Limited proteolysis of human kidney angiotensin-converting enzyme and generation of catalytically active N- and C-terminal domains

Urinary ACE Phenotyping as a Research and Diagnostic Tool: Identification of Sex-Dependent ACE Immunoreactivity

BACKGROUND

Angiotensin-converting enzyme (ACE) is highly expressed in renal proximal tubules, but ACE activity/levels in the urine are at least 100-fold lower than in the blood. Decreased proximal tubular ACE has been associated with renal tubular damage in both animal models and clinical studies. Because ACE is shed into urine primarily from proximal tubule epithelial cells, its urinary ACE measurement may be useful as an index of tubular damage.

OBJECTIVE AND METHODOLOGY

We applied our novel approach-ACE phenotyping-to characterize urinary ACE in volunteer subjects. ACE phenotyping includes (1) determination of ACE activity using two substrates (ZPHL and HHL); (2) calculation of the ratio of hydrolysis of the two substrates (ZPHL/HHL ratio); (3) quantification of ACE immunoreactive protein levels; and (4) fine mapping of local ACE conformation with mAbs to ACE.

PRINCIPAL FINDINGS

In normal volunteers, urinary ACE activity was 140-fold less than in corresponding plasma/serum samples and did not differ between males and females. However, urinary ACE immunoreactivity (normalized binding of 25 mAbs to different epitopes) was strongly sex-dependent for the several mAbs tested, an observation likely explained by differences in tissue ACE glycosylation/sialylation between males and females. Urinary ACE phenotyping also allowed the identification of ACE outliers. In addition, daily variability of urinary ACE has potential utility as a feedback marker for dieting individuals pursuing weight loss.

CONCLUSIONS/SIGNIFICANCE

Urinary ACE phenotyping is a promising new approach with potential clinical significance to advance precision medicine screening techniques.

A new high-resolution crystal structure of the Drosophila melanogaster angiotensin converting enzyme homologue, AnCE

Angiotensin converting enzyme (ACE) is a zinc-dependent dipeptidyl carboxypeptidase with an essential role in blood pressure homeostasis in mammals. ACE has long been targeted in the treatment of hypertension through ACE inhibitors, however current inhibitors are known to cause severe side effects. Therefore, there is a requirement for a new generation of ACE inhibitors and structural information will be invaluable in their development. ACE is a challenging enzyme to work with due to its extensive glycosylation. As such, the Drosophila melanogaster ACE homologue, AnCE, which shares ∼60% sequence similarity with human ACE, can be used as a model for studying inhibitor binding. The presence of ligands originating from the crystallisation condition at the AnCE active site has proved an obstacle to studying the binding of new inhibitor precursors. Here we present the crystal structure of AnCE (in a new crystal form) at 1.85 Å resolution, using crystals grown under different conditions. This new structure may be more suitable for studying the binding of new compounds, with the potential of developing a new generation of improved ACE inhibitors.

ACE for all - a molecular perspective

Angiotensin-I converting enzyme (ACE, EC 3.4.15.1) is a zinc dependent dipeptidyl carboxypeptidase with an essential role in mammalian blood pressure regulation as part of the renin-angiotensin aldosterone system (RAAS). As such, it has long been targeted in the treatment of hypertension through the use of ACE inhibitors. Although ACE has been studied since the 1950s, only recently have the full range of functions of this enzyme begun to truly be appreciated. ACE homologues have been found in a host of other organisms, and are now known to be conserved in insects. Insect ACE homologues typically share over 30 % amino acid sequence identity with human ACE. Given that insects lack a mammalian type circulatory system, they must have crucial roles in other physiological processes. The first ACE crystal structures were reported during the last decade and have enabled these enzymes to be studied from an entirely different perspective. Here we review many of these key developments and the implications that they have had on our understanding of the diverse functions of these enzymes. Specifically, we consider how structural information is being used in the design of a new generation of ACE inhibitors with increased specificity, and how the structures of ACE homologues are related to their functions. The Anopheles gambiae genome is predicted to code for ten ACE homologues, more than any genome studied so far. We have modelled the active sites of some of these as yet uncharacterised enzymes to try and infer more about their potential roles at the molecular level.

Crystal structures of highly specific phosphinic tripeptide enantiomers in complex with the angiotensin-I converting enzyme

Human somatic angiotensin-I converting enzyme (ACE) is a zinc-dependent dipeptidyl carboxypeptidase and a central component of the renin angiotensin aldosterone system (RAAS). Its involvement in the modulation of physiological actions of peptide hormones has positioned ACE as an important therapeutic target for the treatment of hypertension and cardiovascular disorders. Here, we report the crystal structures of the two catalytic domains of human ACE (N- and C-) in complex with FI, the S enantiomer of the phosphinic ACE/ECE-1 (endothelin converting enzyme) dual inhibitor FII, to a resolution of 1.91 and 1.85 Å, respectively. In addition, we have determined the structure of AnCE (an ACE homologue from Drosophila melanogaster) in complex with both isomers. The inhibitor FI (S configuration) can adapt to the active site of ACE catalytic domains and shows key differences in its binding mechanism mostly through the reorientation of the isoxazole phenyl side group at the P₁' position compared with FII (R configuration). Differences in binding are also observed between FI and FII in complex with AnCE. Thus, the new structures of the ACE-inhibitor complexes presented here provide useful information for further exploration of ACE inhibitor pharmacophores involving phosphinic peptides and illustrate the role of chirality in enhancing drug specificity.

A modern understanding of the traditional and nontraditional biological functions of angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE) is a zinc-dependent peptidase responsible for converting angiotensin I into the vasoconstrictor angiotensin II. However, ACE is a relatively nonspecific peptidase that is capable of cleaving a wide range of substrates. Because of this, ACE and its peptide substrates and products affect many physiologic processes, including blood pressure control, hematopoiesis, reproduction, renal development, renal function, and the immune response. The defining feature of ACE is that it is composed of two homologous and independently catalytic domains, the result of an ancient gene duplication, and ACE-like genes are widely distributed in nature. The two ACE catalytic domains contribute to the wide substrate diversity of ACE and, by extension, the physiologic impact of the enzyme. Several studies suggest that the two catalytic domains have different biologic functions. Recently, the X-ray crystal structure of ACE has elucidated some of the structural differences between the two ACE domains. This is important now that ACE domain-specific inhibitors have been synthesized and characterized. Once widely available, these reagents will undoubtedly be powerful tools for probing the physiologic actions of each ACE domain. In turn, this knowledge should allow clinicians to envision new therapies for diseases not currently treated with ACE inhibitors.

Novel mechanism of inhibition of human angiotensin-I-converting enzyme (ACE) by a highly specific phosphinic tripeptide

Human ACE (angiotensin-I-converting enzyme) has long been regarded as an excellent target for the treatment of hypertension and related cardiovascular diseases. Highly potent inhibitors have been developed and are extensively used in the clinic. To develop inhibitors with higher therapeutic efficacy and reduced side effects, recent efforts have been directed towards the discovery of compounds able to simultaneously block more than one zinc metallopeptidase (apart from ACE) involved in blood pressure regulation in humans, such as neprilysin and ECE-1 (endothelin-converting enzyme-1). In the present paper, we show the first structures of testis ACE [C-ACE, which is identical with the C-domain of somatic ACE and the dominant domain responsible for blood pressure regulation, at 1.97Å (1 Å=0.1 nm)] and the N-domain of somatic ACE (N-ACE, at 2.15Å) in complex with a highly potent and selective dual ACE/ECE-1 inhibitor. The structural determinants revealed unique features of the binding of two molecules of the dual inhibitor in the active site of C-ACE. In both structures, the first molecule is positioned in the obligatory binding site and has a bulky bicyclic P(1)' residue with the unusual R configuration which, surprisingly, is accommodated by the large S(2)' pocket. In the C-ACE complex, the isoxazole phenyl group of the second molecule makes strong pi-pi stacking interactions with the amino benzoyl group of the first molecule locking them in a 'hand-shake' conformation. These features, for the first time, highlight the unusual architecture and flexibility of the active site of C-ACE, which could be further utilized for structure-based design of new C-ACE or vasopeptidase inhibitors.

Porcine pulmonary angiotensin I-converting enzyme--biochemical characterization and spatial arrangement of the N- and C-domains by three-dimensional electron microscopic reconstruction

The somatic angiotensin I-converting enzyme (sACE; peptidyl-dipeptidase A; EC 3.4.15.1) was isolated from pig lung and purified to homogeneity. The purified enzyme has a molecular mass of about 180 kDa. Upon proteolytic cleavage, two approximately 90 kDa fragments were obtained and identified by amino-terminal sequence analysis as the N- and C-domains of sACE. Both purified domains were shown to be catalytically active. A 2.3 nm resolution model of sACE was obtained by three-dimensional electron microscopic reconstruction of negatively stained sACE particles, based on atomic X-ray data fitting. Our model shows for the first time the relative orientation of the sACE catalytically active domains and their spatial distance.

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.

Different contributions of the angiotensin-converting enzyme C-domain and N-domain in subjects with the angiotensin-converting enzyme II and DD genotype

BACKGROUND

Angiotensin-converting enzyme (ACE) insertion/deletion (I/D) polymorphism-related differences in ACE concentration do not result in differences in angiotensin levels.

METHODS AND RESULTS

To investigate whether this relates to differences in the contribution of the ACE C-domain and N-domain, we quantified, using the C-domain-selective inhibitors quinaprilat and RXPA380, and the N-domain-selective inhibitor RXP407, the contribution of both domains to the metabolism of angiotensin I, bradykinin, the C-domain-selective substrate Mca-BK(1-8), and the N-domain-selective substrate Mca-Ala in serum of IIs, DDs, and 'hyperACE' subjects (i.e., subjects with increased ACE due to enhanced shedding). During incubation with angiotensin I, the highest angiotensin II levels were observed in sera with the highest ACE activity. This confirms that ACE is rate-limiting with regard to angiotensin II generation. C-domain-selective concentrations of quinaprilat fully blocked angiotensin I-II conversion in DDs, whereas additional N-domain blockade was required to fully block conversion in IIs. Both domains contributed to bradykinin hydrolysis in all subjects, and the inhibition profile of RXP407 when using Mca-Ala was identical in IIs and DDs. In contrast, the RXPA380 concentrations required to block C-domain activity when using Mca-BK (1-8) were three-fold higher in IIs than DDs.

CONCLUSION

The contributions of the C-domain and N-domain differ between DDs and IIs, and RXPA380 is the first inhibitor capable of distinguishing D-allele ACE from I-allele ACE. The lack of angiotensin II accumulation in DDs in vivo is not because of the often quoted concept that ACE is a nonrate-limiting enzyme. It may relate to the fact that in IIs both the N-domain and C- domain generate angiotensin II, whereas in DDs only the C-domain converts angiotensin I.

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.

Homologous substitution of ACE C-domain regions with N-domain sequences: effect on processing, shedding, and catalytic properties

Angiotensin-converting enzyme (ACE) exists as two isoforms: somatic ACE (sACE), comprised of two homologous N and C domains, and testis ACE (tACE), comprised of the C domain only. The N and C domains are both active, but show differences in substrate and inhibitor specificity. While both isoforms are shed from the cell surface via a sheddase-mediated cleavage, tACE is shed much more efficiently than sACE. To delineate the regions of tACE that are important in catalytic activity, intracellular processing, and regulated ectodomain shedding, regions of the tACE sequence were replaced with the corresponding N-domain sequence. The resultant chimeras C1-163Ndom-ACE, C417-579Ndom-ACE, and C583-623Ndom-ACE were processed to the cell surface of transfected Chinese hamster ovary (CHO) cells, and were cleaved at the identical site as that of tACE. They also showed acquisition of N-domain-like catalytic properties. Homology modelling of the chimeric proteins revealed structural changes in regions required for tACE-specific catalytic activity. In contrast, C164-416Ndom-ACE and C191-214Ndom-ACE demonstrated defective intracellular processing and were neither enzymatically active nor shed. Therefore, critical elements within region D164-V416 and more specifically I191-T214 are required for the processing, cell-surface targeting, and enzyme activity of tACE, and cannot be substituted for by the homologous N-domain sequence.

Inhibitory antibodies to human angiotensin-converting enzyme: fine epitope mapping and mechanism of action

Angiotensin I-converting enzyme (ACE), a key enzyme in cardiovascular pathophysiology, consists of two homologous domains (N and C), each bearing a Zn-dependent active site. We modeled the 3D-structure of the ACE N-domain using known structures of the C-domain of human ACE and the ACE homologue, ACE2, as templates. Two monoclonal antibodies (mAb), 3A5 and i2H5, developed against the human N-domain of ACE, demonstrated anticatalytic activity. N-domain modeling and mutagenesis of 21 amino acid residues allowed us to define the epitopes for these mAbs. Their epitopes partially overlap: amino acid residues K407, E403, Y521, E522, G523, P524, D529 are present in both epitopes. Mutation of 4 amino acid residues within the 3A5 epitope, N203E, R550A, D558L, and K557Q, increased the apparent binding of mAb 3A5 with the mutated N-domain 3-fold in plate precipitation assay, but abolished the inhibitory potency of this mAb. Moreover, mutation D558L dramatically decreased 3A5-induced ACE shedding from the surface of CHO cells expressing human somatic ACE. The inhibition of N-domain activity by mAbs 3A5 and i2H5 obeys similar kinetics. Both mAbs can bind to the free enzyme and enzyme-substrate complex, forming E.mAb and E.S.mAb complexes, respectively; however, only complex E.S can form a product. Kinetic analysis indicates that both mAbs bind better with the ACE N-domain in the presence of a substrate, which, in turn, implies that binding of a substrate causes conformational adjustments in the N-domain structure. Independent experiments with ELISA demonstrated better binding of mAbs 3A5 and i2H5 in the presence of the inhibitor lisinopril as well. This effect can be attributed to better binding of both mAbs with the "closed" conformation of ACE, therefore, disturbing the hinge-bending movement of the enzyme, which is necessary for catalysis.

Crystal Structure of the N Domain of Human Somatic Angiotensin I-converting Enzyme Provides a Structural Basis for Domain-specific Inhibitor Design

Human somatic angiotensin I-converting enzyme (sACE) is a key regulator of blood pressure and an important drug target for combating cardiovascular and renal disease. sACE comprises two homologous metallopeptidase domains, N and C, joined by an inter-domain linker. Both domains are capable of cleaving the two hemoregulatory peptides angiotensin I and bradykinin, but differ in their affinities for a range of other substrates and inhibitors. Previously we determined the structure of testis ACE (C domain); here we present the crystal structure of the N domain of sACE (both in the presence and absence of the antihypertensive drug lisinopril) in order to aid the understanding of how these two domains differ in specificity and function. In addition, the structure of most of the inter-domain linker allows us to propose relative domain positions for sACE that may contribute to the domain cooperativity. The structure now provides a platform for the design of "domain-specific" second-generation ACE inhibitors.

The N domain of somatic angiotensin-converting enzyme negatively regulates ectodomain shedding and catalytic activity

sACE (somatic angiotensin-converting enzyme) consists of two homologous, N and C domains, whereas the testis isoenzyme [tACE (testis ACE)] consists of a single C domain. Both isoenzymes are shed from the cell surface by a sheddase activity, although sACE is shed much less efficiently than tACE. We hypothesize that the N domain of sACE plays a regulatory role, by occluding a recognition motif on the C domain required for ectodomain shedding and by influencing the catalytic efficiency. To test this, we constructed two mutants: CNdom-ACE and CCdom-ACE. CNdom-ACE was shed less efficiently than sACE, whereas CCdom-ACE was shed as efficiently as tACE. Notably, cleavage occurred both within the stalk and the interdomain bridge in both mutants, suggesting that a sheddase recognition motif resides within the C domain and is capable of directly cleaving at both positions. Analysis of the catalytic properties of the mutants and comparison with sACE and tACE revealed that the k(cat) for sACE and CNdom-ACE was less than or equal to the sum of the kcat values for tACE and the N-domain, suggesting negative co-operativity, whereas the kcat value for the CCdom-ACE suggested positive co-operativity between the two domains. Taken together, the results provide support for (i) the existence of a sheddase recognition motif in the C domain and (ii) molecular flexibility of the N and C domains in sACE, resulting in occlusion of the C-domain recognition motif by the N domain as well as close contact of the two domains during hydrolysis of peptide substrates.

Localization of an N-domain region of angiotensin-converting enzyme involved in the regulation of ectodomain shedding using monoclonal antibodies

ACE chimeric proteins and N domain monoclonal antibodies (mAbs) were used to determine the influence of the N domain, and particular regions thereof, on the rate of ACE ectodomain shedding. Somatic ACE (having both N and C domains) was shed at a rate of 20%/24 h. Deletion of the C domain of somatic ACE generated an N domain construct (ACEDeltaC) which demonstrated the lowest rate of shedding (12%). However, deletion of the N domain of somatic ACE (ACEDeltaN) dramatically increased shedding (212%). Testicular ACE (tACE) having 36 amino acid residues (heavily O-glycosylated) at the N-terminus of the C domain shows a 4-fold decrease in the rate of shedding (49%) compared to that of ACEDeltaN. When the N-terminal region of the C domain was replaced with the corresponding homologous 141 amino acids of the N domain (N-delACE) the rate of shedding of the ACEDeltaN was only slightly decreased (174%), but shedding was still 3.5-fold more efficient than wild-type testicular ACE. Monoclonal antibodies specific for distinct, but overlapping, N-domain epitopes altered the rate of ACE shedding. The mAb 3G8 decreased the rate of shedding by 30%, whereas mAbs 9B9 and 3A5 stimulated ACE shedding 2- to 4-fold. Epitope mapping of these mAbs in conjunction with a homology model of ACE N domain structure, localized a region in the N-domain that may play a role in determining the relatively low rate of shedding of somatic ACE from the cell surface.

Monoclonal antibodies 1B3 and 5C8 as probes for monitoring the integrity of the C-terminal end of soluble angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE) is a membrane-anchored ectoprotein that is proteolytically cleaved, yielding an enzymatically active soluble ACE. Two mouse monoclonal antibodies, MAbs 1B3 and 5C8, were generated to the C-terminal part of human soluble ACE. MAb 1B3 recognized the catalytically active ACE, as revealed by ELISA and precipitation assays, whereas Western blotting and immunohistochemisty on paraffin- embedded sections using MAb 5C8 detected denatured ACE. MAb 1B3 showed extensive cross-reactivity, recognizing 15 species out of the 16 tested. The binding of this MAb to ACE was greatly affected by conformational changes induced by adsorption on plastic, formalin fixation, and underglycosylation. Furthermore, MAb 1B3 binding to the mutated ACE (Pro1199Leu substitution in the juxtamembrane region, leading to a fivefold increase in serum ACE level) was markedly decreased. MAb 5C8 detected all the known expression sites of full-size ACE using formalin-fixed and paraffin-embedded human tissues. The sequential epitope for MAb 5C8 is formed by the last 11 amino acid residues of soluble ACE (Pro1193-Arg1203), whereas the conformational epitope for 1B3 is formed by a motif within these 11 amino acid residues and, in addition, by at least one stretch that includes Ala837-His839 located distal to the sequential epitope. Our findings demonstrated that MAbs 1B3 and 5C8 are very useful for the study of ACE shedding, for identification of mutations in stalk regions, and for studying alternatively spliced variants of ACE. In addition, binding of MAb 1B3 is a sensitive determinant of the integrity of soluble ACE.

Development and characterization of rat monoclonal antibodies to denatured mouse angiotensin-converting enzyme

Four new rat monoclonal antibodies, generated to denatured mouse somatic angiotensin-converting enzyme (ACE, CD143), detect mouse ACE with high sensitivity in Western blotting. Epitope mapping for the monoclonal antibodies--B12, 4G6 and 5C4--was also performed. Two monoclonal antibodies--B12 and 5C4--are directed to various epitopes on the N-domain--i.e., they recognized only the somatic isoform of mouse ACE. The monoclonal antibody H7 recognized an epitope on the C-domain of mouse ACE. The monoclonal antibody 4G6 was directed to a sequence on the N-domain of mouse ACE, which is homologous to a region of the C-domain and, as a result, also recognizes mouse testicular ACE (tACE) by means of Western blotting. In paraffin-embedded mouse tissues, all monoclonal antibodies detected all known expression sites of somatic ACE (sACE), e.g., the epithelial cells of the kidney proximal tubules, intestine and epididymis, and heterogeneously in endothelial cells. The monoclonal antibodies 4G6 and H7 additionally stained mouse tACE in spermatozoa and in mature spermatids. The monoclonal antibody 4G6 also demonstrated cross-reactivity with sACE from a broad spectrum of animal species, including human, rat, rabbit and bovine. However, this monoclonal antibody did not recognize the testicular isoform of ACE of these species. This set of monoclonal antibodies is useful for identifying even subtle changes in mouse ACE conformation because of denaturation. These monoclonal antibodies are also sensitive tools for the detection of mouse ACE in biological fluids and tissues by using proteomics approaches. Their high reactivity in paraffin-embedded tissues opens up opportunities to study possible changes in the pattern of ACE expression in knockout mouse models and may prove useful for correlating ACE expression in these models with human diseases.

A calorimetric study of the binding of lisinopril, enalaprilat and captopril to angiotensin-converting enzyme

The angiotensin I-converting enzyme (ACE; EC.3.4.15.1) is a dipeptidyl carboxypeptidase that plays a central role in blood pressure regulation. The somatic form of the enzyme is composed of two highly similar domains, usually referred to as N and C domains, each containing one active site. Nevertheless, a 1:1 stoichiometry for the binding of lisinopril, captopril or enalaprilat to somatic pig lung ACE is shown by isothermal titration calorimetry (ITC) and enzymatic assays. The binding of the three inhibitors at neutral pH is very tight and the enthalpy changes are positive, indicating that the binding is entropically driven. The origin of this thermodynamic signature is discussed under the new structural information available.

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.

Epitope-dependent blocking of the angiotensin-converting enzyme dimerization by monoclonal antibodies to the N-terminal domain of ACE: possible link of ACE dimerization and shedding from the cell surface

In a biomembrane modeling system, reverse micelles, somatic ACE forms dimers via carbohydrate-mediated interaction, providing evidence for the existence of a carbohydrate-recognizing domain on the ACE molecule. We localized this putative region on the N-domain of ACE using monoclonal antibodies (mAbs) to seven different epitopes of ACE. Two mAbs, 9B9 and 3G8, directed to distinct, but overlapping, epitopes of the N-domain of ACE shielded the CRD. Only "simple" ACE-antibody complexes were found in the system. Five mAbs allowed the formation of "double" antibody-ACE-ACE-antibody complexes via carbohydrate-mediated interactions. The results were confirmed using the ACE N- and C-domains. Testicular ACE was unable to form carbohydrate-mediated ACE dimers in the reverse micelles, while the N-domain of ACE, obtained by limited proteolysis of the parent full-length ACE, retained the ability to form dimers. Furthermore, mAb 3G8, which blocked ACE dimerization in micelles, significantly inhibited ACE shedding from the surface of ACE-expressing cells. Galactose prevented ACE dimerization in reverse micelles and also affected antibody-induced ACE shedding in an epitope-dependent manner. Restricted glycosylation of somatic ACE, obtained by the treatment of CHO-ACE cells with the glucosidase inhibitor N-butyldeoxynojirimycin, significantly increased the rate of basal ACE shedding and altered antibody-induced ACE shedding. A chemical cross-linking approach was used to show that ACE is present (at least in part) as noncovalently linked dimers on the surface of CHO-ACE cells. These results suggest a possible link between putative ACE dimerization on the cell surface and the proteolytic cleavage (shedding) of ACE.

Monoclonal antibodies to denatured human ACE (CD 143), broad species specificity, reactivity on paraffin sections, and detection of subtle conformational changes in the C-terminal domain of ACE

Two new mouse monoclonal antibodies (mAbs) were generated to denatured human angiotensin-converting enzyme (ACE, CD143). The clones 2E2 and 3C5, each of the IgG1 kappa chain isotype, detect ACE with high sensitivity, respectively, at 20 ng and 2 ng of protein per lane in Western blotting. They both recognize different epitopes on the C-domain of ACE located between amino acid residues 740 and 992. In formalin-fixed and paraffin-embedded human tissues, immunohistochemistry revealed all known expression sites of ACE, e.g. the epithelial brush borders of proximal kidney tubules, epithelial cells of epididymis, endothelial cells, activated macrophages as well as germ cells during spermatogenesis. In contrast to other mAbs to denatured human ACE, mAbs 2E2 and 3C5 demonstrate cross-reactivity with a broad spectrum of animal species such as monkey, rat, rabbit, cattle, dog, cat, and guinea pig. In addition, mAb 2E2 recognized mouse ACE in Western blotting and on paraffin sections. Our findings suggest that mAbs 2E2 and 3C5 are useful for identifying even subtle changes in ACE conformation resulting from denaturation. These mAbs are also sensitive tools for the detection of minimal amounts of ACE in biological fluids and tissues using proteomics approaches. Their reactivity in routinely processed tissues of various species may prove useful for correlation of ACE expression in animal models to human diseases.

Temperature-induced selective death of the C-domain within angiotensin-converting enzyme molecule

Somatic angiotensin-converting enzyme (ACE) consists of two homologous domains, each domain bearing a catalytic site. Differential scanning calorimetry of the enzyme revealed two distinct thermal transitions with melting points at 55.3 and 70.5 degrees C. which corresponded to denaturation of C- and N-domains, respectively. Different heat stability of the domains underlies the methods of acquiring either single active N-domain or active N-domain with inactive C-domain within parent somatic ACE. Selective denaturation of C-domain supports the hypothesis of independent folding of the two domains within the ACE molecule. Modeling of ACE secondary structure revealed the difference in predicted structures of the two domains, which, in turn, allowed suggestion of the region 29-133 in amino acid sequence of the N-part of the molecule as responsible for thermostability of the N-domain.

Is "somatic" angiotensin I-converting enzyme a mechanosensor?

"Somatic" angiotensin I-converting enzyme (ACE) appears to be one of the evolutionary advances that made a closed circulation possible, and may have contributed to the Cambrian "explosion" of species approximately 540 million years ago. It also appears to be at the origin of a large number of common human diseases. A model is proposed in which the duplicated form of ACE ("somatic" ACE) functions as a mechanotransducer, defending downstream vessels and tissues from an increase in pressure. In the model, ACE senses shear stress (blood velocity) in regions of turbulent blood flow. An increase in shear stress strips an autoinhibitor tripeptide, FQP, from the N-terminal active site, thereby activating it. The C-terminal domain is constitutively activated by chloride. This model explains the clinical superiority of hydrophobic ACE inhibitors relative to hydrophilic ones.

Arg(1098) is critical for the chloride dependence of human angiotensin I-converting enzyme C-domain catalytic activity

Angiotensin (Ang) I-converting enzyme (ACE) is a Zn(2+) metalloprotease with two homologous catalytic domains. Both the N- and C-terminal domains are peptidyl dipeptidases. Hydrolysis by ACE of its decapeptide substrate Ang I is increased by Cl(-), but the molecular mechanism of this regulation is unclear. A search for single substitutions to Gln among all conserved basic residues (Lys/Arg) in human ACE C-domain identified R1098Q as the sole mutant that lacked Cl(-) dependence. Cl(-) dependence is also lost when the equivalent Arg in the N-domain, Arg(500), is substituted with Gln. The Arg(1098) to Lys substitution reduced Cl(-) binding affinity by approximately 100-fold. In the absence of Cl(-), substrate binding affinity (1/K(m)) of and catalytic efficiency (k(cat)/K(m)) for Ang I hydrolysis are increased 6.9- and 32-fold, respectively, by the Arg(1098) to Gln substitution, and are similar (<2-fold difference) to the respective wild-type C-domain catalytic constants in the presence of optimal [Cl(-)]. The Arg(1098) to Gln substitution also eliminates Cl(-) dependence for hydrolysis of tetrapeptide substrates, but activity toward these substrates is similar to that of the wild-type C-domain in the absence of Cl(-). These findings indicate that: 1) Arg(1098) is a critical residue of the C-domain Cl(-)-binding site and 2) a basic side chain is necessary for Cl(-) dependence. For tetrapeptide substrates, the inability of R1098Q to recreate the high affinity state generated by the Cl(-)-C-domain interaction suggests that substrate interactions with the enzyme-bound Cl(-) are much more important for the hydrolysis of short substrates than for Ang I. Since Cl(-) concentrations are saturating under physiological conditions and Arg(1098) is not critical for Ang I hydrolysis, we speculate that the evolutionary pressure for the maintenance of the Cl(-)-binding site is its ability to allow cleavage of short cognate peptide substrates at high catalytic efficiencies.

Effects of the N-terminal sequence of ACE on the properties of its C-domain

Angiotensin I-converting enzyme (ACE, kininase II) has 2 active domains (N and C) in a single peptide chain. Because we found its N-domain more stable than its C-domain, we investigated the effect of the amino-terminus of human ACE on the C-domain with a molecular construct expressed in Chinese hamster ovary cells (CHO) cells and transiently in HEK293 cells. This active N-deleted ACE contained only the first 141 amino acids of the human N-domain but not its active center and was linked to the active C-domain containing the transmembrane and cytosolic portions of ACE. The CHO cells were also transfected with human B(2) bradykinin receptor. ACE inhibitors (5 nmol/L or 1 micromol/L) augmented bradykinin (100 nmol/L) effects, elevated B(2) receptor numbers, and resensitized the receptor desensitized by agonist as measured by arachidonic acid release or [Ca(2+)](i) mobilization. Arachidonic acid release was mediated by pertussis toxin-sensitive G alpha(i), and [Ca(2+)](i) mobilization was mediated by pertussis-insensitive G alpha(q) protein receptor complex. The properties of the construct were compared with wild-type ACE and separate N- and C-domains. The N-deleted ACE differed from wild-type in activation by Cl(-) and [SO(4)](2-) ions, hydrolysis ratios of substrates (both short synthetic and endogenous peptides) and heat stability. Thus, the N-terminal peptide of ACE affected the characteristics of the C-domain active center. ACE inhibitors acting on N-deleted ACE, which had only a single C-domain active center anchored to plasma membrane, induced cross-talk between the enzyme and the B(2) receptor (eg, the inhibitors resensitized the receptor) independent of blocking bradykinin inactivation.