The design and properties of N-carboxyalkyldipeptide inhibitors of angiotensin-converting enzyme

The Thyrotropin-Releasing Hormone-Degrading Ectoenzyme, a Therapeutic Target?

Thyrotropin releasing hormone (TRH: Glp-His-Pro-NH2) is a peptide mainly produced by brain neurons. In mammals, hypophysiotropic TRH neurons of the paraventricular nucleus of the hypothalamus integrate metabolic information and drive the secretion of thyrotropin from the anterior pituitary, and thus the activity of the thyroid axis. Other hypothalamic or extrahypothalamic TRH neurons have less understood functions although pharmacological studies have shown that TRH has multiple central effects, such as promoting arousal, anorexia and anxiolysis, as well as controlling gastric, cardiac and respiratory autonomic functions. Two G-protein-coupled TRH receptors (TRH-R1 and TRH-R2) transduce TRH effects in some mammals although humans lack TRH-R2. TRH effects are of short duration, in part because the peptide is hydrolyzed in blood and extracellular space by a M1 family metallopeptidase, the TRH-degrading ectoenzyme (TRH-DE), also called pyroglutamyl peptidase II. TRH-DE is enriched in various brain regions but is also expressed in peripheral tissues including the anterior pituitary and the liver, which secretes a soluble form into blood. Among the M1 metallopeptidases, TRH-DE is the only member with a very narrow specificity; its best characterized biological substrate is TRH, making it a target for the specific manipulation of TRH activity. Two other substrates of TRH-DE, Glp-Phe-Pro-NH2 and Glp-Tyr-Pro-NH2, are also present in many tissues. Analogs of TRH resistant to hydrolysis by TRH-DE have prolonged central efficiency. Structure-activity studies allowed the identification of residues critical for activity and specificity. Research with specific inhibitors has confirmed that TRH-DE controls TRH actions. TRH-DE expression by β2-tanycytes of the median eminence of the hypothalamus allows the control of TRH flux into the hypothalamus-pituitary portal vessels and may regulate serum thyrotropin secretion. In this review we describe the critical evidences that suggest that modification of TRH-DE activity in tanycytes, and/or in other brain regions, may generate beneficial consequences in some central and metabolic disorders and identify potential drawbacks and missing information needed to test these hypotheses.

Extrapolative prediction using physically-based QSAR

Surflex-QMOD integrates chemical structure and activity data to produce physically-realistic models for binding affinity prediction . Here, we apply QMOD to a 3D-QSAR benchmark dataset and show broad applicability to a diverse set of targets. Testing new ligands within the QMOD model employs automated flexible molecular alignment, with the model itself defining the optimal pose for each ligand. QMOD performance was compared to that of four approaches that depended on manual alignments (CoMFA, two variations of CoMSIA, and CMF). QMOD showed comparable performance to the other methods on a challenging, but structurally limited, test set. The QMOD models were also applied to test a large and structurally diverse dataset of ligands from ChEMBL, nearly all of which were synthesized years after those used for model construction. Extrapolation across diverse chemical structures was possible because the method addresses the ligand pose problem and provides structural and geometric means to quantitatively identify ligands within a model’s applicability domain. Predictions for such ligands for the four tested targets were highly statistically significant based on rank correlation. Those molecules predicted to be highly active (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {pK}_i \ge 7.5$$\end{document}pKi≥7.5) had a mean experimental \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {pK}_i$$\end{document}pKi of 7.5, with potent and structurally novel ligands being identified by QMOD for each target.

Inhibitor and substrate binding by angiotensin-converting enzyme: quantum mechanical/molecular mechanical molecular dynamics studies

Angiotensin-converting enzyme (ACE) is an important zinc-dependent hydrolase responsible for converting the inactive angiotensin I to the vasoconstrictor angiotensin II and for inactivating the vasodilator bradykinin. However, the substrate binding mode of ACE has not been completely understood. In this work, we propose a model for an ACE Michaelis complex based on two known X-ray structures of inhibitor-enzyme complexes. Specifically, the human testis angiotensin-converting enzyme (tACE) complexed with two clinic drugs were first investigated using a combined quantum mechanical and molecular mechanical (QM/MM) approach. The structural parameters obtained from the 550 ps molecular dynamics simulations are in excellent agreement with the X-ray structures, validating the QM/MM approach. Based on these structures, a model for the Michaelis complex was proposed and simulated using the same computational protocol. Implications to ACE catalysis are discussed.

On the origin of the catalytic power of carboxypeptidase A and other metalloenzymes

Zinc metalloenzymes play a major role in key biological processes and carboxypeptidase-A (CPA) is a major prototype of such enzymes. The present work quantifies the energetics of the catalytic reaction of CPA and its mutants using the empirical valence bond (EVB) approach. The simulations allow us to quantify the origin of the catalytic power of this enzyme and to examine different mechanistic alternatives. The first step of the analysis used experimental information to determine the activation energy of each assumed mechanism of the reference reaction without the enzyme. The next step of the analysis involved EVB simulations of the reference reaction and then a calibration of the simulations by forcing them to reproduce the energetics of the reference reaction, in each assumed mechanism. The calibrated EVB was then used in systematic simulations of the catalytic reaction in the protein environment, without changing any parameter. The simulations reproduced the observed rate enhancement in two feasible general acid-general base mechanisms (GAGB-1 and GAGB-2), although the calculations with the GAGB-2 mechanism underestimated the catalytic effect in some treatments. We also reproduced the catalytic effect in the R127A mutant. The mutation calculations indicate that the GAGB-2 mechanism is significantly less likely than the GAGB-1 mechanism. It is also found, that the enzyme loses all its catalytic effect without the metal. This and earlier studies show that the catalytic effect of the metal is not some constant electrostatic effect, that can be assessed from gas phase studies, but a reflection of the dielectric effect of the specific environment.

A slow, tight-binding inhibitor of the zinc-dependent deacetylase LpxC of lipid A biosynthesis with antibiotic activity comparable to ciprofloxacin

The zinc-dependent enzyme LpxC catalyzes the deacetylation of UDP-3-O-acyl-GlcNAc, the first committed step of lipid A biosynthesis. Lipid A is an essential component of the outer membranes of most Gram-negative bacteria, including Escherichia coli, Salmonella enterica, and Pseudomonas aeruginosa, making LpxC an attractive target for antibiotic design. The inhibition of LpxC by a novel N-aroyl-l-threonine hydroxamic acid (CHIR-090) from a recent patent application (International Patent WO 2004/062601 A2 to Chiron and the University of Washington) is reported here. CHIR-090 possesses remarkable antibiotic activity against both E. coli and P. aeruginosa, comparable to that of ciprofloxacin. The biological activity of CHIR-090 is explained by its inhibition of diverse LpxC orthologues at low nanomolar concentrations, including that of Aquifex aeolicus, for which structural information is available. The inhibition of A. aeolicus LpxC by CHIR-090 occurs in two steps. The first step is rapid and reversible, with a K(i) of 1.0-1.7 nM, depending upon the method of assay. The second step involves the conversion of the EI complex with a half-life of about a minute to a tightly bound form. The second step is functionally irreversible but does not result in the covalent modification of the enzyme, as judged by electrospray ionization mass spectrometry. CHIR-090 is the first example of a slow, tight-binding inhibitor for LpxC and may be the prototype for a new generation of LpxC inhibitors with therapeutic applicability.

Ace revisited: a new target for structure-based drug design

Angiotensin-converting enzyme (ACE) is a chloride-dependent metalloenzyme that catalyses the hydrolytic cleavage of dipeptides from the carboxyl terminus of many regulatory oligopeptides. ACE is central to the renin–angiotensin system that regulates blood pressure, fluid homeostasis, and renal and vascular function. It is therefore a major target for cardiovascular therapies.

ACE inhibitors (for example, captopril, enalaprilat and lisinopril) have been on the market for more than 20 years. Side effects of treatment with ACE inhibitors include cough and angioedema.

ACE comprises an N- and a C-domain, each containing an active site with distinct substrates and activation properties. The design of domain-selective inhibitors might produce new drugs with improved safety and efficacy — this endeavour will be facilitated by the recent determination of the three-dimensional structure of ACE.

The C-domain seems to be primarily responsible for the regulation of blood pressure. Data indicate that C-domain-selective inhibitors will have less severe side effects than current-generation inhibitors, which generally target both the N- and C-domains.

In contrast to the C-domain, the N-domain seems to have relatively low affinity for the peptides that control blood pressure. It preferentially hydrolyses at least three other physiologically important peptides, so targeted inhibition of the N-domain might have novel therapeutic applications.

Current-generation angiotensin-converting enzyme (ACE) inhibitors are widely used for cardiovascular diseases, including high blood pressure, heart failure, heart attack and kidney failure, and have combined annual sales in excess of US $6 billion. However, the use of these ACE inhibitors, which were developed in the late 1970s and early 1980s, is hampered by common side effects. Moreover, we now know that ACE actually consists of two parts (called the N- and C-domains) that have different functions. Therefore, the design of specific domain-selective ACE inhibitors is expected to produce next-generation drugs that might be safer and more effective. Here we discuss the structural features of current inhibitors and outline how next-generation ACE inhibitors could be designed by using the three-dimensional molecular structure of human testis ACE. The ACE structure provides a unique opportunity for rational drug design, based on a combination of in silico modelling using existing inhibitors as scaffolds and iterative lead optimization to drive the synthetic chemistry.

Lipopolysaccharide endotoxins

Bacterial lipopolysaccharides (LPS) typically consist of a hydrophobic domain known as lipid A (or endotoxin), a nonrepeating "core" oligosaccharide, and a distal polysaccharide (or O-antigen). Recent genomic data have facilitated study of LPS assembly in diverse Gram-negative bacteria, many of which are human or plant pathogens, and have established the importance of lateral gene transfer in generating structural diversity of O-antigens. Many enzymes of lipid A biosynthesis like LpxC have been validated as targets for development of new antibiotics. Key genes for lipid A biosynthesis have unexpectedly also been found in higher plants, indicating that eukaryotic lipid A-like molecules may exist. Most significant has been the identification of the plasma membrane protein TLR4 as the lipid A signaling receptor of animal cells. TLR4 belongs to a family of innate immunity receptors that possess a large extracellular domain of leucine-rich repeats, a single trans-membrane segment, and a smaller cytoplasmic signaling region that engages the adaptor protein MyD88. The expanding knowledge of TLR4 specificity and its downstream signaling pathways should provide new opportunities for blocking inflammation associated with infection.

N5-(L-1-carboxyethyl)-L-ornithine synthase: physical and spectral characterization of the enzyme and its unusual low pKa fluorescent tyrosine residues

N5-(L-1-carboxyethyl)-L-ornithine synthase [E.C. 1.5.1.24] (CEOS) from Lactococcus lactis has been cloned, expressed, and purified from Escherichia coli in quantities sufficient for characterization by biophysical methods. The NADPH-dependent enzyme is a homotetramer (Mr approximately equal to 140,000) and in the native state is stabilized by noncovalent interactions between the monomers. The far-ultraviolet circular dichroism spectrum shows that the folding pattern of the enzyme is typical of the alpha,beta family of proteins. CEOS contains one tryptophan (Trp) and 19 tyrosines (Tyr) per monomer, and the fluorescence spectrum of the protein shows emission from both Trp and Tyr residues. Relative to N-acetyltyrosinamide, the Tyr quantum yield of the native enzyme is about 0.5. All 19 Tyr residues are titratable and, of these, two exhibit the uncommonly low pKa of approximately 8.5, 11 have pKa approximately 10.75, and the remaining six titrate with pKa approximately 11.3. The two residues with pKa approximately 8.5 contribute approximately 40% of the total tyrosine emission, implying a relative quantum yield >1, probably indicating Tyr-Tyr energy transfer. In the presence of NADPH, Tyr fluorescence is reduced by 40%, and Trp fluorescence is quenched completely. The latter result suggests that the single Trp residue is either at the active site, or in proximity to the sequence GSGNVA, that constitutes the beta alphabeta fold of the nucleotide-binding domain. Chymotrypsin specifically cleaves native CEOS after Phe255. Although inactivated by this single-site cleavage of the subunit, the enzyme retains the capacity to bind NADPH and tetramer stability is maintained. Possible roles in catalysis for the chymotrypsin sensitive loop and for the low pKa Tyr residues are discussed.

RXP 407, a phosphinic peptide, is a potent inhibitor of angiotensin I converting enzyme able to differentiate between its two active sites

The human somatic angiotensin converting enzyme (ACE) contains two homologous domains, each bearing a zinc-dependent active site. All of the synthetic inhibitors of this enzyme used in clinical applications interact with these two active sites to a similar extent. Recently, several lines of evidence have suggested that the N-terminal active site of ACE might be involved in specific hydrolysis of some important physiological substrates, like Acetyl-Seryl-Aspartyl-Lysyl-Proline, a negative regulator of hematopoietic stem cell differentiation and proliferation. These findings have stimulated studies aimed at identifying new ACE inhibitors able to block only one of the two active sites of this enzyme. By screening phosphinic peptide libraries, we discovered a phosphinic peptide Ac-Asp-(L)Phepsi(PO2-CH2)(L)Ala-Ala-NH2, called RXP 407, which is able to differentiate the two ACE active sites, with a dissociation constant three orders of magnitude lower for the N-domain of the enzyme. The usefulness of a combinatorial chemistry approach to develop new lead structures is underscored by the unusual chemical structure of RXP 407, as compared with classical ACE inhibitors. As a highly potent and selective inhibitor of the N-terminal active site of wild ACE (Ki = 12 nM), RXP 407, which is metabolically stable in vivo, may lead to a new generation of ACE inhibitors able to block in vivo only a subset of the different functions regulated by ACE.

Angiotensin II: biosynthesis, molecular recognition, and signal transduction

1. Angiotensin II is a well-known vasopressive octapeptide that is the principal end-product of the renin-angiotensin system. In addition to its tonic effect on vascular smooth muscle cells, it also stimulates aldosterone secretion from the adrenals and promotes sodium reabsorption through renal tubular cells. 2. These physiological functions have been appreciated for some time, but as details of the molecular and cell biology of the angiotensin response mechanism become understood, it is increasingly apparent that the hormone has a much broader repertoire. Its functional variability is made possible by (i) different enzymatic routes for its generation, (ii) different receptors distributed in different tissues, (iii) different mechanisms for receptor regulation, and (iv) different signal transduction pathways. 3. This insight is the direct consequence of advances in pharmacology that led first to inhibitors of angiotensin converting enzyme and later to angiotensin II receptor antagonists. This review looks at the current status of angiotensin biochemistry and physiology and provides a basis for anticipation of future developments.

Identification of positive and negative transcriptional regulatory elements of the rabbit angiotensin-converting enzyme gene

The two tissue-specific mRNAs encoding the isozymes of rabbit angiotensin-converting enzyme (ACE) are generated from the same gene by alternative choice of two transcription initiation sites 5.7 kb apart. In the current study, we have characterized the regulatory sites controlling the transcription of the larger pulmonary isozyme mRNA. For this purpose, reporter genes driven by varying lengths of upstream region of the ACE gene were transfected into ACE-producing cells. Our results demonstrated that the transcription of this gene is primarily driven by positive elements within the first 274 bp DNA upstream of the transcription initiation site. The reporter gene driven by this region was expressed in two ACE-producing cells but not in two ACE-non-producing cells thereby establishing its tissue specificity. Our experiments also revealed the existence of a strong negative element located between -692 and -610 positions. This element suppressed the expression of the reporter gene in a dose-dependent and position and orientation-independent fashion thus suggesting that it is a true silencer element. It could also repress the expression of a reporter gene driven by the heterologous strong promoter of the beta-actin gene. The repressing effects of the negative element could be partially overcome by cotransfecting the isolated negative element along with the reporter gene containing the negative element. This result was possibly due to the functional removal of a limiting trans-acting factor which binds to this element. Electrophoretic mobility shift assays revealed that the negative element can form several complexes with proteins present in the nuclear extract of an ACE-producing cell line. At least part of the negative element is strongly conserved in the upstream regions of the human and mouse ACE genes.

Human small intestinal angiotensin-converting enzyme: intracellular transport, secretion and glycosylation

Human intestinal angiotensin-converting enzyme (ACE) exists in the brush-border membrane as a monomeric protein of apparent molecular mass 184 kDa. It is associated with the membrane via a hydrophobic segment and has a transmembrane orientation [Naim (1992) Biochem. J. 286, 451-457]. In addition to the membrane-bound form (ACEm), hydrophilic forms of ACE (ACEsec) can be identified in biosynthetically labelled intestinal cells. Thus the culture medium of biosynthetically labelled human biopsy samples contains an ACE molecule which has an apparent molecular mass similar to that of its membrane-bound counterpart. The secreted ACEsec forms follow a precursor/product relationship with the mature ACE molecule. The effect of the monomeric structure of ACE in its intracellular transport and secretion was investigated by pulse-chase experiments on human biopsy samples labelled with [35S]methionine. The results reveal 2-3-fold slower transport of ACE from the endoplasmic reticulum (ER) to the Golgi as compared with the homodimeric proteins dipeptidylpeptidase IV and aminopeptidase N. Further, the transport kinetics of ACE are comparable with those of human sucrase-isomaltase and human maltase-glucoamylase, two brush-border disaccharidases that do not form homodimers in the ER of human small-intestinal cells. These findings strongly suggest that homodimerization of brush-border proteins may influence the rate of transport of these proteins from the ER to the Golgi. The effect of glycosylation on the transport and secretion of ACE was investigated by utilizing several inhibitors of glycan processing. Here, secretion of ACE molecules continued to take place, albeit to a considerably lesser extent. In fact, approx. 2-fold less ACE molecules were secreted in the presence of inhibitors of ER glucosidases I and II and cis-Golgi mannosidase-I, suggesting that carbohydrate processing is important in the attainment of a transport-competent conformation.

Endopeptidase 3.4.24.11 converts N-1-(R,S)carboxy-3-phenylpropyl-Ala-Ala-Phe-p-carboxyanilide into a potent inhibitor of angiotensin-converting enzyme

It was reported recently that N-1-(R,S)carboxy-3-phenylpropyl-Ala-Ala-Phe-p-carboxyanilide (CPP-A-A-F-pAB), an inhibitor of endopeptidase 3.4.24.15 (E-24.15), also inhibits angiotensin-converting enzyme (ACE) from rabbit lung. We have found that this compound is without effect on ACE purified from pig kidney, at a concentration some 1000-fold greater than the Ki reported for inhibition of the enzyme from lung. However, preincubation of CPP-A-A-F-pAB with neutral endopeptidase 3.4.24.11 (E-24.11) does result in potent inhibitory effects on ACE. We have shown this to be due to formation of a fragment, CPP-A-A, the structure of which is closely related to ACE inhibitors such as enalaprilat. CPP-A-A was found to be a potent inhibitor of pig ACE. Under the conditions used it had an IC50 value of 1.6 x 10(-8) M, compared with the value obtained for captopril of 7.5 x 10(-10) M. These results have important implications for studies of E-24.15 when using CPP-A-A-F-pAB in vivo or in crude tissue extracts where E-24.11 might also be present.

Sperm-specific expression of angiotensin-converting enzyme (ACE) is mediated by a 91-base-pair promoter containing a CRE-like element

The gene encoding the testis isozyme of angiotensin-converting enzyme (testis ACE) is one example of the many genes expressed uniquely during spermatogenesis. This protein is expressed by developing germ cells late in their development and results from the activation of a sperm-specific promoter that is located within intron 12 of the gene encoding the somatic isozyme of ACE. In vitro transcription, DNase footprinting, gel shift assays, and transgenic mouse studies have been used to define the minimal testes ACE promoter and to characterize DNA-protein interactions mediating germ cell-specific expression. These studies show that proper cell- and stage-specific expression of testis ACE requires only a small portion of the immediate upstream sequence extending to -91. A critical motif within this core promoter is a cyclic AMP-responsive element sequence that interacts with a testis-specific transactivating factor. Since this putative cyclic AMP-responsive element has been conserved within the testis ACE promoters of different species and is found at the same site in other genes that are expressed specifically in the testis, it may provide a common mechanism for the recognition of sperm-specific promoters.

Sperm-specific expression of angiotensin-converting enzyme (ACE) is mediated by a 91-base-pair promoter containing a CRE-like element

The gene encoding the testis isozyme of angiotensin-converting enzyme (testis ACE) is one example of the many genes expressed uniquely during spermatogenesis. This protein is expressed by developing germ cells late in their development and results from the activation of a sperm-specific promoter that is located within intron 12 of the gene encoding the somatic isozyme of ACE. In vitro transcription, DNase footprinting, gel shift assays, and transgenic mouse studies have been used to define the minimal testes ACE promoter and to characterize DNA-protein interactions mediating germ cell-specific expression. These studies show that proper cell- and stage-specific expression of testis ACE requires only a small portion of the immediate upstream sequence extending to -91. A critical motif within this core promoter is a cyclic AMP-responsive element sequence that interacts with a testis-specific transactivating factor. Since this putative cyclic AMP-responsive element has been conserved within the testis ACE promoters of different species and is found at the same site in other genes that are expressed specifically in the testis, it may provide a common mechanism for the recognition of sperm-specific promoters.

A comparison of the zinc contents and substrate specificities of the endothelial and testicular forms of porcine angiotensin converting enzyme and the preparation of isoenzyme-specific antisera

Angiotensin converting enzyme (ACE; EC 3.4.15.1) was purified from porcine kidney and lung (endothelial isoenzyme) and testis (testicular isoenzyme) by affinity chromatography on lisinopril-2.8 nm-Sepharose. Atomic-absorption spectroscopy revealed that ACE purified from kidney and lung contained 2.58 and 2.35 atoms of zinc per molecule of enzyme (M(r) 147,000) respectively. In contrast, ACE purified from testis contained only 1.58 atoms of zinc per molecule of enzyme (M(r) 80,000). Thus it would appear that both putative zinc-binding sites in endothelial ACE contain zinc and may therefore be catalytically active. No differences were observed in the pattern of products generated on hydrolysis of benzoyl (Bz)-Gly-His-Leu, substance P, luteinizing-hormone-releasing hormone (LH-RH) and its analogue, des-Gly10-LH-RH-ethylamide, by kidney and testicular ACE. There was also no difference in the initial rates of hydrolysis of Bz-Gly-His-Leu or substance P by the two isoenzymes, although LH-RH and its analogue were hydrolysed twice as rapidly by kidney ACE. It is therefore unlikely that the N-terminal catalytic site in porcine endothelial ACE is predominantly responsible for the atypical cleavage of LH-RH generating the N-terminal tripeptide. Two polyclonal antisera were raised to the affinity-purified forms of pig kidney and testicular ACE. Isoenzyme-specific antisera were then isolated from these by absorbing out those antibodies recognizing determinants on the other isoenzyme. Immunoelectrophoretic blot analyses and immunofluorescent staining of sections of pig kidney were used to demonstrate the specificity of the antisera. Immunofluorescent staining of sections of pig testis with the antiserum specific to testicular ACE localized testicular ACE solely to the lumen of the seminiferous tubules, whereas the antiserum specific to endothelial ACE revealed the presence of this isoenzyme only in blood vessels. The antiserum to endothelial ACE, which recognizes determinants in the unique N-terminal domain, was investigated as a possible specific inhibitor of the N-terminal catalytic site. Although this antiserum failed to inhibit testicular ACE, the effect on the activity of endothelial ACE appeared to be due to inhibition of both the N- and C-terminal catalytic sites.

Angiotensin-converting enzyme of the human small intestine. Subunit and quaternary structure, biosynthesis and membrane association

Angiotensin-converting enzyme (ACE) was isolated from detergent-derived extracts of human intestinal brush-border membranes (BBMs) by immunoprecipitation using a monoclonal antibody. Analysis of the immunoprecipitates by SDS/PAGE revealed a polypeptide of apparent M(r) 184,000 under reducing and non-reducing conditions, indicating that ACE does not contain intermolecular disulphide bridges. The quaternary structure of ACE was examined using cross-linking experiments with dithiobis[succinimidylpropionate] (DSP) and density gradient centrifugation on sucrose gradients. Both approaches demonstrated that ACE is assembled in the membrane as a monomer. By contrast, the control glycoprotein aminopeptidase N (ApN) exists as a dimer. Biosynthetic labelling experiments in intestinal tissue explants demonstrated that the 184,000-M(r) protein is generated from a single-polypeptide, mannose-rich precursor of ACE (M(r) 175,000) by modification of the carbohydrate side-chains in the Golgi apparatus. The mode of association of the mature form of the enzyme with BBMs was investigated by hydrophobic labelling of right-side-out brush-border vesicles with the photoactivatable carbene-generating reagent 125I-labelled 3-(trifluoromethyl)-3-(m[formylamino]phenyl)diazirine (125I-labelled TID), followed by treatment with trypsin at dilutions that do not cause substantial degradation of ACE. These studies demonstrated that ACE is associated with the membrane via a hydrophobic segment. Furthermore, treatment of 35S-labelled inside-out membrane vesicles with trypsin revealed that ACE possesses a cytoplasmic tail, and therefore has a transmembraneous orientation.

Use of alternative polyadenylation sites for tissue-specific transcription of two angiotensin-converting enzyme mRNAs

The pulmonary isozyme of rabbit angiotensin-converting enzyme (ACE) is encoded by an mRNA of about 5 kb. cDNA clones corresponding to different parts of this mRNA have been isolated and the complete nucleotide sequences of both the coding and non-coding regions of the mRNA have been determined. The encoded protein has 1309 residues with a 33 amino acids-long signal peptide at the amino terminus and a potential membrane-anchoring domain near the carboxyl terminus. There is a strong sequence homology between two regions of the rabbit cDNA and between the rabbit, human, and mouse cDNAs. Comparison of the nucleotide sequences of the 3' untranslated regions of rabbit pulmonary and testicular ACE cDNAs revealed that the testicular cDNA is nested within the pulmonary cDNA at the 3' end. A rabbit genomic clone encompassing this region was isolated and partially sequenced. It was shown that the gene contains two potential polyadenylation sites 628 bp apart within one exon. Northern analyses with an appropriate oligonucleotide probe confirmed that the proximal polyadenylation site is used exclusively for terminating the testicular mRNA whereas the distal one is used exclusively for the pulmonary mRNA. These results demonstrated that the transcription of the two mRNAs encoding the two ACE isozymes not only initiates at two alternative tissue-specific sites which are 5.7 kb apart but the mRNAs also get polyadenylated at two alternative sites which are 628 bp apart.

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.

Transcription of testicular angiotensin-converting enzyme (ACE) is initiated within the 12th intron of the somatic ACE gene

Angiotensin-converting enzyme (ACE) is a zinc-containing dipeptidyl carboxypeptidase that catalyzes the conversion of angiotensin I to the potent vasoconstrictor angiotensin II. By analyzing cDNA and genomic DNA, we have constructed a consensus sequence encoding the testis isozyme of mouse ACE. Testis ACE cDNA contains 2,435 base pairs and encodes a protein of 732 amino acids. The N-terminal 66 amino acids are unique to the testis isozyme, while the remaining 666 are identical to the carboxyl half of mouse somatic ACE. The overall conservation of amino acid sequence between the testis isozymes of the mouse, rabbit, and human is 78 to 84%. The conservation of amino acids for the N-terminal domain uniquely expressed within the testis is 63 to 67% between these species. Primer extension and RNase protection experiments show that RNA transcription of the testis ACE isozyme begins 16 or 17 bases upstream from the translation start site. A sequence element resembling a TATA box is found 25 bases 5' of the transcription start site. To create its unique isozyme of ACE, the testis begins mRNA transcription in the middle of the exonic-intronic structure of somatic ACE, within a sequence treated as an intron by somatic tissues. Testis ACE is not the result of alternative RNA splicing but seems due to the start of transcription at a unique site within the ACE gene.

Transcription of testicular angiotensin-converting enzyme (ACE) is initiated within the 12th intron of the somatic ACE gene

Angiotensin-converting enzyme (ACE) is a zinc-containing dipeptidyl carboxypeptidase that catalyzes the conversion of angiotensin I to the potent vasoconstrictor angiotensin II. By analyzing cDNA and genomic DNA, we have constructed a consensus sequence encoding the testis isozyme of mouse ACE. Testis ACE cDNA contains 2,435 base pairs and encodes a protein of 732 amino acids. The N-terminal 66 amino acids are unique to the testis isozyme, while the remaining 666 are identical to the carboxyl half of mouse somatic ACE. The overall conservation of amino acid sequence between the testis isozymes of the mouse, rabbit, and human is 78 to 84%. The conservation of amino acids for the N-terminal domain uniquely expressed within the testis is 63 to 67% between these species. Primer extension and RNase protection experiments show that RNA transcription of the testis ACE isozyme begins 16 or 17 bases upstream from the translation start site. A sequence element resembling a TATA box is found 25 bases 5' of the transcription start site. To create its unique isozyme of ACE, the testis begins mRNA transcription in the middle of the exonic-intronic structure of somatic ACE, within a sequence treated as an intron by somatic tissues. Testis ACE is not the result of alternative RNA splicing but seems due to the start of transcription at a unique site within the ACE gene.

Novel activity of angiotensin-converting enzyme. Hydrolysis of cholecystokinin and gastrin analogues with release of the amidated C-terminal dipeptide

ACE (angiotensin-converting enzyme; peptidyl dipeptidase A; EC 3.4.15.1), cleaves C-terminal dipeptides from active peptides containing a free C-terminus. We investigated the hydrolysis of cholecystokinin-8 [CCK-8; Asp-Tyr(SO3H)-Met-Gly-Trp-Met-Asp-Phe-NH2] and of various gastrin analogues by purified rabbit lung ACE. Although these peptides are amidated at their C-terminal end, they were metabolized by ACE to several peptide fragments. These fragments were analysed by h.p.l.c., isolated and identified by comparison with synthetic fragments, and by amino acid analysis. The initial and major site of hydrolysis was the penultimate peptide bond, which generated a major product, the C-terminal amidated dipeptide Asp-Phe-NH2. As a secondary cleavage, ACE subsequently released di- or tri-peptides from the C-terminal end of the remaining N-terminal fragments. The cleavage of CCK-8 and gastrin analogues was inhibited by ACE inhibitors (Captopril and EDTA), but not by other enzyme inhibitors (phosphoramidon, thiorphan, bestatin etc.). Hydrolysis of [Leu15]gastrin-(14-17)-peptide [Boc (t-butoxycarbonyl)-Trp-Leu-Asp-Phe-NH2] in the presence of ACE was found to be dependent on the chloride-ion concentration. Km values for the hydrolysis of CCK-8, [Leu15]gastrin-(11-17)-peptide and Boc-[Leu15]gastrin-(14-17)-peptide at an NaCl concentration of 300 mM were respectively 115, 420 and 3280 microM, and the catalytic constants were about 33, 115 and 885 min-1. The kcat/Km for the reactions at 37 degrees C was approx. 0.28 microM-1.min-1, which is approx. 35 times less than that reported for the cleavage of angiotensin I. These results suggest that ACE might be involved in the metabolism in vivo of CCK and gastrin short fragments.

Pig kidney angiotensin converting enzyme. Purification and characterization of amphipathic and hydrophilic forms of the enzyme establishes C-terminal anchorage to the plasma membrane

Angiotensin converting enzyme from pig kidney was isolated by affinity chromatography after solubilization from the membrane by one of four different procedures. Solubilization with Triton X-100, trypsin or by an endogenous activity in microvillar membranes all generated hydrophilic forms of the enzyme as assessed by phase separation in Triton X-114 and failure to incorporate into liposomes. Only when solubilization and purification was effected by Triton X-100 in the presence of EDTA (10 mM) could an amphipathic form of the enzyme (membrane- or m-form) be generated. The m-form of angiotensin converting enzyme (ACE) appeared slightly larger (Mr approx. 180,000) than the hydrophilic forms (Mr approx. 175,000) after SDS/polyacrylamide-gel electrophoresis, and the m-form incorporated into liposomes, consistent with retention of the membrane anchor. The m-form of ACE showed an N-terminal sequence identical with that of preparations of enzyme isolated after solubilization with detergent alone (d-form), with trypsin (t-form) or by the endogenous mechanism (e-form). These data imply that ACE is anchored to the plasma membrane via its C-terminus, in contrast with the N-terminal anchorage of endopeptidase-24.11. No release of ACE from the membrane could be detected with a variety of phospholipases, including bacterial phosphatidylinositol-specific phospholipases C, although an endogenous EDTA-sensitive membrane-associated hydrolase was capable of releasing a soluble, hydrophilic, form of the enzyme.

Renin inhibitors

Pharmacological intervention in the renin-angiotensin system (RAS) by inhibition of angiotensin-converting enzyme (ACE) is an effective therapy for the majority of hypertensive patients and a major advance in the treatment of hypertension and congestive heart failure. The success achieved with ACE inhibitors has increased interest in inhibitors of renin. Renin catalyzes the first and rate-limiting step of the RAS and, unlike ACE, has a high specificity for its endogenous protein substrate. A therapeutic agent that inhibits this specific reaction could have advantages over antihypertensive drugs with less specific modes of action. Although inhibitors of renin have been studied for over two decades, only recently has substantial progress been made toward potent, low molecular weight inhibitors likely to become useful therapeutic agents. Recent advances in the development of renin inhibitors, especially progress toward clinically useful inhibitors, is reviewed.

The behaviour of leucine aminopeptidase towards thionopeptides

Thionoleucine S-anilide (Leut-anilide), Leut-Gly-OEt and Leut-Phe-OMe were synthesized and shown to be competitive inhibitors of leucine aminopeptidase from pig kidney. The kinetics of inhibition were determined in the presence of leucine 4-methylcoumarin-7-amide as substrate. Although the compounds showed only moderate inhibitory potency, it was found that all were resistant to hydrolysis by the enzyme, in contrast with the reported behaviour of some thionopeptide analogues of substrates for other Zn2+-peptidases such as carboxypeptidase A and angiotensin-converting enzyme.

BW A575C, a chemically novel agent with angiotensin converting enzyme inhibitor and beta-adrenoceptor-blocking properties

BW A575C (N-(1-(S)-carboxy-5-[4(3-isopropylamino-2-(R, S)-hydroxypropoxy)indole-2- carboxamido]pentyl)-(R, S)-alanyl-(S)-proline) is a chemically novel agent which exhibits in a single molecule both angiotensin converting enzyme (ACE) inhibition and beta-adrenoceptor-blocking properties. BW A575C produced a competitive blockade of heart rate responses to isoprenaline in a guinea-pig right atrial preparation (pKB 7.18 +/- 0.05, cf. pindolol 8.9 +/- 0.7). BW A575C inhibited a partially purified preparation of ACE obtained from rabbit lung (IC50 10.7 +/- 2.1 nM, cf. enalaprilat, 4.4 +/- 0.8 nM). Intravenous administration of BW A575C (1-100 micrograms kg-1 min-1) to the pithed rat inhibited in a dose-dependent fashion both angiotensin I-induced pressor responses and isoprenaline-induced tachycardia. Dose-ratios obtained from such studies demonstrated that, in this preparation, BW A575C was approximately 100 times more active as an ACE inhibitor than as a beta-adrenoceptor blocking agent. Intravenous administration of BW A575C (1 mg kg-1) to the conscious rat inhibited angiotensin I-induced pressor responses, being approximately equipotent to enalapril and 10 times more potent than captopril. At the same dose, BW A575C had a similar duration of action as an ACE inhibitor to enalapril. Intravenous administration of BW A575C (1 mg kg-1) to either conscious dogs or rats inhibited both angiotensin I-induced pressor responses and isoprenaline-induced heart rate responses. Dose-ratios obtained from such studies demonstrated that in these species, BW A575C was 2-10 times more active as an ACE inhibitor than as a beta-adrenoceptor blocking agent.

The broad substrate specificity of human angiotensin I converting enzyme

Angiotensin I converting enzyme (kininase II; ACE) has been described as a peptidyldipeptidase or dipeptidyl carboxypeptidase (EC 3.4.15.1) of the pulmonary endothelial cells, which liberates angiotensin II or inactivates kinins. However, ACE has a much wider distribution and substrate specifity; it is concentrated in human epithelial cells (e.g. brush border of the kidney, placenta, intestine and choroid plexus), neuroepithelial cells (subfornical organ, pallidonigral dendrites, median eminence) and male genital tract (testes, prostate, epididymides, seminal plasma). Its substrates include enkaphalins, the C-terminal extended proenkephalins and a protected chemotactic tripeptide. Recent, mostly in vitro studies with purified ACE, indicate that ACE also cleaves peptides by other than peptidyldipeptidase action. Homogeneous human ACE inactivated substance P in spite of its blocked C-terminus (Met11-NH2) primarily by releasing the C-terminal tripeptide. A blocked C-terminal tripeptide, Arg-Pro-Gly-NH2 was also released from the luteinizing hormone releasing hormone (LHRH). Although ACE shares many properties with carboxypeptidases, it surprisingly cleaves the N-terminal tripeptide greater than Glu1-His2-Trp3 from LHRH. Because human ACE hydrolyzes a variety of peptide hormones, actions of its inhibitors may go well beyond blocking the conversion of angiotensin I.