Substrate dependence of angiotensin I-converting enzyme inhibition: captopril displays a partial selectivity for inhibition of N-acetyl-seryl-aspartyl-lysyl-proline hydrolysis compared with that of angiotensin I

Diverse biological functions of the renin-angiotensin system

The renin-angiotensin system (RAS) has been widely known as a circulating endocrine system involved in the control of blood pressure. However, components of RAS have been found to be localized in rather unexpected sites in the body including the kidneys, brain, bone marrow, immune cells, and reproductive system. These discoveries have led to steady, growing evidence of the existence of independent tissue RAS specific to several parts of the body. It is important to understand how RAS regulates these systems for a variety of reasons: It gives a better overall picture of human physiology, helps to understand and mitigate the unintended consequences of RAS-inhibiting or activating drugs, and sets the stage for potential new therapies for a variety of ailments. This review fulfills the need for an updated overview of knowledge about local tissue RAS in several bodily systems, including their components, functions, and medical implications.

Structural basis for the inhibition of human angiotensin-1 converting enzyme by fosinoprilat

Human angiotensin I-converting enzyme (ACE) has two isoforms, somatic ACE (sACE) and testis ACE (tACE). The functions of sACE are widespread, with its involvement in blood pressure regulation most extensively studied. sACE is composed of an N-domain (nACE) and a C-domain (cACE), both catalytically active but have significant structural differences, resulting in different substrate specificities. Even though ACE inhibitors are used clinically, they need much improvement because of serious side effects seen in patients (~ 25-30%) with long-term treatment due to nonselective inhibition of nACE and cACE. Investigation into the distinguishing structural features of each domain is therefore of vital importance for the development of domain-specific inhibitors with minimal side effects. Here, we report kinetic data and high-resolution crystal structures of both nACE (1.75 Å) and cACE (1.85 Å) in complex with fosinoprilat, a clinically used inhibitor. These structures allowed detailed analysis of the molecular features conferring domain selectivity by fosinoprilat. Particularly, altered hydrophobic interactions were observed to be a contributing factor. These experimental data contribute to improved understanding of the structural features that dictate ACE inhibitor domain selectivity, allowing further progress towards designing novel 2nd-generation domain-specific potent ACE inhibitors suitable for clinical administration, with a variety of potential future therapeutic benefits. DATABASE: The atomic coordinates and structure factors for nACE-fosinoprilat and cACE-fosinoprilat structures have been deposited with codes 7Z6Z and 7Z70, respectively, in the RCSB Protein Data Bank, www.pdb.org.

Protein Hydrolysates from Brewing By-Products as Natural Alternatives to ACE-Inhibitory Drugs for Hypertension Management

Simple Summary

Hypertension is the predominant risk factor for cardiovascular disease, which is the leading cause of mortality and morbidity worldwide. The search for natural compounds with antihypertensive properties, such as bioactive peptides from brewing by-products (spent grain and yeast), which are less likely to cause severe side effects compared with anti-hypertensive drugs, is of major importance to reduce cardiovascular events. Since oral intake of these peptides may modify their expected effects, the aim of the present study was to simulate oral administration and evaluate the impact of gastrointestinal digestion, intestinal absorption, and liver metabolism on the effectiveness of those bioactive peptides and determine their potential to be used as supplements or nutraceuticals as well as anti-hypertensive drugs before moving forward to animal studies. Results showed that peptides derived from the brewing industry maintain or present higher antihypertensive activity after simulation of oral administration, validating the usefulness of these peptides to reduce the risk, ameliorate, or treat primary hypertension. In conclusion, this study reinforces, through in vitro studies, the benefits of oral administrated brewing bioactive peptides to directly manage hypertension by lowering blood pressure, thus being promising compounds.

Abstract

The treatment of hypertension is of major importance to reduce the risk of cardiovascular disease, the leading cause of death worldwide. Angiotensin-converting enzyme (ACE) inhibitors are anti-hypertensive drugs associated with several side effects. Natural products, namely bioactive peptides from brewing by-products, brewers’ spent grain (BSG), and yeast (BSY), are promising alternatives since they can inhibit ACE in vitro. However, the oral intake of these peptides may modify their expected inhibitory effect owing to possible changes in active peptides’ bioavailability, which have not been assessed so far. The goal of this study was to simulate oral administration to evaluate BSG/BSY peptides’ effectiveness by submitting protein hydrolysates sequentially to simulated gastrointestinal digestion, intestinal absorption (Caco-2 cells), and liver metabolism (HepG2 cells). MTT assay was used to assess BSG/BSY protein hydrolysates safeness. The ACE-inhibitory potential of initial and final protein hydrolysates (BSY, BSG, and a new product, MIX) were tested using a fluorometric assay and compared with captopril (1 µM, an ACE-inhibitory drug). Simulation of oral administration greatly increased BSY and MIX protein hydrolysates’ ACE-inhibitory capacity, though final MIX and BSG revealed greater ACE-inhibitory potential than captopril. Notwithstanding, all final protein hydrolysates presented ACE-inhibitory capacity, thus being promising compounds to manage hypertension.

High-volume endurance exercise training stimulates hematopoiesis by increasing ACE NH2-terminal activity

One of the health benefits of endurance exercise training (ET) is the stimulation of hematopoiesis. However, the mechanisms underlying ET-induced hematopoietic adaptations are understudied. N-Acetyl-Seryl-Aspartyl-Lysyl-Proline (Ac-SDKP) inhibits proliferation of early hematopoietic progenitor cells. The angiotensin I-converting enzyme (ACE) NH2-terminal promotes hematopoiesis by inhibiting the anti-hematopoietic effect of Ac-SDKP. Here we demonstrate for the first time the role of ACE NH2-terminal in ET-induced hematopoietic adaptations. Wistar rats were subjected to 10 weeks of moderate-(T1) and high-(T2) volume swimming-training. Although both protocols induced classical ET-associated adaptations, only T2 increased plasma ACE NH2-domain activity (by 40%, P=0.0003) and reduced Ac-SDKP levels (by 50%, P<0.0001). T2 increased the number of hematopoietic stem cells (HSCs; ∼200%, P=0.0008), early erythroid progenitor colonies (∼300%, P<0.0001) and reticulocytes (∼500%, P=0.0007), and reduced erythrocyte lifespan (∼50%, P=0.022). Following, Wistar rats were subjected to T2 or T2 combined with ACE NH2-terminal inhibition (captopril (Cap) treatment: 10 mg.kg-1.day-1). T2 combined with ACE NH2-terminal inhibition prevented Ac-SDKP decrease and attenuated ET-induced hematopoietic adaptations. Altogether, our findings show that ET-induced hematopoiesis was at least partially associated with increased ACE NH2-terminal activity and reduction in the hematopoietic inhibitor Ac-SDKP.

COVID-19 outbreak: history, mechanism, transmission, structural studies and therapeutics

PURPOSE

The coronavirus outbreak emerged as a severe pandemic, claiming more than 0.8 million lives across the world and raised a major global health concern. We survey the history and mechanism of coronaviruses, and the structural characteristics of the spike protein and its key residues responsible for human transmissions.

METHODS

We have carried out a systematic review to summarize the origin, transmission and etiology of COVID-19. The structural analysis of the spike protein and its disordered residues explains the mechanism of the viral transmission. A meta-data analysis of the therapeutic compounds targeting the SARS-CoV-2 is also included.

RESULTS

Coronaviruses can cross the species barrier and infect humans with unexpected consequences for public health. The transmission rate of SARS-CoV-2 infection is higher compared to that of the closely related SARS-CoV infections. In SARS-CoV-2 infection, intrinsically disordered regions are observed at the interface of the spike protein and ACE2 receptor, providing a shape complementarity to the complex. The key residues of the spike protein have stronger binding affinity with ACE2. These can be probable reasons for the higher transmission rate of SARS-CoV-2. In addition, we have also discussed the therapeutic compounds and the vaccines to target SARS-CoV-2, which can help researchers to develop effective drugs/vaccines for COVID-19. The overall history and mechanism of entry of SARS-CoV-2 along with structural study of spike-ACE2 complex provide insights to understand disease pathogenesis and development of vaccines and drugs.

A SARS-CoV-2-Human Protein-Protein Interaction Map Reveals Drug Targets and Potential Drug-Repurposing

An outbreak of the novel coronavirus SARS-CoV-2, the causative agent of COVID-19 respiratory disease, has infected over 290,000 people since the end of 2019, killed over 12,000, and caused worldwide social and economic disruption1,2. There are currently no antiviral drugs with proven efficacy nor are there vaccines for its prevention. Unfortunately, the scientific community has little knowledge of the molecular details of SARS-CoV-2 infection. To illuminate this, we cloned, tagged and expressed 26 of the 29 viral proteins in human cells and identified the human proteins physically associated with each using affinity-purification mass spectrometry (AP-MS), which identified 332 high confidence SARS-CoV-2-human protein-protein interactions (PPIs). Among these, we identify 67 druggable human proteins or host factors targeted by 69 existing FDA-approved drugs, drugs in clinical trials and/or preclinical compounds, that we are currently evaluating for efficacy in live SARS-CoV-2 infection assays. The identification of host dependency factors mediating virus infection may provide key insights into effective molecular targets for developing broadly acting antiviral therapeutics against SARS-CoV-2 and other deadly coronavirus strains.

Advances in Structural Biology of ACE and Development of Domain Selective ACE-inhibitors

BACKGROUND

The Angiotensin-I converting enzyme (ACE) is one of the most important components of the renin-angiotensin-aldosterone system controlling blood pressure and renal functions. Inhibitors of ACE are first line therapeutics used in the treatment of hypertension and related cardiovascular diseases. Somatic ACE consists of two homologous catalytic domains, the C- and N-domains. Recent findings have shown that although both domains are highly homologous in structure, they may have different physiological functions. The C-domain is primarily involved in the control of blood pressure, in contrast to the N-domain that is engaged in the regulation of hematopoietic stem cell proliferation. The currently available ACE inhibitors have some adverse effects that can be attributed to the non-selective inhibition of both domains. In addition, specific Ndomain inhibitors have emerged as potential antifibrotic drugs. Therefore, ACE is still an important drug target for the development of novel domain-selective drugs not only for the cardiovascular system but also for other systems.

OBJECTIVE

Detailed structural information about interactions in the protein-ligand complex is crucial for rational drug design. This review highlights the structural information available from crystallographic data which is essential for the development of domain selective inhibitors of ACE.

METHODS

Over eighty crystal complexes of ACE are placed into the Protein Database. An overview of X-ray ACE complexes with various inhibitors in C- and N-domains and an analysis of their binding mode have given mechanistic explanation of the structural determinants of selective ligand binding. In addition, ACE domain selective inhibitors with dual modes of action in complexes with ACE are also discussed.

CONCLUSION

Selectivity of ACE inhibitors for the N- and C-domain is controlled by subtle differences in the amino-acids forming the active site. Reported studies of crystal complexes of inhibitors in the C- and N-domains revealed that most selective inhibitors interact with non-conserved amino-acids between domains and have distinct interactions with the residues in the S2 and S2' subsites of the ACE catalytic site. Moreover, unusual binding of the second molecule of inhibitors in the binding cavity opens new possibilities of exploiting more distant regions of the catalytic center in structure-based design of novel drugs.

Effects of natural peptides from food proteins on angiotensin converting enzyme activity and hypertension

Cardiovascular diseases are the leading cause of death. The underlying pathophysiology is largely contributed by an overactivation of the renin-angiotensin-aldosterone-system (RAAS). Herein, angiotensin II (AngII) is a key mediator not only in blood pressure control and vascular tone regulation, but also involved in inflammation, endothelial dysfunction, atherosclerosis, hypertension and congestive heart failure. Since more than three decades suppression of AngII generation by inhibition of the angiotensin-converting enzyme (ACE) or blockade of the AngII-receptor has shown clinical benefit by reducing hypertension, atherosclerosis and other inflammation-associated cardiovascular diseases. Besides pharmaceutical ACE-inhibitors some natural peptides derived from food proteins reduce in vitro ACE activity. Several animal studies and a few human clinical trials have shown antihypertensive effects of such peptides, which might be attractive as food additives to prevent age-related RAAS activation. However, their inhibitory potency on in vitro ACE activity does not always correlate with an antihypertensive impact. While some peptides with high inhibitory activity on ACE-activity in vitro show no antihypertensive effect in vivo, other peptides with only a moderate ACE inhibitory activity in vitro cause such effects. The explanation for this conflicting phenomenon between inhibitory activity and antihypertensive effect remains unclear to date. This review shall critically address the effects of natural peptides derived from different food proteins on the cardiovascular system and the possible underlying mechanisms. A central aspect will be to point to conceptual gaps in the current understanding of the action of these peptides with respect to in vivo blood pressure lowering effects.

Angiotensin-converting enzyme defines matrikine-regulated inflammation and fibrosis

The neutrophil chemoattractant proline-glycine-proline (PGP) is generated from collagen by matrix metalloproteinase-8/9 (MMP-8/9) and prolyl endopeptidase (PE), and it is concomitantly degraded by extracellular leukotriene A₄ hydrolase (LTA₄H) to limit neutrophilia. Components of cigarette smoke can acetylate PGP, yielding a species (AcPGP) that is resistant to LTA₄H-mediated degradation and can, thus, support a sustained neutrophilia. In this study, we sought to elucidate if an antiinflammatory system existed to degrade AcPGP that is analogous to the PGP-LTA₄H axis. We demonstrate that AcPGP is degraded through a previously unidentified action of the enzyme angiotensin-converting enzyme (ACE). Pulmonary ACE is elevated during episodes of acute inflammation, as a consequence of enhanced vascular permeability, to ensure the efficient degradation of AcPGP. Conversely, we suggest that this pathway is aberrant in chronic obstructive pulmonary disease (COPD) enabling the accumulation of AcPGP. Consequently, we identify a potentially novel protective role for AcPGP in limiting pulmonary fibrosis and suggest the pathogenic function attributed to ACE in idiopathic pulmonary fibrosis (IPF) to be a consequence of overzealous AcPGP degradation. Thus, AcPGP seemingly has very divergent roles: it is pathogenic in its capacity to drive neutrophilic inflammation and matrix degradation in the context of COPD, but it is protective in its capacity to limit fibrosis in IPF.

ACE degrades the collagen-derived matrikine, acetylate proline–glycine–proline, to limit pulmonary inflammation and promote repair.

Nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) and its inhibitors

Ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1, EC 3.1.4.1) is a metalloenzyme that belongs to the NPP family, which comprises seven subtypes (NPP1-7). NPP1 hydrolyzes a wide range of phosphodiester bonds, e.g. in nucleoside triphosphates, (cyclic) dinucleotides, and nucleotide sugars yielding nucleoside 5'-monophosphates as products. Its main substrate is ATP which is cleaved to AMP and diphosphate. The enzyme is involved in various biological processes including bone mineralization, soft-tissue calcification, insulin receptor signalling, cancer cell proliferation and immune modulation. Therefore, NPP1 inhibitors have potential as novel drugs, e.g. for (immuno)oncology. In the last two decades several inhibitors of NPP1 derived from nucleotide- or non-nucleotide scaffolds have been developed. The most potent and selective NPP1-inhibitory substrate analog is adenosine 5'-α,β-methylene-γ-thiotriphosphate (Ki = 20 nM vs. p-Nph-5'-TMP, human membrane-bound NPP1). Non-nucleotide-derived NPP1 inhibitors comprise polysulfonates, polysaccharides, polyoxometalates and small heterocyclic compounds. The polyoxometalate [TiW11CoO40]8- (PSB-POM141) is the most potent and selective NPP1 inhibitor described to date (Ki = 1.46 nM vs. ATP, human soluble NPP1); it displays an allosteric mechanism of inhibition and represents a useful pharmacological tool for evaluating the potential of NPP1 as a novel drug target.

Substrate-Dependence of Competitive Nucleotide Pyrophosphatase/Phosphodiesterase1 (NPP1) Inhibitors

Nucleotide pyrophosphatase/phosphodiesterase type 1 (NPP1) is a membrane glycoprotein involved in the hydrolysis of extracellular nucleotides. Its major substrate is ATP which is converted to AMP and diphosphate. NPP1 was proposed as a new therapeutic target in brain cancer and immuno-oncology. Several NPP1 inhibitors have been reported to date, most of which were evaluated vs. the artificial substrate p-nitrophenyl 5′-thymidine monophosphate (p-Nph-5′-TMP). Recently, we observed large discrepancies in inhibitory potencies for a class of competitive NPP1 inhibitors when tested vs. the artificial substrate p-Nph-5′-TMP as compared to the natural substrate ATP. Therefore, the goal of the present study was to investigate whether inhibitors of human NPP1 generally display substrate-dependent inhibitory potency. Systematic evaluation of nucleotidic as well as non-nucleotidic NPP1 inhibitors revealed significant differences in determined K_i values for competitive, but not for non- and un-competitive inhibitors when tested vs. the frequently used artificial substrate p-Nph-5′-TMP as compared to ATP. Allosteric modulation of NPP1 by p-Nph-5′-TMP may explain these discrepancies. Results obtained using the AMP derivative p-nitrophenyl 5′-adenosine monophosphate (p-Nph-5′-AMP) as an alternative artificial substrate correlated much better with those employing the natural substrate ATP.

Chitosan oligosaccharide: Biological activities and potential therapeutic applications

Chitosan oligosaccharide (COS) is an oligomer of β-(1➔4)-linked d-glucosamine. COS can be prepared from the deacetylation and hydrolysis of chitin, which is commonly found in the exoskeletons of arthropods and insects and the cell walls of fungi. COS is water soluble, non-cytotoxic, readily absorbed through the intestine and mainly excreted in the urine. Of particular importance, COS and its derivatives have been demonstrated to possess several biological activities including anti-inflammation, immunostimulation, anti-tumor, anti-obesity, anti-hypertension, anti-Alzheimer's disease, tissue regeneration promotion, drug and DNA delivery enhancement, anti-microbial, anti-oxidation and calcium-absorption enhancement. The mechanisms of actions of COS have been found to involve the modulation of several important pathways including the suppression of nuclear factor kappa B (NF-κB) and mitogen-activated protein kinases (MAPK) and the activation of AMP-activated protein kinase (AMPK). This review summarizes the current knowledge of the preparation methods, pharmacokinetic profiles, biological activities, potential therapeutic applications and safety profiles of COS and its derivatives. In addition, future research directions are discussed.

Structural basis of Ac-SDKP hydrolysis by Angiotensin-I converting enzyme

Angiotensin-I converting enzyme (ACE) is a zinc dipeptidylcarboxypeptidase with two active domains and plays a key role in the regulation of blood pressure and electrolyte homeostasis, making it the principal target in the treatment of cardiovascular disease. More recently, the tetrapetide N-acetyl-Ser–Asp–Lys–Pro (Ac-SDKP) has emerged as a potent antifibrotic agent and negative regulator of haematopoietic stem cell differentiation which is processed exclusively by ACE. Here we provide a detailed biochemical and structural basis for the domain preference of Ac-SDKP. The high resolution crystal structures of N-domain ACE in complex with the dipeptide products of Ac-SDKP cleavage were obtained and offered a template to model the mechanism of substrate recognition of the enzyme. A comprehensive kinetic study of Ac-SDKP and domain co-operation was performed and indicated domain interactions affecting processing of the tetrapeptide substrate. Our results further illustrate the molecular basis for N-domain selectivity and should help design novel ACE inhibitors and Ac-SDKP analogues that could be used in the treatment of fibrosis disorders.

Angiotensin-I converting enzyme (ACE): structure, biological roles, and molecular basis for chloride ion dependence

Somatic angiotensin-I converting enzyme (sACE) has an essential role in the regulation of blood pressure and electrolyte fluid homeostasis. It is a zinc protease that cleaves angiotensin-I (AngI), bradykinin, and a broad range of other signalling peptides. The enzyme activity is provided by two homologous domains (N- and C-), which display clear differences in substrate specificities and chloride activation. The presence of chloride ions in sACE and its unusual role in activity was identified early on in the characterisation of the enzyme. The molecular mechanisms of chloride activation have been investigated thoroughly through mutagenesis studies and shown to be substrate-dependent. Recent results from X-ray crystallography structural analysis have provided the basis for the intricate interactions between ACE, its substrate and chloride ions. Here we describe the role of chloride ions in human ACE and its physiological consequences. Insights into the chloride activation of the N- and C-domains could impact the design of improved domain-specific ACE inhibitors.

Pharmacological evidence of bradykinin regeneration from extended sequences that behave as peptidase-activated B2 receptor agonists

While bradykinin (BK) is known to be degraded by angiotensin converting enzyme (ACE), we have recently discovered that Met-Lys-BK-Ser-Ser is paradoxically activated by ACE. We designed and evaluated additional "prodrug" peptides extended around the BK sequence as potential ligands that could be locally activated by vascular or blood plasma peptidases. BK regeneration was estimated using the contractility of the human umbilical vein as model of vascular functions mediated by endogenous B2 receptors (B2Rs) and the endocytosis of the fusion protein B2R-green fluorescent protein (B2R-GFP) expressed in Human Embryonic Kidney 293 cells. Of three BK sequences extended by a C-terminal dipeptide, BK-His-Leu had the most desirable profile, exhibiting little direct affinity for the receptor but a significant one for ACE (as shown by competition of [(3)H]BK binding to B2R-GFP or of [(3)H]enalaprilat to recombinant ACE, respectively). The potency of the contractile effect of this analog on the vein was reduced 18-fold by the ACE inhibitor enalaprilat, pharmacologically evidencing BK regeneration in situ. BK-Arg, a potential substrate of arginine carboxypeptidases, had a low affinity for B2Rs and its potency as a contractile agent was reduced 15-fold by tissue treatment with an inhibitor of these enzymes, Plummer's inhibitor. B2R-GFP internalization in response to 100 nM of the extended peptides recapitulated these findings, as enalaprilat selectively inhibited the effect of BK-His-Leu and Plummer's inhibitor, that of BK-Arg. The two peptidase inhibitors did not affect BK-induced effects in either assay. The novel C-terminally extended BKs had no or very little affinity for the kinin B1 receptor (competition of [(3)H]Lys-des-Arg(9)-BK binding). The feasibility of peptidase-activated B2R agonists is illustrated by C-terminal extensions of the BK sequence.

Surface plasmon resonance analysis of the binding mechanism of pharmacological and peptidic inhibitors to human somatic angiotensin I-converting enzyme

Somatic angiotensin I-converting enzyme (ACE) possesses two catalytic domains and plays a major role in the regulation of blood pressure, thus representing a therapeutic target for the treatment of hypertension. We present a comprehensive surface plasmon resonance (SPR) study of the interaction of human somatic ACE with the pharmacological inhibitors captopril and lisinopril, the bradykinin potentiating peptide BPP-11b, and the food peptidic inhibitors from bovine αs2-casein, F(174)ALPQYLK(181) and F(174)ALPQY(179). SPR binding curves recorded with the high potency inhibitors captopril, lisinopril, and BPP-11b were evaluated both by regression analysis and by kinetic distribution analysis. The results indicated that captopril and lisinopril bound ACE with two K(D)'s differing by a factor 10-20 and >30, respectively (lowest K(D) = 0.1-0.3 nM for both inhibitors). This shows, for the first time in a direct binding assay with the two-domain enzyme, the existence of two binding modes of the pharmacological inhibitors, presumably with the two ACE domains. The BPP-11b-ACE binding curves were complex but showed a predominant interaction with K(D) in the nanomolar range. The caseinopeptides, known to inhibit ACE with an IC₅₀ of 4.3 μM, bound to ACE with K(D) = 3-4 μM. Mapping of the F(174)ALPQY(179) binding site on ACE by sequential binding studies using captopril or BPP-11b indicated that it bound to (or near) the two active sites of ACE, in agreement with the stoichiometry of 2 determined from data fitting. Our results provide a detailed characterization of ACE-inhibitor binding modes and validate SPR for predicting the inhibitory potential of new compounds.

Measuring angiotensin-I converting enzyme inhibitory activity by micro plate assays: comparison using marine cryptides and tentative threshold determinations with captopril and losartan

To determine the angiotensin-I converting enzyme (ACE) inhibitory activity of marine cryptides, different methods were tested. ACE inhibition was measured using two synthetic substrates, (N-[3-(2-furyl) acryloyl]-Phe-Gly-Gly (FAPGG) and N-hippuryl-His-Leu hydrate salt (HHL)), and a natural one, angiotensin-I. The IC50 value (defined as the concentration of inhibitory molecule needed to inhibit 50% of the ACE activity) of the reference synthetic inhibitor captopril was in the nanomolar range (1.79-15.1 nM) when synthetic substrates were used, whereas it exhibited IC50 of micromolar range (16.71 μM) with angiotensin-I. We chose losartan, an antagonist of angiotensin-II receptor as negative control for the ACE inhibition. Losartan was also able to inhibit ACE whatever the substrate tested, with IC50 of micromolar range (17.13-146 μM). We defined this value as a limit above which molecules are not showing in vitro ACE inhibitory activity. Val-Trp (VW), Val-Tyr (VY), Lys-Tyr (KY), Lys-Trp (KW), Ile-Tyr (IY), Ala-Pro (AP), Val-Ile-Tyr (VIY), Leu-Lys-Pro (LKP), Gly-Pro-Leu (GPL), Ala-Lys-Lys (AKK), and Val-Ala-Pro (VAP) were tested as inhibitors of ACE with synthetic and natural substrates. IC50 displayed were substrate-dependent. With FAPGG as substrate, IW, VAP, KY, IY, AP, AKK, and VIY show IC50 values over the IC50 value of losartan and should not be considered as inhibitors of ACE. VY, VW, KW, and LKP exhibited IC50 value lower than the IC50 value of losartan for all substrates tested and were thus considered as good candidates for effectively decreasing hypertension. It appears that the comparison of IC50 is not consistent when IC50 values are obtained with different substrates and different methods. In vitro ACE inhibitory activity assays should always include various ACE substrates and references such as captopril and a negative control to obtain data reliable to discriminate ACE inhibitory peptides.

A bradykinin-potentiating peptide (BPP-10c) from Bothrops jararaca induces changes in seminiferous tubules

Background

The testis-specific isoform of angiotensin-converting enzyme (tACE) is exclusively expressed in germ cells during spermatogenesis. Although the exact role of tACE in male fertility is unknown, it clearly plays a critical function in spermatogenesis. The dipeptidase domain of tACE is identical to the C-terminal catalytic domain of somatic ACE (sACE). Bradykinin potentiating peptides (BPPs) from snake venoms are the first natural sACE inhibitors described and their structure–activity relationship studies were the basis for the development of antihypertensive drugs such as captopril. In recent years, it has been showed that a number of BPPs – including BPP-10c – are able to distinguish between the N- and C-active sites of sACE, what is not applicable to captopril. Considering the similarity between tACE and sACE (and since BPPs are able to distinguish between the two active sites of sACE), the effects of the BPP-10c and captopril on the structure and function of the seminiferous epithelium were characterized in the present study. BPP-10c and captopril were administered in male Swiss mice by intraperitoneal injection (4.7 μmol/kg for 15 days) and histological sections of testes were analyzed. Classification of seminiferous tubules and stage analysis were carried out for quantitative evaluation of germ cells of the seminiferous epithelium. The blood-testis barrier (BTB) permeability and distribution of claudin-1 in the seminiferous epithelium were analyzed by hypertonic fixative method and immunohistochemical analyses of testes, respectively.

Results

The morphology of seminiferous tubules from animals treated with BPP-10c showed an intense disruption of the epithelium, presence of atypical multinucleated cells in the lumen and degenerated germ cells in the adluminal compartment. BPP-10c led to an increase in the number of round spermatids and total support capacity of Sertoli cell in stages I, V, VII/VIII of the seminiferous epithelium cycle, without affecting BTB permeability and the distribution of claudin-1 in the seminiferous epithelium. Interestingly, no morphological or morphometric alterations were observed in animals treated with captopril.

Conclusions

The major finding of the present study was that BPP-10c, and not captopril, modifies spermatogenesis by causing hyperplasia of round spermatids in stages I, V, and VII/VIII of the spermatogenic cycle.

Target-directed catalytic metallodrugs

Most drugs function by binding reversibly to specific biological targets, and therapeutic effects generally require saturation of these targets. One means of decreasing required drug concentrations is incorporation of reactive metal centers that elicit irreversible modification of targets. A common approach has been the design of artificial proteases/nucleases containing metal centers capable of hydrolyzing targeted proteins or nucleic acids. However, these hydrolytic catalysts typically provide relatively low rate constants for target inactivation. Recently, various catalysts were synthesized that use oxidative mechanisms to selectively cleave/inactivate therapeutic targets, including HIV RRE RNA or angiotensin converting enzyme (ACE). These oxidative mechanisms, which typically involve reactive oxygen species (ROS), provide access to comparatively high rate constants for target inactivation. Target-binding affinity, co-reactant selectivity, reduction potential, coordination unsaturation, ROS products (metal-associated vs metal-dissociated; hydroxyl vs superoxide), and multiple-turnover redox chemistry were studied for each catalyst, and these parameters were related to the efficiency, selectivity, and mechanism(s) of inactivation/cleavage of the corresponding target for each catalyst. Important factors for future oxidative catalyst development are 1) positioning of catalyst reduction potential and redox reactivity to match the physiological environment of use, 2) maintenance of catalyst stability by use of chelates with either high denticity or other means of stabilization, such as the square planar geometric stabilization of Ni- and Cu-ATCUN complexes, 3) optimal rate of inactivation of targets relative to the rate of generation of diffusible ROS, 4) targeting and linker domains that afford better control of catalyst orientation, and 5) general bio-availability and drug delivery requirements.

Targeted catalytic inactivation of angiotensin converting enzyme by lisinopril-coupled transition-metal chelates

A series of compounds that target reactive transition-metal chelates to somatic angiotensin converting enzyme (sACE-1) have been synthesized. Half-maximal inhibitory concentrations (IC(50)) and rate constants for both inactivation and cleavage of full-length sACE-1 have been determined and evaluated in terms of metal chelate size, charge, reduction potential, coordination unsaturation, and coreactant selectivity. Ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), and tripeptide GGH were linked to the lysine side chain of lisinopril by 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide hydrochloride/N-hydroxysuccinimide coupling. The resulting amide-linked chelate-lisinopril (EDTA-lisinopril, NTA-lisinopril, DOTA-lisinopril, and GGH-lisinopril) conjugates were used to form coordination complexes with iron, cobalt, nickel, and copper, such that lisinopril could mediate localization of the reactive metal chelates to sACE-1. ACE activity was assayed by monitoring cleavage of the fluorogenic substrate Mca-RPPGFSAFK(Dnp)-OH, a derivative of bradykinin, following preincubation with metal chelate-lisinopril compounds. Concentration-dependent inhibition of sACE-1 by metal chelate-lisinopril complexes revealed IC(50) values ranging from 44 to 4500 nM for Ni-NTA-lisinopril and Ni-DOTA-lisinopril, respectively, versus 1.9 nM for lisinopril. Stronger inhibition was correlated with smaller size and lower negative charge of the attached metal chelates. Time-dependent inactivation of sACE-1 by metal chelate-lisinopril complexes revealed a remarkable range of catalytic activities, with second-order rate constants as high as 150,000 M(-1) min(-1) (Cu-GGH-lisinopril), while catalyst-mediated cleavage of sACE-1 typically occurred at much lower rates, indicating that inactivation arose primarily from side chain modification. Optimal inactivation of sACE-1 was observed when the reduction potential for the metal center was poised near 1000 mV, reflecting the difficulty of protein oxidation. This class of metal chelate-lisinopril complexes possesses a range of high-affinity binding to ACE, introduces the advantage of irreversible catalytic turnover, and marks an important step toward the development of multiple-turnover drugs for selective inactivation of sACE-1.

Novel antihypertensive hexa- and heptapeptides with ACE-inhibiting properties: from the in vitro ACE assay to the spontaneously hypertensive rat

Bioactive ACE inhibiting peptides are gaining interest in hypertension treatment. We have designed and screened six synthetic heptapeptides (PACEI48 to PACEI53) based on two hexapeptide leads (PACEI32 and PACEI34) to improve ACE inhibitory properties and assess their antihypertensive effects. ACE activity was assayed in vitro and ex vivo. Selected peptides were administered to spontaneously hypertensive rats (SHRs) and normotensive Wistar-Kyoto (WKY) rats. In vitro cytotoxicity was assessed with the MTT reduction test. The six heptapeptides at low micromolar concentration produced different degrees of in vitro inhibition of ACE activity using the synthetic substrate HHL or the natural substrate angiotensin I; and ex vivo inhibition of ACE-dependent, angiotensin I-induced vasoconstriction, but not angiotensin II-induced vasoconstriction. Oral administration of the hexapeptide PACEI32L, and the heptapeptides PACEI50L and PACEI52L, induced reductions in systolic blood pressure lasting up to 3h in SHRs but not in WKY rats. Intravenous injection of PACEI32L and PACEI50L, but not PACEI52L, induced acute transient reductions in mean blood pressure of SHRs. d-Amino acid peptides showed five-fold less ACE inhibitory potency, no inhibitory effect on angiotensin I-induced vasoconstriction, and antihypertensive effect in SHRs after i.v. injection, but not after oral administration. The toxicity of peptides to reduce the viability of cultured cells was in the millimolar range. In conclusion, we have obtained novel rationally designed heptapeptides with improved ACE inhibitory properties when compared to lead hexapeptides. One selected hexapeptide and two heptapeptides show oral antihypertensive effects in SHRs and appear safe in cytotoxicity assays.

Simultaneous determination of ACE activity with 2 substrates provides information on the status of somatic ACE and allows detection of inhibitors in human blood

Angiotensin I-converting enzyme (ACE), a key enzyme in cardiovascular pathophysiology, consists of 2 homologous domains, each bearing a Zn-dependent active site. The ratio of the rates of hydrolysis of 2 synthetic substrates, Z-Phe-His-Leu (ZPHL) and Hip-His-Leu (HHL), is characteristic for each type of ACE: somatic 2-domain 1, N-domain 4.5, and C-domain 0.7 (Williams et al, 1996). We hypothesized that inactivation or selective inhibition of the C-domain within the somatic ACE should increase the ratio from 1 toward higher values, whereas inactivation or inhibition of the N-domain should decrease the ratio to lower values. Temperatures in the 40-60 degrees C range cause preferential inactivation of the C-domain in somatic ACE and an increase in the ZPHL/HHL ratio. Determination of the ZPHL/HHL ratio in freshly 100-fold concentrated urine ruled out the existence of the N-domain fragment in human urine, which appears only as a result of long storage. Inhibition of ACE by common inhibitors also increases the ZPHL/HHL ratio for 2-domain ACE, thus providing a sensitive method for the detection of ACE inhibitors in biological fluids. Therefore, simultaneous measurements of ACE activity with 2 substrates (ZPHL and HHL) and calculation of their ratio allow us to monitor the status of the ACE molecule and detect ACE inhibitors (endogenous and exogenous) in human blood and other biological fluids. This method should find wide application in monitoring clinical trials with ACE inhibitors and in the development of the approach for personalized medicine by these effective drugs.

Angiotensin-converting enzyme C-terminal catalytic domain is the main site of angiotensin I cleavage in vivo

Angiotensin-converting enzyme (ACE) plays a central role in the production of the vasoconstrictor angiotensin II. ACE is a single polypeptide, but it contains 2 homologous and independent catalytic domains, each of which binds zinc. To understand the in vivo role of these 2 domains, we used gene targeting to create mice with point mutations in the ACE C-domain zinc-binding motif. Such mice, termed ACE13/13, produce a full-length ACE protein with tissue expression identical to wild-type mice. Analysis of ACE13/13 mice showed that they produce ACE having only N-domain catalytic activity, as determined by the hydrolysis of domain specific substrates and by chloride sensitivity. ACE13/13 mice have blood pressure and blood angiotensin II levels similar to wild-type mice. However, plasma renin concentration is increased 2.6-fold and blood angiotensin I levels are increased 7.5-fold. Bradykinin peptide levels are not different from wild-type levels. ACE13/13 mice have a reduced increase of blood pressure after intravenous infusion of angiotensin I. ACE13/13 mice have a normal renal structure, but they are not able to concentrate urine after dehydration as effectively as wild-type mice. This study shows that the C-domain of ACE is the predominant site of angiotensin I cleavage in vivo. Although mice lacking C-domain activity have normal physiology under laboratory conditions, they respond less well to the stress of dehydration.

Characterization of the first angiotensin-converting like enzyme in bacteria: Ancestor ACE is already active

Angiotensin-converting enzyme (ACE) is a metallopeptidase that converts angiotensin I into angiotensin II. ACE is crucial in the control of cardiovascular and renal homeostasis and fertility in mammals. In vertebrates, both transmembrane and soluble ACE, containing one or two active sites, have been characterized. So far, only soluble, single domain ACEs from invertebrates have been cloned, and these have been implicated in reproduction in insects. Furthermore, an ACE-related carboxypeptidase was recently characterized in Leishmania, a unicellular eukaryote, suggesting the existence of ACE in more distant organisms.

Interestingly, in silico databank analysis revealed that bacterial DNA sequences could encode putative ACE-like proteins, strikingly similar to vertebrates' enzymes. To gain more insight into the bacterial enzymes, we cloned the putative ACE from the phytopathogenic bacterium, Xanthomonas axonopodis pv. citri, named XcACE. The 2 kb open reading frame encodes a 672-amino-acid soluble protein containing a single active site. In vitro expression and biochemical characterization revealed that XcACE is a functional 72 kDa dipeptidyl-carboxypeptidase. As in mammals, this metalloprotease hydrolyses angiotensin I into angiotensin II. XcACE is sensitive to ACE inhibitors and chloride ions concentration. Variations in the active site residues, highlighted by structural modelling, can account for the different substrate selectivity and inhibition profile compared to human ACE. XcACE characterization demonstrates that ACE is an ancestral enzyme, provoking questions about its appearance and structure/activity specialisation during the course of evolution.

ACE activity during the hypotension produced by standardized aqueous extract of Cecropia glaziovii Sneth: a comparative study to captopril effects in rats

To evaluate the effect of the standardized aqueous extract (AE) of Cecropia glaziovii Sneth on the plasma angiotensin I converting enzyme (ACE-EC 3.4.15.1) activity, rats were treated with a single dose of AE (1 g/kg, p.o.) or repeatedly (0.5 g/kg/bid, p.o.) for 60 days. Captopril (50 mg/kg, p.o.) was used as positive control on the same animals. The effects on the blood pressure were recorded directly from the femoral artery (single dose), or indirectly by the tail cuff method (repeated doses) in conscious rats. The plasma ACE activity was determined spectrofluorimetrically using Hypuril-Hystidine-Leucine as substrate. The arterial blood pressure, heart rate and plasma ACE activity were not significantly modified within 24 h after a single dose administration of AE. Comparatively, blood pressure in captopril treated rats was reduced by 7-16% and heart rate was increased by 10-20% from 30 min to 24 h after drug administration. ACE activity after captopril presented a dual response: an immediate inhibition peaking at 30 min and a slow reversal to 32% up-regulation after 24 h. To correlate the drug effects upon repeated administration of either compound, normotensive rats were separated in three groups: animals with high ACE (48.8+/-2.6 nmol/min/ml), intermediate ACE (39.4+/-1.4 nmol/min/ml) and low ACE (23.5+/-0.6 nmol/min/ml) activity, significantly different among them. Repeated treatment with AE reduced the mean systolic blood pressure (121.7+/-0.5 mm Hg) by 20 mm Hg after 14 days. The hypotension was reversed upon washout 60 days afterwards. Likely, repeated captopril administration decreased blood pressure by 20 mm Hg throughout treatment in all groups. After 30 days treatment with AE (0.5 g/kg/bid, p.o.) the plasma ACE activity was unchanged in any experimental group. After captopril (50 mg/kg/bid, p.o.) administration the plasma ACE activity was inhibited by 50% within 1 h treatment but it was up-regulated by 120% after 12 h in all groups. It is concluded that the hypotension produced by prolonged treatment with AE of C. glaziovii is unrelated to ACE inhibition.

ACE phenotyping as a first step toward personalized medicine for ACE inhibitors. Why does ACE genotyping not predict the therapeutic efficacy of ACE inhibition?

Angiotensin (Ang)-converting enzyme (ACE) inhibitors are widely used for the treatment of cardiovascular diseases. Not all patients respond to ACE inhibitors, and it has been suggested that genetic variation might be a useful marker to predict the therapeutic efficacy of these drugs. In particular, the ACE insertion (I)/deletion (D) polymorphism has been investigated in this regard. Despite a decade of intensive research involving the genotyping of thousands of patients, we still do not know whether ACE genotyping helps in predicting the success of ACE inhibition. This review critically addresses the concept that predictive information on therapeutic efficacy of ACE inhibitors might be obtained based on ACE genotyping. It answers the following questions: Do higher ACE levels really result in higher Ang II levels? Is ACE the only converting enzyme in humans? Does ACE inhibition affect ACE expression? Why does ACE have 2 catalytically active domains? What is the relevance of ACE inhibitor-induced signaling through membrane-bound ACE? The review ends with the proposal that ACE phenotyping may prove to be a better first step toward personalized medicine for ACE inhibitors than ACE genotyping.

Glycosides and amino acyl esters of carbohydrates as potent inhibitors of angiotensin converting enzyme

About 12 glycosides prepared through amyloglucosidase catalysis and 23 amino acyl esters of carbohydrates prepared through lipase catalysis in organic solvents showed angiotensin converting enzyme (ACE) inhibition activity. Both amino acyl esters of carbohydrates and glycosides exhibited IC50 values for ACE inhibition in the 0.5 mM to 15.7 mM range. Eugenyl-D-glucoside (IC50: 0.5+/-0.04 mM), L-isoleucyl-D-glucose (IC50: 0.7+/-0.067 mM), vanillyl-D-sorbitol (IC50: 0.8+/-0.09 mM), L-histidyl-D-fructose (IC50: 0.9+/-0.087 mM), L-tryptophanyl-D-fructose (IC50: 0.9+/-0.092 mM), octyl-D-glucoside (IC50: 1.0+/-0.093 mM), vanillyl-D-mannoside (IC50: 1.0+/-0.089 mM), L-valyl-D-mannitol (IC50: 1.0+/-0.092 mM) and L-phenylalanyl-D-glucose (IC50: 1.0+/-0.089 mM) were the compounds, which showed the best ACE inhibitory activities.

Relevance of renin expression by real-time PCR in acute myeloid leukemia

The search for useful molecular markers in the diagnosis of AML and in the follow-up of minimal residual disease (MRD) has been the focus of many recent studies. Previous research showed that, while normal bone marrow cells lack expression of renin, myeloid blasts have been reported to do so. The aim was to study the expression of the renin gene by the use of real-time quantitative PCR (RQ-PCR) at diagnosis in acute myeloid leukemia patients (AML) and to assess its possible relevance in the prognosis and outcome of such patients. This study analysed 76 samples from patients with AML, with follow-up of positive patients. Thirty-one patients (41%) were positive for renin gene expression at diagnosis. All renin-positive patients at diagnosis showed no expression during complete remission (CR), but expression recurred in those experiencing relapse and persisted when the disease was refractory to treatment. Although the results suggest that the sub-group of renin-positive AML patients might have a worse outcome and a higher relapse rate (at 5 years, the projected rate of disease-free survival was 18.5 +/- 9.8% for renin-positive and 23.5 +/- 8.8% for renin-negative patients), no significant differences were found. It is believed that further studies should aim to validate whether such a difference exists, using a much larger and homogeneus group of patients.

Combined use of selective inhibitors and fluorogenic substrates to study the specificity of somatic wild-type angiotensin-converting enzyme

Somatic angiotensin-converting enzyme (ACE) contains two homologous domains, each bearing a functional active site. Studies on the selectivity of these ACE domains towards either substrates or inhibitors have mostly relied on the use of mutants or isolated domains of ACE. To determine directly the selectivity properties of each ACE domain, working with wild-type enzyme, we developed an approach based on the combined use of N-domain-selective and C-domain-selective ACE inhibitors and fluorogenic substrates. With this approach, marked differences in substrate selectivity were revealed between rat, mouse and human somatic ACE. In particular, the fluorogenic substrate Mca-Ala-Ser-Asp-Lys-DpaOH was shown to be a strict N-domain-selective substrate of mouse ACE, whereas with rat ACE it displayed marked C-domain selectivity. Similar differences in selectivity between these ACE species were also observed with a new fluorogenic substrate of ACE, Mca-Arg-Pro-Pro-Gly-Phe-Ser-Pro-DpaOH. In support of these results, changes in amino-acid composition in the binding site of these three ACE species were pinpointed. Together these data demonstrate that the substrate selectivity of the N-domain and C-domain depends on the ACE species. These results raise concerns about the interpretation of functional studies performed in animals using N-domain and C-domain substrate selectivity data derived only from human ACE.

Kinetic probes for inter-domain co-operation in human somatic angiotensin-converting enzyme

s-ACE (the somatic form of angiotensin-converting enzyme) consists of two homologous domains (N- and C-domains), each bearing a catalytic site. Negative co-operativity between the two domains has been demonstrated for cow and pig ACEs. However, for the human enzyme there are conflicting reports in the literature: some suggest possible negative co-operativity between the domains, whereas others indicate independent functions of the domains within s-ACE. We demonstrate here that a 1:1 stoichiometry for the binding of the common ACE inhibitors, captopril and lisinopril, to human s-ACE is enough to abolish enzymatic activity towards FA {N-[3-(2-furyl)acryloyl]}-Phe-GlyGly, Cbz (benzyloxycarbonyl)-Phe-His-Leu or Hip (N-benzoylglycyl)-His-Leu. The kinetic parameters for the hydrolysis of seven tripeptide substrates by human s-ACE appeared to represent average values for parameters obtained for the individual N- and C-domains. Kinetic analysis of the simultaneous hydrolysis of two substrates, Hip-His-Leu (S1) and Cbz-Phe-His-Leu (S2), with a common product (His-Leu) by s-ACE at different values for the ratio of the initial concentrations of these substrates (i.e. sigma=[S2]0/[S1]0) demonstrated competition of these substrates for binding to the s-ACE molecule, i.e. binding of a substrate at one active site makes the other site unavailable for either the same or a different substrate. Thus the two domains within human s-ACE exhibit strong negative co-operativity upon binding of common inhibitors and in the hydrolysis reactions of tripeptide substrates.

[Expression of angiotensin I converting enzyme in pulmonary hypertension]

We studied angiotensin I converting enzyme (ACE) expression in lung tissue from patients with different forms of pulmonary hypertension (PH) in comparison with that from morphologically normal lung specimens. ACE antigen expression was analysed by immunohistochemistry in morphologically normal lung tissue from 33 patients (19 males, 32-77 years; 14 females, 34-93 years) and compared to that in specimens from 94 patients (67 males, 30-97 years; 27 females: 27-90 years) with different clinically proven forms of PH (according to Venedig classification). Type specific vessel expression pattern as described for normal lung tissue was generally intensified in arteries, arterioles and capillaries of the lung specimens with PH. Specimens with PH due to left heart disease and chronic obstructive pulmonary disease (COPD) showed only very weak or no augmented arterial ACE expression, while PH due to collagenoses or interstitial lung disease showed significantly higher ACE expression. In human PH there is--comparable to animal models--a raised ACE expression in pulmonary lung vessels, with differences between the various forms of PH. These differences in ACE expression may be relevant for subtly differentiated therapeutic anti-ACE therapy regimes.

Hinge peptide combinatorial libraries for inhilbitors of botulinum neurotoxins and saxitoxin: Deconvolution strategy

Abstract Combinatorial library screening offers a rapid process for identifying potential therapies to toxins. Hinge peptide libraries, which rely on conformational diversity rather than traditional molecular diversity, reduce the need for huge numbers of syntheses and screening steps and greatly expedite the discovery process of active molecules. Hinge peptide libraries having the structures: Acetyl-X1-X2-hinge-X3-X4-NH2 (capped) and X1-hinge-X2-X3 (uncapped), where X1 through X4 are near-equimolar mixtures of twelve L-amino acids and hinge = 4-aminobutyric acid, were screened for inhibitory activity in bioassays for botulinum neurotoxins A and B (BoNT/A, BoNT/B) and saxitoxin. The zinc protease activity of the reduced light chains of BoNT/A and /B was assayed by measuring the cleavage of synthetic substrates. Saxitoxin activity was measured by the restoration of the viability of neuroblastoma cells treated with ouabain and veratridine. Deconvolution of libraries was accomplished by fixing one position at a time beginning with the C-terminus. Primary library subsets in which position 4 was fixed showed moderate levels of inhibition for BoNT/A. Secondary library subsets showed stronger inhibition in the bioassays. In each of the bioassays, inhibitory potency was stronger when the second position to be fixed was on the opposite side of the hinge, rather than on the same side with respect to the C-terminus, suggesting that the hinge facilitates the interaction of side chains. Inhibitors for all three of the toxins studied were discovered within library subsets, although not necessarily in primary subsets. These studies demonstrate that (1) the best strategy for deconvoluting hinge peptide libraries is by fixing residues alternately on each side of the hinge moiety, and (2) it is essential to investigate secondary subsets even when primary subsets are inactive. The present findings support the concept that the increased flexibility imposed by the inclusion of a central hinge residue in small peptides increases the opportunity for side chain interactions, providing a distinct advantage for hinge peptide libraries over conventional peptide libraries. Hinge peptide libraries are a rich source of novel ligands for modulation of biomechanisms. The library subsets uncovered in this study may possess peptides that will lead to effective therapies to neurotoxin poisoning.

Amyloid beta-protein is degraded by cellular angiotensin-converting enzyme (ACE) and elevated by an ACE inhibitor

Human genetic data have associated angiotensin-converting enzyme (ACE) with Alzheimer disease (AD), and purified ACE has been reported to cleave synthetic amyloid beta-protein (Abeta) in vitro. Whether deficiency in ACE activity, arising from genetic alteration or pharmacological inhibition, can decrease Abeta degradation and allow Abeta accumulation in intact cells is unknown. We cloned ACE from human neuroblastoma cells and showed that it had posttranslational processing and enzymatic activity typical of the endogenous protease. Cellular expression of ACE promoted degradation of naturally secreted Abeta40 and Abeta42, leading to significant clearance of both species. Using site-directed mutagenesis, we determined that both active sites within ACE contribute to Abeta clearance, and an ACE construct bearing mutations in each catalytic domain had no effect on Abeta levels. Pharmacological inhibition of ACE with a widely prescribed drug, captopril, promoted the accumulation of cell-derived Abeta in the media of beta-amyloid precursor-protein expressing cells. Together, these results show that ACE can lower the levels of secreted Abeta in living cells and that this effect is blocked by inhibiting the protease's activity with an ACE inhibitor. This work, combined with the genetic studies, supports the hypothesis that ACE may modulate the susceptibility to and progression of AD via degradation of Abeta. Our data encourage further analyses of the ACE gene for disease association and raise the question of whether currently prescribed ACE inhibitors could elevate cerebral Abeta levels in humans.

Selective Angiotensin-Converting Enzyme C-Domain Inhibition Is Sufficient to Prevent Angiotensin I–Induced Vasoconstriction

Somatic angiotensin-converting enzyme (ACE) contains 2 domains (C-domain and N-domain) capable of hydrolyzing angiotensin I (Ang I) and bradykinin. Here we investigated the effect of the selective C-domain and N-domain inhibitors RXPA380 and RXP407 on Ang I-induced vasoconstriction of porcine femoral arteries (PFAs) and bradykinin-induced vasodilation of preconstricted porcine coronary microarteries (PCMAs). Ang I concentration-dependently constricted PFAs. RXPA380, at concentrations >1 mumol/L, shifted the Ang I concentration-response curve (CRC) 10-fold to the right. This was comparable to the maximal shift observed with the ACE inhibitors (ACEi) quinaprilat and captopril. RXP407 did not affect Ang I at concentrations < or =0.1 mmol/L. Bradykinin concentration-dependently relaxed PCMAs. RXPA380 (10 micromol/L) and RXP407 (0.1 mmol/L) potentiated bradykinin, both inducing a leftward shift of the bradykinin CRC that equaled approximately 50% of the maximal shift observed with quinaprilat. Ang I added to blood plasma disappeared with a half life (t(1/2)) of 42+/-3 minutes. Quinaprilat increased the t(1/2) approximately 4-fold, indicating that 71+/-6% of Ang I metabolism was attributable to ACE. RXPA380 (10 micromol/L) and RXP407 (0.1 mmol/L) increased the t(1/2) approximately 2-fold, thereby suggesting that both domains contribute to conversion in plasma. In conclusion, tissue Ang I-II conversion depends exclusively on the ACE C-domain, whereas both domains contribute to conversion by soluble ACE and to bradykinin degradation at tissue sites. Because tissue ACE (and not plasma ACE) determines the hypertensive effects of Ang I, these data not only explain why N-domain inhibition does not affect Ang I-induced vasoconstriction in vivo but also why ACEi exert blood pressure-independent effects at low (C-domain-blocking) doses.

Effects of Palladacycle Complex on Hematopoietic Progenitor Cells Proliferation In Vivo and In Vitro and Its Relation with the Inhibitory Properties of This Compound on the Angiotensin‐I Converting Enzyme Activity

In the present study, we introduce a new class of organometallic compound, the Biphosphinic Palladacycle Complex [Pd (C2, N-S(-)(dmpa)(dppf)] Cl (BPC), as an angiotensin-I converting-enzyme inhibitor (ACEI) with hematological regulation properties. When BPC was assayed as a competitive inhibitor over the hydrolysis of Abz-YRK (Dnp)-P-OH (Km = 7.0 microM), it showed a Kiapp = 0.2259 ng and a Ki value of 94.12 pg. Using murine long-term bone marrow cultures (LTBMCs) and clonal culture techniques, we also evaluated the capacity of this drug (1.18 microM) to module haematopoietic progenitor cells proliferation in vitro and in vivo. Our results demonstrated that BPC produces no toxicity to bone marrow cells, as determined by the unchanged cell number in the non-adherent layer at weeks 1, 2, and 8 and the increased number of adherent cells present in the BPC-treated LTBMCs. However, the proportion of CFU-Cs in the non-adherent cell layer was reduced at weeks 5, 6, 8, and 9. In vivo studies using the dose of 1 mg/kg of BPC, administered by subcutaneous route, presented similar result as those found in vitro, in the number of CFU-Cs. This latter finding may be explained by the inhibitory effects of this drug on the ACE activity, which probably result in increased levels of its substrate AcSDKP, a negative regulator of hematopoiesis.

Evidence for the negative cooperativity of the two active sites within bovine somatic angiotensin-converting enzyme

The somatic isoform of angiotensin-converting enzyme (ACE) consists of two homologous domains (N- and C-domains), each bearing a catalytic site. We have used the two-domain ACE form and its individual domains to compare characteristics of different domains and to probe mutual functioning of the two active sites within a bovine ACE molecule. The substrate Cbz-Phe-His-Leu (N-carbobenzoxy-L-phenylalanyl-L-histidyl-L-leucine; from the panel of seven) was hydrolyzed faster by the N-domain, the substrates FA-Phe-Gly-Gly (N-(3-[2-furyl]acryloyl)-L-phenylalanyl-glycyl-glycine) and Hip-His-Leu (N-benzoyl-glycyl-L-histidyl-L-leucine) were hydrolyzed by both domains with equal rates, while other substrates were preferentially hydrolyzed by the C-domain. The inhibitor captopril ((2S)-1-(3-mercapto-2-methylpropionyl)-L-proline) bound to the N-domain more effectively than to the C-domain, whereas lisinopril ((S)-N(alpha)-(1-carboxy-3-phenylpropyl)-L-lysyl-L-proline) bound to equal extent with all ACE forms. However, active site titration with lisinopril assayed by hydrolysis of FA-Phe-Gly-Gly revealed that 1 mol of inhibitor/mol of enzyme abolished the activity of either two-domain or single-domain ACE forms, indicating that a single active site functions in bovine somatic ACE. Neither of the k(cat) values obtained for somatic enzyme was the sum of k(cat) values for individual domains, but in every case the value of the catalytic constant of the hydrolysis of the substrate by the two-domain ACE represented the mean quantity of the values of the corresponding catalytic constants obtained for single-domain forms. The results indicate that the two active sites within bovine somatic ACE exhibit strong negative cooperativity.

Roles of the two active sites of somatic angiotensin-converting enzyme in the cleavage of angiotensin I and bradykinin: insights from selective inhibitors

Somatic angiotensin-converting enzyme (ACE) contains two homologous domains, each bearing a functional active site. The in vivo contribution of each active site to the release of angiotensin II (Ang II) and the inactivation of bradykinin (BK) is still unknown. To gain insights into the functional roles of these two active sites, the in vitro and in vivo effects of compounds able to selectively inhibit only one active site of ACE were determined, using radiolabeled Ang I or BK, as physiological substrates of ACE. In vitro studies indicated that a full inhibition of the Ang I and BK cleavage requires a blockade of the two ACE active sites. In contrast, in vivo experiments in mice demonstrated that the selective inhibition of either the N-domain or the C-domain of ACE by these inhibitors prevents the conversion of Ang I to Ang II, while BK protection requires the inhibition of the two ACE active sites. Thus, in vivo, the cleavage of Ang I and BK by ACE appears to obey to different mechanisms. Remarkably, in vivo the conversion of Ang I seems to involve the two active sites of ACE, free of inhibitor. Based on these findings, it might be suggested that the gene duplication of ACE in vertebrates may represent a means for regulating the cleavage of Ang I differently from that of BK.

Angiotensin I-converting enzyme and metabolism of the haematological peptide N-acetyl-seryl-aspartyl-lysyl-proline

1. Angiotensin I-converting enzyme (ACE) has two homologous active N- and C-terminal domains and displays activity towards a broad range of substrates. The tetrapeptide N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP) has been shown to be hydrolysed in vitro by ACE and to be a preferential substrate for its N-terminal active site. This peptide reversibly prevents the recruitment of pluripotent haematopoietic stem cells and normal early progenitors into the S-phase. 2. Angiotensin I-converting enzyme inhibitors, given as a single dose to normal subjects or during long-term treatment in hypertensive patients, result in plasma AcSDKP levels five- to six-fold higher and urine concentrations 40-fold higher than those of control subjects and/or patients. Thus, AcSDKP is a natural peptide hydrolysed by the N-terminal domain of ACE in vivo. In addition, ACE may be implicated in the process of haematopoietic stem cell regulation by permanently degrading this natural circulating inhibitor of cell entry into the S-phase. 3. Besides hydrolysis by ACE, the second very effective mechanism by which AcSDKP is cleared from plasma is glomerular filtration. Because of its high sensitivity and specificity, the measurement of AcSDKP in plasma and urine provides a valuable tool in screening specific inhibitors of the N-terminal domain of ACE and in monitoring ACE inhibition during chronic treatment. 4. The long-term consequences of AcSDKP accumulation are not known. During chronic ACE inhibition in rats, AcSDKP levels slightly increase in organs with high ACE content (kidneys, lungs). To significantly increase its concentration in target haematopoietic organs (the extracellular fraction of bone marrow), AcSDKP has to be infused on top of a captopril-based treatment. 5. A selective inhibitor of the N-domain of ACE in vitro and in vivo has been identified recently. The phosphinic peptide RXP 407 does not interfere with blood pressure regulation, but does increase, dose dependently, plasma concentrations of AcSDKP in mice, in contrast with lisinopril, which affects the metabolism of both AcSDKP and angiotensin I. N-Terminal-selective ACE inhibitors may be used to selectively control AcSDKP metabolism in target haematopoietic organs. This new therapeutic strategy may be of value for protecting haematopoietic cells from the toxicity of cancer chemotherapy.

Bradykinin potentiation by angiotensin-(1-7) and ACE inhibitors correlates with ACE C- and N-domain blockade

ACE inhibitors block B(2) receptor desensitization, thereby potentiating bradykinin beyond blocking its hydrolysis. Angiotensin (Ang)-(1-7) also acts as an ACE inhibitor and, in addition, may stimulate bradykinin release via angiotensin II type 2 receptors. In this study we compared the bradykinin-potentiating effects of Ang-(1-7), quinaprilat, and captopril. Porcine coronary arteries, obtained from 32 pigs, were mounted in organ baths, preconstricted with prostaglandin F(2alpha), and exposed to quinaprilat, captopril, Ang-(1-7), and/or bradykinin. Bradykinin induced complete relaxation (pEC(50)=8.11+/-0.07, mean+/-SEM), whereas quinaprilat, captopril, and Ang-(1-7) alone were without effect. Quinaprilat shifted the bradykinin curve to the left in a biphasic manner: a 5-fold shift at concentrations that specifically block the C-domain (0.1 to 1 nmol/L) and a 10-fold shift at concentrations that block both domains. Captopril and Ang-(1-7) monophasically shifted the bradykinin curve to the left, by a factor of 10 and 5, respectively. A 5-fold shift was also observed when Ang-(1-7) was combined with 0.1 nmol/L quinaprilat. Repeated exposure of porcine coronary arteries to 0.1 micromol/L bradykinin induced B(2) receptor desensitization. The addition of 10 micromol/L quinaprilat or Ang-(1-7) to the bath, at a time when bradykinin alone was no longer able to induce relaxation, fully restored the relaxant effects of bradykinin. Angiotensin II type 1 or 2 receptor blockade did not affect any of the observed effects of Ang-(1-7). In conclusion, Ang-(1-7), like quinaprilat and captopril, potentiates bradykinin by acting as an ACE inhibitor. Bradykinin potentiation is maximal when both the ACE C- and N-terminal domains are inhibited. The inhibitory effects of Ang-(1-7) are limited to the ACE C-domain, raising the possibility that Ang-(1-7) synergistically increases the blood pressure-lowering effects of N-domain-specific ACE inhibitors.

Captopril inhibits in vitro and in vivo the proliferation of primitive haematopoietic cells induced into cell cycle by cytotoxic drug administration or irradiation but has no effect on myeloid leukaemia cell proliferation

Angiotensin I-converting enzyme (ACE) has been shown to be involved in the catabolism of the tetrapeptide acetyl-Ser-Asp-Lys-Pro (AcSDKP). As AcSDKP is a physiological inhibitor of haematopoietic stem cell proliferation, we investigated the in vitro and in vivo effects of captopril, one of the specific inhibitors of ACE, on the proliferation of primitive haematopoietic cells. Regenerating bone marrow cells obtained from mice given one injection of cytosine arabinoside (100 mg/kg) as well as SA2 myeloid leukaemia cells were incubated in vitro for 24 h with 10-6 M captopril. Captopril significantly reduced the proportion of high proliferative potential colony-forming cells (HPP-CFC-1) in S-phase, whereas it had no effect on the proportion of SA2 leukaemic colony-forming cells in S-phase. When given in vivo to mice 1 h after 2 Gy gamma-irradiation or cytosine arabinoside (AraC) injection, captopril (100 mg/kg) was shown to prevent HPP-CFC-1 entry into S-phase induced by these cytotoxic treatments. The observed effects correlated with a reduction in ACE degradative activity and an increase in the level of endogenous AcSDKP both in the supernatants of captopril-treated bone marrow cells and in plasma of treated animals. The present findings suggest that AcSDKP might mediate the observed in vitro and in vivo inhibitory effects of captopril on primitive haematopoietic cell proliferation.

In vitro and in vivo inhibition of the 2 active sites of ACE by omapatrilat, a vasopeptidase inhibitor

The vasopeptidase inhibitor omapatrilat inhibits both neutral endopeptidase and angiotensin-converting enzyme (ACE). The in vitro and in vivo inhibitory potency of omapatrilat and the specific ACE inhibitor fosinopril toward the 2 active sites of ACE (called N- and C-domains) was investigated with the use of 3 substrates: angiotensin I, which is equally cleaved by the 2 ACE domains; hippuryl-histidyl-leucine, specific synthetic substrate of the C-domain in high- salt conditions; and a newly synthesized specific substrate of the N-domain designed by acetylating the lysine residue of AcSDKP. In vitro, omapatrilat was 5 times more potent than fosinoprilat in inhibiting angiotensin I hydrolysis. Omapatrilat inhibited similarly both N- and C-domain hydrolysis, whereas fosinoprilat was slightly more specific for the N-domain. The in vivo selective inhibitory potency of single oral doses of 10 mg omapatrilat and 20 mg fosinopril were investigated in a double-blind, placebo-controlled, cross-over study in 9 mildly sodium-depleted normotensive subjects. In accordance with the in vitro results, fosinopril appeared to be more specific for the N-domain than the C-domain in vivo, since plasma and urine AcSDKP concentrations were significantly higher than those observed with omapatrilat. This study shows that it is possible to assess separately in vitro and in vivo the selectivity of ACE or ACE/neutral endopeptidase inhibitors. A differential selectivity may explain some peculiar properties observed with some ACE inhibitors.