-1 foam cells in a concentration-dependent manner and with a higher intensity than the control contrast agents DOTA-Gd or P1135 (DOTA-Gd coupled to a scrambled peptide). In THP-1 macrophages, P947 inhibited largely (70%) and almost completely (95%) MMP and APN activities, respectively, which strongly suggested an MMP- and APN-dependent binding of P947 to these cells. This enzyme-specific binding was confirmed with in vitro MRI. Indeed, the T1 value of THP-1 cells decreased from 2.094 s (macrophages w/o P947) to 2.004 s (macrophages with 1 mM of P947). In addition, the Gd content measured by ICP-MS was 11.01 ± 1.05 fg Gd/macrophage when cells were incubated in the presence of P947 and only 5.18 ± 0.43 fg Gd/macrophage with the control product P1135. The difference of Gd concentration between both contrast agents corresponded to a specific accumulation of 5.83 fg Gd/cell, which may be detected by MRI. MR imaging in the atherosclerosis rabbit model showed enhancement of the aortic wall after P947 injection with a significant increase of CNR values from 0.21 ± 0.02 (before injection) to 0.37 ± 0.07 (after injection), demonstrating the efficacy of the contrast agent to detect the atherosclerotic plaques in vivo. Taken together, these data suggest that P947 may be an interesting contrast agent for in vivo molecular MR imaging of MMPs, ACE, and APN activities present in vulnerable atherosclerotic plaques.

Identification of an endothelin-converting enzyme-2-specific fluorigenic substrate and development of an in vitro and ex vivo enzymatic assay

Endothelin-converting enzyme-2 (ECE-2) is a membrane-bound zinc-dependent metalloprotease that shares a high degree of sequence homology with ECE-1, but displays an acidic pH optimum characteristic of maturing enzymes acting late in the secretory pathway. Although ECE-2, like ECE-1, can cleave the big endothelin intermediate to produce the vasoconstrictive endothelin peptide, its true physiological function remains to be elucidated, a task that is hampered by the lack of specific tools to study and discriminate ECE-2 from ECE-1, i.e. specific substrates and/or specific inhibitors. To fill this gap, we searched for novel ECE-specific peptide substrates. To this end, peptides derived from the big endothelin intermediate were tested using ECE-1 and ECE-2, leading to the identification of an ECE-1-specific substrate. Moreover, screening of our proprietary fluorigenic peptide Fluofast® libraries using ECE-1 and ECE-2 allowed the identification of Ac-SKG-Pya-F-W-Nop-GGK-NH(2) (PL405), as a specific and high affinity ECE-2 substrate. Indeed, ECE-2 cleaved PL405 at the Pya-F amide bond with a specificity constant (k(cat)/K(m)) of 8.1 ± 0.9 × 10(3) M(-1) s(-1). Using this novel substrate, we also characterized the first potent (K(i) = 7.7 ± 0.3 nM) and relatively selective ECE-2 inhibitor and developed a quantitative fluorigenic ECE-2 assay. The assay was used to study the ex vivo ECE-2 activity in wild type and ECE-2 knock-out tissues and was found to truly reflect ECE-2 expression patterns. The PL405 assay is thus the first tool to study ECE-2 inhibition using high throughput screening or for ex vivo ECE-2 quantification.

Toward an optimal joint recognition of the S1' subsites of endothelin converting enzyme-1 (ECE-1), angiotensin converting enzyme (ACE), and neutral endopeptidase (NEP)

The formation of vasoconstrictors (e.g., angiotensin II and endothelin) and the inactivation of vasodilators (e.g., bradykinin and atrial natriuretic) by membrane-bound zinc metallopeptidases are key mechanisms in the control of blood pressure and fluid homeostasis. The way in which these peptides modulate physiological functions has been intensively studied. With the aim to develop compounds that can jointly block the three metallopeptidases-neutral endopeptidase (NEP, neprilysin), angiotensin-converting enzyme (ACE), and endothelin-converting enzyme (ECE-1)-we studied the common structural specificity of the S1' subsites of these peptidases. Various mercaptoacyl amino acids of the general formula HS-CH2-CH(R1')CO-Trp-OH, possessing more or less constrained R1' side chains, were designed. The mercapto-acyl synthons contain one or two asymmetrical centers. The K(i) values of the separated stereoisomers of the most efficient inhibitors were used to determine the stereochemical preference of each enzyme. A guideline for the joint inhibition of the three peptidases was obtained with the (2R,3R) isomer of compound 13b. Its K(i) values on NEP, ACE, and ECE were 0.7, 43, and 26 nM, respectively.

Neutral endopeptidase is expressed on the follicular granulosa cells of rabbit ovaries

Neutral endopeptidase (NEP) is a zinc metallopeptidase ubiquitously distributed in various tissues in mammals. This peptidase is involved in the post-secretory metabolism of various neuropeptides and peptide hormones in vivo, such as enkephalins, bradykinin, atrial natriuretic peptide, substance P and endothelins. In this paper we show that NEP is expressed in ovaries as a 110-kDa glycosylated integral membrane protein with enzymatic properties similar to those of the kidney protein. Using immunohistochemistry, we localize the peptidase in the granulosa cells of follicles at all stages of maturation, with the exception of atretic follicles. We also observe immunoreactive staining in the epithelia that lines the blood vessels in the medulla and the surface of the ovary. The co-localization of NEP and bioactive peptides known to be physiological substrates of NEP in other tissues suggests an important role for this protein in processes such as follicle maturation, ovulation, and/or regulation of ovarian blood flow, by modulating the physiological function of these peptides.

Highly sensitive and selective fluorescence assays for rapid screening of endothelin-converting enzyme inhibitors

The highly potent vasoconstrictor peptide endothelin (ET) is generated from an inactive precursor, big endothelin (bET), by endothelin-converting enzyme (ECE). ECE is a phosphoramidon-sensitive zinc metallopeptidase, which is closely related to neprilysin (neutral endopeptidase). It is possible that compounds which inhibit the formation of ET may be used as new drugs for the treatment of cardiovascular diseases. Such an approach requires a fast, simple and selective assay to measure ECE activity, allowing rapid screening of inhibitors. We describe here two new ECE substrates based on the concept of 'intramolecularly quenched fluorescence' which may fulfill this aim. One, S(1) [Pya(21)-Nop(22)-bET-1(19--35)], is the (19--35) fragment of the natural peptide big-ET-1(1--38), which is modified by introducing the fluorescent amino acid, pyrenylalanine (Pya), in position 21 and a quencher, p-nitrophenylalanine (Nop), in position 22. The second substrate (S(2)) is a small peptide, Ac-Ser-Gly-Pya-Lys-Ala-Phe-Ala-Nop-Gly-Lys-NH(2), from a biased substrate peptide library. The recombinant, hECE-1c, cleaved both Pya(21)-Nop(22)-bET-1(19--35) and the natural substrate selectively between residues 21 and 22, whereas cleavage occurred between alanine and phenylalanine in the small peptide. In both cases, this generated intense fluorescence emission. The synthesis and kinetic parameters of these substrates are described. These assays, which can be used directly on tissue homogenates, are the most sensitive and selective described to date for ECE, and are easily automated for a high-throughput screening of inhibitors.

Phosphinic derivatives as new dual enkephalin-degrading enzyme inhibitors: synthesis, biological properties, and antinociceptive activities

The development of dual inhibitors of the two zinc metallopeptidases, neprilysin (neutral endopeptidase) and aminopeptidase N involved in the inactivation of the opioid peptides, enkephalins, represents an attractive physiological approach in the search for new analgesics devoid of the major drawbacks of morphine. Phosphinic compounds, corresponding to the general formula H(3)N(+)-CH(R(1))-P(O)(OH)-CH(2)-CH(R(2))-CONH-CH(R(3))-COO(-), able to act as transition-state analogues and to fit the S(1), S(1)', and S(2)' subsites of both enzymes were designed. Selection of the R(1), R(2), and R(3) residues for optimal recognition of these enzymes led to the first dual competitive inhibitors with K(i) values in the nanomolar range for neprilysin and aminopeptidase N. These compounds induce potent analgesic responses after intracerebroventricular or intravenous administrations in mice (hot plate test), and several of them were shown to be, at least, 10 times more potent than the previously described dual inhibitors.

Investigation of subsite preferences in aminopeptidase A (EC 3.4.11.7) led to the design of the first highly potent and selective inhibitors of this enzyme

The study of the physiological roles of the membrane-bound zinc-aminopeptidase A (glutamyl aminopeptidase, EC 3.4.11.7) needs the design of efficient and selective inhibitors of this enzyme. An acute exploration of aminopeptidase A active site was performed by a combinatorial approach using (3-amino-2-mercapto-acyl)dipeptides able to fit its S(1), S(1)', and S(2)' subsites. This analysis confirmed that the S(1) subsite is optimally blocked by a glutamate or isosteric residues and demonstrated that the S(1)' subsite is hydrophobic whereas the S(2)' subsite recognizes preferentially negatively charged residues derived from aspartic acid. The optimization of these structural parameters led to the synthesis of nanomolar and subnanomolar inhibitors of aminopeptidase A such as H(3)N(+)CH(CH(2)CH(2)SO(3)(-))CH(SH)CO-Ile-(3-COOH)Pro that exhibits a K(i) of 0.87 nM. The best compounds were synthesized by a stereochemically controlled route. These first described highly potent inhibitors could allow studies about the role of physiological substrates of APA such as angiotensin II and cholecystokinin CCK(8) in the central nervous system.

Characterization of neutral endopeptidase in vascular cells, modulation of vasoactive peptide levels

We characterized neutral endopeptidase activity and protein in the three aortic layers and in corresponding cultured primary cells. Neutral endopeptidase was expressed in all three layers of rat aorta with higher protein level and activity in the adventitia than in the media and intimal endothelium. Neutral endopeptidase was also found in primary cultured fibroblasts, smooth muscle and endothelial cells derived from the corresponding layers. Neutral endopeptidase activity and protein were higher in the fibroblasts and smooth muscle cells than in endothelial cells. Neutral endopeptidase inhibition prevented atrial natriuretic peptide (ANP) degradation in endothelial and smooth muscle cells. It potentiated ANP-stimulated cyclic GMP production in these cells. Neutral endopeptidase inhibition also reduced bradykinin degradation and potentiated bradykinin-stimulated release of arachidonic acid in fibroblasts and endothelial cells. Our data demonstrate the presence and functional activity of neutral endopeptidase in all three cell layers of rat aorta as well as in primary cells of the vessel. The data suggest that local concentrations of vasoactive peptides in the vessel wall might be regulated by the neutral endopeptidase cleavage pathway in the immediate vicinity of their target cells.

Secretion of a type II integral membrane protein induced by mutation of the transmembrane segment

Signal peptide/membrane anchor (SA) domains of type II membrane proteins initiate the translocation of downstream polypeptides across the endoplasmic reticulum (ER) membrane. In contrast with signal peptides, however, SA domains are not cleaved by signal peptidase and thus anchor the protein in the membrane. In the present study we have introduced mutations in the SA domain of neprilysin (neutral endopeptidase-24.11; NEP) to identify structural elements that would favour the processing of SA domains by signal peptidase. Mutants of full-length and truncated (without cytoplasmic domain) protein were constructed by substitution of the sequences SQNS, QQTT or YPGY for VTMI starting at position 15 of the NEP SA domain. In addition, a Pro residue was substituted for Thr at position 16 of the SA domain. The rationale for the use of these sequences was decided from our previous observation that substitution in the NEP SA domain of the sequence SQNS, which is polar and has alpha-helix-breaking potential, could promote SA domain processing under certain conditions (Roy, Chatellard, Lemay, Crine and Boileau (1993) J. Biol. Chem. 268. 2699-2704; Yang. Chatellard, Lazure, Crine and Boileau (1994) Arch. Biochem. Biophys. 315, 382-386). The QQTT sequence is polar but, according to secondary structure predictions, is compatible with the alpha-helix structure of the NEP SA domain. The YPGY sequence and single Pro residue are less polar and have alpha-helix-breaking potential. The predicted effects of these mutations on the structure of the NEP SA domain were confirmed by CD analysis of 42-residue peptides encompassing the hydrophobic segment and flanking regions. Wild-type and mutated proteins were expressed in COS-I cells and their fate (membrane-bound or secreted) was determined by immunoblotting and by endoglycosidase digestions. Our biochemical and structural data indicate that: (I) the cytosolic domain of NEP restricts the conformation of the SA domain because mutants not secreted in their full-length form are secreted in their truncated form; (2) alpha-helix-breaking residues are not a prerequisite for cleavage; (3) the presence, in close proximity to a putative signal peptidase cleavage site, of a polar sequence that maintains the alpha-helical structure of the SA domain is sufficient to promote cleavage. Furthermore pulse chase studies suggest that cleavage is performed in the ER by signal peptidase and indicate that cleavage is not a limiting step in the biosynthesis of the soluble form of the protein.

The roles of the prosequence of thermolysin in enzyme inhibition and folding in vitro

The zinc endopeptidase thermolysin (EC 3.4.24.27), an extracellular enzyme from Bacillus thermoproteolyticus, is synthesized as a preproprotein, with the prosequence (204 residues) being two-thirds the size of the mature enzyme (316 residues). This prosequence, expressed in and purified from Escherichia coli, inhibited thermolysin in vitro with an IC50 value of 14 nM. It also inhibited a closely related enzyme produced by Bacillus stearothermophillus, albeit with a 16-fold higher IC50 value (220 nM). The IC50 value for thermolysin inhibition was also increased 15-fold (210 nm) by a monoclonal antibody that recognizes an epitope close to, but not forming a part of, the active site. At a prosequence concentration of 5 microM a mammalian, thermolysin-like enzyme, neutral endopeptidase 24.11, was not inhibited. The prosequence appeared to act as a mixed, noncompetitive inhibitor of thermolysin activity, with a Ki value of 6 nM for its interaction with the enzyme alone and a Ki' value of 20 nM for its interaction with the enzyme-substrate complex. In addition, when thermolysin was denatured in 6 M guanidinium hydrochloride at acid pH and then brought to neutral pH by rapid dilution, the prosequence was found to facilitate the recovery of active enzyme in a stoichiometric manner.

Identification of the cysteine residues implicated in the formation of alpha 2 and alpha/beta dimers of rat meprin

Meprin (endopeptidase-24.18; EC 3.4.24.18) is a multisubunit zinc-metallopeptidase found in the brush-border membranes of rodent kidney and human intestine. The alpha and beta subunits of meprin are disulphide-linked to form either soluble alpha 2 homodimers or membrane-associated alpha/beta heterodimers. The aim of the present study was to identify the cysteine residue(s) implicated in the formation of alpha 2 and alpha/beta dimers and to investigate the effects of dimerization on intracellular transport and processing of the alpha subunit. Three cysteine residue candidates for the formation of disulphide bonds in the alpha subunit were selected by hydrophobic cluster analysis. These residues, located at positions 309, 560 and 562, were mutated to serine residues. When the resulting alpha subunit mutants were expressed alone in COS-1 cells, the alpha C560S and alpha C562S mutants were found to be secreted as alpha 2 homodimers whereas the alpha C309S mutant was found as monomers in the culture medium. In double-transfection experiments with the wild-type beta subunit, the alpha C560S and alpha C562S mutants behaved exactly as the wild-type alpha subunit and formed membrane-bound alpha/beta heterodimers. In contrast, the alpha C309S mutant was not retained at the cell surface but rather secreted as monomers in the culture medium, as was found in the simple transfection experiment. These results show that, despite the normal expression level and folding of the protein in a transport-competent from, the alpha C309S mutant is unable to form alpha 2 homodimers or alpha/beta heterodimers. This suggests that Cys309 is the unique residue of the alpha subunit implicated in the alpha 2 and alpha/beta dimerizations. Hydrophobic cluster analysis of the alpha and beta subunit sequences predicts that Cys309 is similar to Cys306 of the beta subunit. We mutated the latter residue to a serine and expressed the beta C306S mutant and the wild-type alpha subunit in the same COS-1 cells. No beta 2 or alpha/beta dimers were observed on immunoblotting, showing that Cys306 of the beta subunit is required for the formation of intermolecular disulphide bonds both in beta 2 homodimers and in alpha/beta heterodimers. Taken together, these results suggest that the alpha/beta heterodimeric form of meprin is held together by a single disulphide bond linking Cys309 in the alpha subunit to Cys306 in the beta subunit.

Design of orally active dual inhibitors of neutral endopeptidase and angiotensin-converting enzyme with long duration of action

Mercaptoacyl dipeptides, containing a glycine linked to a C-terminal 5-phenylproline, have been synthesized in order to obtain new highly efficient dual inhibitors of the two zinc metallopeptidases, neutral endopeptidase (NEP) and angiotensin-converting enzyme (ACE), which are involved in the control of blood pressure and fluid homeostasis. These compounds have been designed (i) to fit optimally the ACE pharmacophore previously described (Fournié-Zaluski, M. C.; et al. J. Med. Chem. 1994, 37, 1070-1083), through interaction with the S1, S1', and S2' subsites of this enzyme, (ii) and to interact with the S1' and S2' subsites of NEP with the 5-phenylproline moiety outside the catalytic domain (Coric, P.; et al. J. Med. Chem. 1996, 39, 1210-1219). Replacement of Gly by Ala in these mercaptoacyl dipeptides induced an about 100-fold decrease in ACE inhibition. This shows that, in agreement with molecular modeling studies, a steric constraint as weak as a methyl group hinders optimal ACE active site recognition. Among these compounds, the dual inhibitor 26 (RB 106) (Ki, ACE = 0.35 nM; NEP = 1.6 nM) showed excellent pharmacokinetic properties with an almost complete in vivo inhibition of NEP and ACE for more than 4 h after oral administration in mice of a low dose (2.6 x 10(-5) mol/kg) of the inhibitor. Moreover, RB 106 remained active 12 h after oral administration. In spontaneous hypertensive rats, a chronic treatment of orally administered RB 106 (25 mg/kg/day) induced a prolonged hypotensive effect (-28 mmHg) still significant 2 days after the end of the treatment. In DOCA salt rats, a hypotensive response and a significant natriuresis were observed after i.v. administration. RB 106, which is one of the most potent dual inhibitors described to date, could have interesting clinical applications in long term treatment of congestive heart failure and myocardial ischemia.

Optimal recognition of neutral endopeptidase and angiotensin-converting enzyme active sites by mercaptoacyldipeptides as a means to design potent dual inhibitors

An interesting approach for the treatment of congestive heart failure and chronic hypertension could be to avoid the formation of angiotensin II by inhibiting angiotensin converting enzyme (ACE) and to protect atrial natriuretic factor by blocking neutral endopeptidase 24.11 (NEP). This is supported by recent results obtained with potent dual inhibitors of the two zinc metallopeptidases, such as RB 105, HSCH2CH(CH3)PhCONHCH(CH3)COOH (Fournié-Zaluski et al. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 4072-4076), which reduces blood pressure in experimental models of hypertension, independently of the salt and renin angiotensin system status. In order to develop new dual inhibitors with improved affinities, long duration of action, and/or better bioavailabilities, various series of mercaptoacyldipeptides corresponding to the general formula HSCH(R1)CONHCH(R1')CON(R)CH(R2')COOH have been synthesized. The introduction of well-selected beta-branched chains in positions R1 and R1', associated with a tyrosine or a cyclic amino acid in the C-terminal position, led to potent dual inhibitors of NEP and ACE such as 21 [N-[(2S)-2-mercapto-3-methylbutanoyl]-Ile-Tyr] and 22 [N-[(2S)-2-mercapto-3-phenylpropanoyl]Ala-Pro] which have IC50 values in the nanomolar range for NEP and subnanomolar range for ACE. These compounds could have different modes of binding to the two peptidases. In NEP, the dual inhibitors seem to interact only with the S1' and S2' subsites, whereas additional interactions with the S1 binding subsite of ACE probably account for their subnanomolar inhibitory potencies for this enzyme. The localization of the Pro residue of 22 outside the NEP active site is supported by biochemical data using (Arg102,Glu)NEP and molecular modeling studies with thermolysin used as model of NEP. One hour after oral administration in mice of a single dose (2.7 x 10(-5) mol/kg), 21 inhibited 80% and 36% of kidney NEP and lung ACE, respectively, while 22 inhibited 40% of kidney NEP and 56% of lung ACE.

Proteolytic processing of the alpha-subunit of rat endopeptidase-24.18 by furin

Endopeptidase-24.18 (EC 3.4.24.18; meprin) is a multisubunit metallopeptidase of the astacin family. It is found in brush-border membranes of rodent kidney and human intestine. The membrane-bound enzyme is composed of alpha/beta dimers. Molecular cloning has shown that both subunits have a similar structural domain organization. Soluble alpha 2 dimers have also been observed in vivo and in transfected cells. The structures of all known alpha-subunits contain, upstream from the transmembrane domain, the sequence RXKR, which corresponds to the RXK/RR consensus sequence for specific cleavage by furin. In order to investigate the involvement of this putative cleavage site in the secretion process of endopeptidase-24,.18 alpha-subunit, we expressed in COS-1 cells rat alpha-subunits in which residues R655 or S656 (within the sequence R652PKRS656) were mutated to valine or leucine respectively. In contrast to the wild-type protein, the alpha R655V and alphaS656L mutants were not secreted in the culture medium. Moreover, when cells expressing the alpha-subunit were infected with a furin-encoding vaccinia virus, immunoblotting showed a shift of the major cell-associated form of endopeptidase-24.18 alpha-subunit from 98 kDa to 85 kDa and an increase in the amounts of secreted alpha-subunit. This shift in molecular mass was not observed with the mutant alpha-subunits. As observed for the 98 kDa species, the 85 kDa cell-associated protein was sensitive to endoglycosidase H treatment, suggesting that the proteolytic cleavage occurred in the endoplasmic reticulum or in an early Golgi compartment. Similar experiments using PACE4 and PC5 instead of furin showed that these enzymes were not able to generate the 85 kDa species. We conclude that furin is most probably the cellular enzyme involved in the proteolysis resulting in secretion of rat endopeptidase-24.18 alpha-subunit.

Neutral endopeptidase can hydrolyze beta-amyloid(1-40) but shows no effect on beta-amyloid precursor protein metabolism

High performance liquid chromatographic analyses of incubations of beta-amyloid(1-40) with neutral endopeptidase revealed at least nine product peaks, indicating that neutral endopeptidase can cleave beta-amyloid at multiple sites. Mass spectroscopic analysis of hydrolyzed beta-amyloid identified at least five cleavage sites, between residues Glu3-Phe4, Gly9-Trp10, Phe19-Phe20, Ala30-Ile31, and Gly33-Leu34. In contrast, amyloid precursor protein metabolism in Neuro2A cells was unaffected by the expression of recombinant neutral endopeptidase in the same cells or by the addition of a secreted form of neutral endopeptidase to spent Neuro2A cell media.

Expression of rat endopeptidase-24.18 in COS-1 cells: membrane topology and activity

Endopeptidase-24.18 (E-24.18; EC 3.4.24.18) is a metallopeptidase of the astacin family and is highly expressed in kidney brush-border membranes of rodents. Rat E-24.18 consists of two disulphide-linked alpha/beta dimers [(alpha/beta)2]. In order to investigate the mechanisms of assembly and the importance of each subunit in the enzymic process, the cloned cDNAs for the rat alpha and beta subunits were transiently expressed either alone or together in COS-1 cells. Immunoblotting of cell extracts and spent culture media showed that, when expressed alone, the alpha subunit is secreted, whereas the beta subunit is membrane-bound. In alpha/beta-transfected cells, the alpha subunit remained membrane-bound, but could be released from the cell surface after papain treatment or after incubation with 10 mM dithiothreitol. Furthermore, mutants of the alpha subunit in which the putative C-terminal anchor domain was deleted could still form cell-associated alpha/beta dimers. These results are consistent with a topological model of E-24.18 in which the beta subunit is anchored in the plasma membrane and the alpha subunit is retained at the cell surface through disulphide bridge(s) with the beta subunit. Both the alpha and beta recombinant subunits expressed in COS-1 cells showed little azocasein-degrading activity. However, activity of either individual subunits of alpha/beta dimers was increased after mild trypsin digestion, suggesting that in COS-1 cells the enzymes are synthesized as zymogens. Finally, inactivation of the alpha subunit by site-directed mutagenesis of Glu-157, which is believed to play a role in catalysis, showed that both subunits participate in the enzymic activity of the heterodimer.

Dual inhibition of angiotensin-converting enzyme and neutral endopeptidase by the orally active inhibitor mixanpril: a potential therapeutic approach in hypertension

In the treatment of cardiovascular disease, it could be of therapeutic interest to associate the hypotensive effects due to the inhibition of angiotensin II formation with the diuretic and natriuretic responses induced by the protection of the endogenous atrial natriuretic peptide (ANP). Investigation of this hypothesis requires an orally active compound able to simultaneously inhibit angiotensin-converting enzyme (ACE) and neutral endopeptidase (NEP), which is involved in renal ANP metabolism. Such compounds have been rationally designed by taking into account the structural characteristics of the active site of both peptidases. Among them, RB 105, N-[(2S,3R)-2-mercaptomethyl-1-oxo-3-phenylbutyl]-(S)-alanine, inhibited NEP and ACE with Ki values of 1.7 +/- 0.3 nM and 4.2 +/- 0.5 nM, respectively. Intravenous infusion of RB 105 in conscious spontaneously hypertensive rats prevented the pressor response to exogenous angiotensin I and potentiated the natriuretic response to ANP. Infusion of RB 105, at 2.5, 5, 10, 25, and 50 mg/kg per hr decreased blood pressure dose-dependently in conscious catheterized spontaneously hypertensive rats and increased diuresis and natriuresis. Infusion of RB 105 as a bolus of 25 mg/kg followed by 25 mg/kg per hr similarly decreased blood pressure and increased natriuresis in three different models of hypertension (renovascular, deoxycorticosterone acetate-salt, and spontaneously hypertensive rats). Mixanpril, a lipophilic prodrug of RB 105 (ED50 values when given orally to mice, 0.7 mg/kg for NEP; 7 mg/kg for ACE), elicited dose-dependent hypotensive effects of long duration in spontaneously hypertensive rats after oral administration [-37 mmHg for 50 mg/kg twice a day (1 mmHg = 133 Pa) and is therefore the first dual NEP/ACE inhibitor potentially useful for clinical investigations.

Characterization of neutral endopeptidase 24.11 in dog glomeruli

Neutral endopeptidase (NEP; also known as neprilysin and enkephalinase; EC 3.4.24.11) is a cell-surface metallopeptidase that is present in many mammalian tissues. It is particularly abundant on the brush-border membranes of the kidney proximal tubule. In this paper, the presence of NEP in purified glomeruli from dog kidney was assessed by measuring phosphoramidon- and thiorphan-sensitive [D-Ala2,Leu5]enkephalin-degrading activity. Using this assay, the Km and kcat. of the glomerular enzyme were found to be identical to those of the tubular enzyme. By Western blotting the apparent M(r) of the glomerular enzyme was found to be 104,000, compared with 94,000 for the tubular enzyme. This might be due to a different glycosylation pattern, since endoglycosidase F treatment of NEP obtained from both tissues yielded deglycosylated enzymes with similar electrophoretic mobilities. The glomerular enzyme also appears to be membrane-bound, since it was retained in the detergent-rich phase after phase separation with Triton X-114. Autoradiography experiments performed with RB104, a new highly selective and potent NEP inhibitor, showed that NEP was expressed in both glomeruli and proximal tubules. The presence in glomeruli of NEP and some other brush-border peptidases (dipeptidyl-dipeptidase IV, aminopeptidase N and angiotensin I-converting enzyme) suggests that cell-surface peptidases might play an important role as regulators of plasma-derived peptides in this part of the nephron.

Detection of neutral endopeptidase-24.11/CD10 by flow cytometry and photomicroscopy using a new fluorescent inhibitor

Neutral endopeptidase (NEP; E.C. 3.4.24.11) is a mammalian ectopeptidase identified as the common acute lymphoblastic leukemia antigen (CALLA or CD10). In order to investigate its cellular processing and its role in B lymphocyte differentiation, a fluorescent derivative of the mercapto NEP inhibitor thiorphan, N-[fluoresceinyl]-N'-[1-(6-(3-mercapto-2-benzyl-1-oxopropyl) amino-1-hexyl]thiocarbamide (FTI), has been synthesized. The fluorescent characteristics of fluorescein were conserved in FTI after linkage with the thiol NEP inhibitor. FTI inhibited NEP with an IC50 value of 10 nM and a good selectivity compared to that of aminopeptidase N (greater than 100 microM) and angiotensin converting enzyme (32 microM). The FTI probe was shown to detect membrane-bound NEP using photomicroscopy on cultured cells or flow cytometry techniques. Using NEP-expressing MDCK cells and episcopic fluorescence microscopy, a specific labeling was obtained with 100 nM FTI which was completely displaced by 10 microM HACBOGly, a specific and potent inhibitor of NEP. Therefore, FTI can be considered a suitable tool for following cellular NEP traffic. In flow cytometry, the fluorescent probe FTI, used at concentrations as low as 1 nM with Reh6 cells, could be very useful for detecting NEP/CALLA on lymphoid cells. In addition, the recognition of FTI is independent of tissues and species, a major advantage of inhibitors over monoclonal antibodies.

Development of [125I]RB104, a potent inhibitor of neutral endopeptidase 24.11, and its use in detecting nanogram quantities of the enzyme by "inhibitor gel electrophoresis"

Neutral endopeptidase 24.11, also known as the common acute lymphoblastic leukemia antigen, is a zinc metallopeptidase involved in the inactivation of biologically active peptides, such as the enkephalins and atrial natriuretic peptide. The highly potent radiolabeled inhibitor 2-((3-[125I]iodo-4-hydroxy)phenylmethyl)-4-N-[3-(hydroxyamino-3-oxo-1- phenylmethyl)propyl]amino-4-oxobutanoic acid ([125I]RB104; Ki = 30 pM) has been developed for the enzyme. [125I]RB104 is highly specific, its Ki for another widely distributed zinc peptidase, angiotensin-converting enzyme, being 15 microM. In binding studies using rat brain slices, [125I]RB104 was shown to have a high affinity (Kd = 300 +/- 20 pM) and high specific binding at the Kd concentration (90%). With rat brain homogenates the Kd of [125I]RB104 was 26.8 +/- 0.9 pM, close to the kinetically derived Kd, 7.0 +/- 0.8 pM. Using the inhibitor, we have developed a simple, rapid, and quantitative technique to detect low nanogram quantities of the endopeptidase directly from tissue extracts after SDS/PAGE. The method has been used to show the presence of low quantities of the enzyme in rabbit bone marrow. Apart from its sensitivity, "inhibitor gel electrophoresis" using [125I]RB104 has the advantage over immunohistochemical methods of being able to label the enzyme in all tissues and species. It will therefore be of great value in determining the exact role of this important regulatory peptidase in a number of biological systems. Moreover, this one-step characterization of neutral endopeptidase 24.11 could be extended to other zinc metallopeptidases such as angiotensin-converting enzyme or collagenases, and inhibitors with affinities as high as RB104 could open the way to visualization of zinc metallopeptidases in different tissues by electron microscopy.

Secretion of a functional soluble form of neutral endopeptidase-24.11 from a baculovirus-infected insect cell line

Neutral endopeptidase (NEP; EC 3.4.24.11) is an integral membrane protein found at the plasma membrane of many cell types. A secreted form of NEP (sec-NEP) was recently obtained by transfection of COS-1 cells with a recombinant expression vector consisting of the cDNA encoding the signal peptide of pro-opiomelanocortin fused in-frame to the cDNA sequence of the complete ectodomain of rabbit NEP [Lemay, Waksman, Roques, Crine & Boileau (1989) J. Biol. Chem. 264, 15620-15623]. In order to produce large quantities of this enzyme for structural studies we have expressed this recombinant soluble form of NEP at high yields using a baculovirus/insect-cell system. A recombinant Autographa californica nuclear polyhedrosis-virus genome containing the sec-NEP sequence was used to infect host Spodoptera frugiperda Sf9 cells. Infected cells secreted an N-glycosylated soluble form of neutral endopeptidase which was enzymically active. The yield was about 80 nmol of enzyme/litre of culture. The soluble form of the recombinant enzyme purified by immunoaffinity showed the same catalytic properties as the wild-type enzyme extracted from the kidney brush-border membranes. Treatment of the recombinant enzyme with endo-beta-N-acetylglucosaminidase H showed, however, that invertebrate cells did not glycosylate the enzyme to the same extent as did mammalian cells. Our findings demonstrate that insect cells can be used as hosts for the production of the soluble form of neutral endopeptidase. We also conclude that neither a full complement of carbohydrate side chains nor the membrane anchor appear to be essential for the production and targeting to the cell surface of a fully functional enzyme in this expression system.

Inhibitors of the enkephalin degrading enzymes. Modulation of activity of hydroxamate containing compounds by modifications of the C-terminal residue

To further characterize the S'2 subsite of both the neutral endopeptidase (EC 3.4.24.11, NEP) and aminopeptidase N (EC 3.4.11.2, APN), two enzymes physiologically involved in enkephalin metabolism, a new series of hydroxamate inhibitors containing a cyclic amino acid as the P'2 component were synthesized. These amino acids differ by the size of the cycle, the relative position of the functional groups, and their absolute configuration. Highly efficient inhibitors of NEP were obtained whatever the modification on the P'2 component, while for APN inhibition, a cyclic beta-amino acid was preferred. The most active inhibitors contained a trans cyclopentyl beta-amino acid and a cis or a trans cyclohexyl beta-amino acid. When injected intracerebroventricularly in mice, these two latter compounds elicited potent antinociceptive responses on both the jump latency and the fore paw lick times.

Enkephalin-degrading enzyme inhibitors. Crucial role of the C-terminal residue on the inhibitory potencies of retro-hydroxamate dipeptides

The retro-inversion of the amide bond in kelatorphan and analogs, the first series of complete inhibitors of enkephalin metabolism, led to compounds highly efficient only against the neutral endopeptidase 24-11 (NEP). In order to increase the recognition of the aminopeptidase N (APN) and dipeptidylaminopeptidase (DAP), without loss of affinity for NEP, the malonyl group of these retro-inhibitors was replaced by diversely substituted succinyl moieties. All the molecules synthesized are highly efficient NEP inhibitors with Ki's in the 0.2-1 nM range, indicating that NEP possesses a relatively large and not very selective S'2 subsite. In contrast, inhibition of DAP activity is crucially dependent on the size and the position of the substituent in the succinyl moiety. Inhibitory potencies in the nanomolar range are obtained with compounds containing a benzyl group in the alpha-position related to the retro amide bond. Finally, a relatively modest inhibition of APN was observed with Ki's in the 0.5-1 microM range for compounds with benzyl or cyclohexyl group in P'2 position. However, these data demonstrate that efficient and complete inhibition of enkephalin degradation can be obtained with hydroxamate dipeptides containing a retro amide bond. The analgesic potency of the most active inhibitors was measured using the hot plate test in mice. Significant antinociceptive responses were obtained but these effects were rather weaker than those expected from the in vitro inhibitory potencies of these compounds on the three enkephalin-degrading enzymes.

Monoclonal antibodies as probes for the transmembrane structure of neutral endopeptidase 24.11 ('enkephalinase')

The neutral endopeptidase (EC 3.4.24.11) ('enkephalinase') is a membrane-bound metalloendopeptidase that is present in large amounts in the microvilli of the kidney proximal tubules. By immunizing mice with purified rabbit kidney brush-border membranes, we have obtained four different monoclonal antibodies that recognize this enzyme in dot-blot and Western-blot assays and can be used for immunoprecipitation of neutral endopeptidase from crude kidney solubilizates. One of these monoclonal antibodies (2B12) allows the labeling of proximal tubule cells with colloidal gold particles. This monoclonal antibody also binds to native brush-border membrane vesicles (which are mostly in the right-side-out configuration) and recognizes an epitope which is destroyed after reduction and alkylation of the protein. By contrast, all three other monoclonal antibodies (21G10, 23B11 and 22E2) compete for another epitope of neutral endopeptidase that is not exposed at the extracytoplasmic surface either in intact cells or in sealed brush-border vesicles. Permeabilization of the vesicles with digitonin, however, restores the full binding activity. Binding of these antibodies is not altered by prior reduction and alkylation of the protein. Taken together, these results strongly suggest that the 2B12 monoclonal antibody binds a conformational epitope located on the ectodomain of the enzyme, whereas the three others (21G10, 23B11 and 22E2) bind to a common or to overlapping epitopes located on the cytosolic domain. These results also demonstrate unambiguously the transmembrane nature of neutral endopeptidase.

Exploration of the catalytic site of endopeptidase 24.11 by site-directed mutagenesis. Histidine residues 583 and 587 are essential for catalysis

Direct comparison of the primary structure of neutral endopeptidase (NEP, EC 3.4.24.11) with that of thermolysin, a bacterial metalloendopeptidase with a similar specificity, has revealed very few similarities between the two sequences, except for two conserved short segments. In thermolysin, these segments contain several of the residues involved in catalysis, including two zinc coordinating histidines (His-142 and His-146) and a third histidine (His-231) involved in stabilizing the transition state through hydrogen bonding. The role of the corresponding histidines in NEP (His-583, His-587 and His-637) was explored by site-directed mutagenesis of NEP cDNA and expression of the mutated cDNA in COS-1 cells. Substitution of either His-583 or His-587 of NEP for Phe completely abolished the activity and Zn-directed inhibitor recognition of the recombinant enzyme, suggesting that these residues play a role similar to His-142 and His-146 of thermolysin as zinc ligands. In contrast, substitution of His-637 for a phenylalanine residue was without effect on enzyme activity.