A deeply recessed active site in angiotensin-converting enzyme is indicated from the binding characteristics of biotin-spacer-inhibitor reagents

The Plethora of Angiotensin-Converting Enzyme-Processed Peptides in Mouse Plasma

Angiotensin-converting enzyme (ACE) converts angiotensin I into the potent vasoconstrictor angiotensin II, which regulates blood pressure. However, ACE activity is also essential for other physiological functions, presumably through processing of peptides unrelated to angiotensin. The goal of this study was to identify novel natural substrates and products of ACE through a series of mass-spectrometric experiments. This included comparing the ACE-treated and untreated plasma peptidomes of ACE-knockout (KO) mice, validation with select synthetic peptides, and a quantitative in vivo study of ACE substrates in mice with distinct genetic ACE backgrounds. In total, 244 natural peptides were identified ex vivo as possible substrates or products of ACE, demonstrating high promiscuity of the enzyme. ACE prefers to cleave substrates with Phe or Leu at the C-terminal P2' position and Gly in the P6 position. Pro in P1' and Iso in P1 are typical residues in peptides that ACE does not cleave. Several of the novel ACE substrates are known to have biological activities, including a fragment of complement C3, the spasmogenic C3f, which was processed by ACE ex vivo and in vitro. Analyses with N-domain-inactive (NKO) ACE allowed clarification of domain selectivity toward substrates. The in vivo ACE-substrate concentrations in WT, transgenic ACE-KO, NKO, and CKO mice correspond well with the in vitro observations in that higher levels of the ACE substrates were observed when the processing domain was knocked out. This study highlights the vast extent of ACE promiscuity and provides a valuable platform for further investigations of ACE functionality.

Conformational fingerprint of blood and tissue ACEs: Personalized approach

Background

The pattern of binding of monoclonal antibodies (mAbs) to 18 epitopes on human angiotensin I-converting enzyme (ACE)–“conformational fingerprint of ACE”–is a sensitive marker of subtle conformational changes of ACE due to mutations, different glycosylation in various cells, the presence of ACE inhibitors and specific effectors, etc.

Methodology/Principal findings

We described in detail the methodology of the conformational fingerprinting of human blood and tissue ACEs that allows detecting differences in surface topography of ACE from different tissues, as well detecting inter-individual differences. Besides, we compared the sensitivity of the detection of ACE inhibitors in the patient’s plasma using conformational fingerprinting of ACE (with only 2 mAbs to ACE, 1G12 and 9B9) and already accepted kinetic assay and demonstrated that the mAbs-based assay is an order of magnitude more sensitive. This approach is also very effective in detection of known (like bilirubin and lysozyme) and still unknown ACE effectors/inhibitors which nature and set could vary in different tissues or different patients.

Conclusions/Significance

Phenotyping of ACE (and conformational fingerprinting of ACE as a part of this novel approach for characterization of ACE) in individuals really became informative and clinically relevant. Appreciation (and counting on) of inter-individual differences in ACE conformation and accompanying effectors make the application of this approach for future personalized medicine with ACE inhibitors more accurate. This (or similar) methodology can be applied to any enzyme/protein for which there is a number of mAbs to its different epitopes.

Activity-Based Protein Profiling: From Chemical Novelty to Biomedical Stalwart

Biological systems often respond to environmental changes by rapidly altering the activity of specific enzymes: for example through desequesterization of enzyme activities by dissociation from inhibitors, activation/deactivation through posttranslational modification, or relocation of the enzyme to different organelles. This means that expression levels of enzymes do not necessarily correlate with the activities observed for these enzymes. In this chapter we review some of the approaches used to selectively image only the active sub-populations of given enzymes, the so-called activity-based protein profiling. A focus lies on recent developments that are taking this approach from chemical novelty to biochemical stalwart.

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.

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.

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.

ACE2 X-ray structures reveal a large hinge-bending motion important for inhibitor binding and catalysis

The angiotensin-converting enzyme (ACE)-related carboxypeptidase, ACE2, is a type I integral membrane protein of 805 amino acids that contains one HEXXH + E zinc-binding consensus sequence. ACE2 has been implicated in the regulation of heart function and also as a functional receptor for the coronavirus that causes the severe acute respiratory syndrome (SARS). To gain further insights into this enzyme, the first crystal structures of the native and inhibitor-bound forms of the ACE2 extracellular domains were solved to 2.2- and 3.0-Å resolution, respectively. Comparison of these structures revealed a large inhibitor-dependent hinge-bending movement of one catalytic subdomain relative to the other (∼16°) that brings important residues into position for catalysis. The potent inhibitor MLN-4760 ((S,S)-2-{1-carboxy-2-[3-(3,5-dichlorobenzyl)-3H-imidazol4-yl]-ethylamino}-4-methylpentanoic acid) makes key binding interactions within the active site and offers insights regarding the action of residues involved in catalysis and substrate specificity. A few active site residue substitutions in ACE2 relative to ACE appear to eliminate the S₂′ substrate-binding subsite and account for the observed reactivity change from the peptidyl dipeptidase activity of ACE to the carboxypeptidase activity of ACE2.

Crystal structure of the human angiotensin-converting enzyme-lisinopril complex

Angiotensin-converting enzyme (ACE) has a critical role in cardiovascular function by cleaving the carboxy terminal His-Leu dipeptide from angiotensin I to produce a potent vasopressor octapeptide, angiotensin II. Inhibitors of ACE are a first line of therapy for hypertension, heart failure, myocardial infarction and diabetic nephropathy. Notably, these inhibitors were developed without knowledge of the structure of human ACE, but were instead designed on the basis of an assumed mechanistic homology with carboxypeptidase A. Here we present the X-ray structure of human testicular ACE and its complex with one of the most widely used inhibitors, lisinopril (N2-[(S)-1-carboxy-3-phenylpropyl]-L-lysyl-L-proline; also known as Prinivil or Zestril), at 2.0 A resolution. Analysis of the three-dimensional structure of ACE shows that it bears little similarity to that of carboxypeptidase A, but instead resembles neurolysin and Pyrococcus furiosus carboxypeptidase--zinc metallopeptidases with no detectable sequence similarity to ACE. The structure provides an opportunity to design domain-selective ACE inhibitors that may exhibit new pharmacological profiles.

Hydrolysis by somatic angiotensin-I converting enzyme of basic dipeptides from a cholecystokinin/gastrin and a LH-RH peptide extended at the C-terminus with gly-Arg/Lys-arg, but not from diarginyl insulin

Endoproteolytic cleavage of protein prohormones often generates intermediates extended at the C-terminus by Arg-Arg or Lys-Arg, the removal of which by a carboxypeptidase (CPE) is normally an important step in the maturation of many peptide hormones. Recent studies in mice that lack CP activity indicate the existence of alternative tissue or plasma enzymes capable of removing C-terminal basic residues from prohormone intermediates. Using inhibitors of angiotensin I-converting enzyme (ACE) and CP, we show that both these enzymes in mouse serum can remove the basic amino acids from the C-terminus of CCK5-GRR and LH-RH-GKR, but only CP is responsible for converting diarginyl insulin to insulin. ACE activity removes C-terminal dipeptides to generate the Gly-extended peptides, whereas CP hydrolysis gives rise to CCK5-GR and LH-RH-GK, both of which are susceptible to the dipeptidyl carboxypeptidase activity of ACE. Somatic ACE has two similar protein domains (the N-domain and the C-domain), each with an active site that can display different substrate specificities. CCK5-GRR is a high-affinity substrate for both the N-domain and C-domain active sites of human sACE (Km of 9.4 microm and 9.0 microm, respectively) with the N-domain showing greater efficiency (kcat : Km ratio of 2.6 in favour of the N-domain). We conclude that somatic forms of ACE should be considered as alternatives to CPs for the removal of basic residues from some Arg/Lys-extended peptides.

Tubuloglomerular feedback in ACE-deficient mice

In these experiments, we used a strain of angiotensin converting enzyme (ACE) germline null mutant mice, generated by J. H. Krege and co-workers (J. H. Krege, S. W. M. John, L. L. Langenbach, J. B. Hodgin, J. R. Hagaman, E. S. Bachman, J. C. Jennette, D. A. O'Brien, and O. Smithies. Nature 375: 146-148, 1995), to examine the effect of chronic ACE deficiency on the magnitude of tubuloglomerular feedback (TGF) responses. The genotype was determined by PCR on DNA extracted from the tail and was verified after each experiment by assessment of the blood pressure response to an injection of ANG I. To assess TGF responsiveness, we determined the change in stop-flow pressure (PSF) caused by increasing NaCl concentration at the macula densa by using micropuncture techniques. When loop of Henle flow rate was increased from 0 to 40 nl/min, PSF fell from a mean of 42.3 +/- 1.95 to 33.6 +/- 2.09 mmHg (n = 6, P = 0.005) in wild-type mice (+/+), fell from 40.6 +/- 2.35 to 38.6 +/- 1.93 mmHg in heterozygous (+/-) mice (n = 7, P = 0.014), and did not change in homozygous ACE (-/-) mice [36.7 +/- 2.02 mmHg vs. 36.4 +/- 2.01 mmHg; n = 4, P = not significant (NS)]. During an infusion of ANG II at a dose that did not significantly elevate blood pressure (70 ng. kg-1. min-1), TGF response magnitude (PSF 0 - PSF 40) increased from 6.5 +/- 1.4 to 9.8 +/- 1.19 mmHg in +/+ (P = 0.006), from 1.14 +/- 0.42 to 4.6 +/- 1.3 mmHg in +/- (P = 0.016), and from 0.42 +/- 0.25 to 4.02 +/- 1.06 in -/- mice (P = 0.05). Absence of TGF responses in ACE null mutant mice and restoration of near-normal responses during an acute infusion of ANG II supports previous conclusions that ANG II is an essential component in the signal transmission pathway that links the macula densa with the glomerular vascular pole.

Enthalpy of captopril-angiotensin I-converting enzyme binding

High-sensitivity titration calorimetry is used to measure changes in enthalpy, heat capacity and protonation for the binding of captopril to the angiotensin I-converting enzyme (ACE; EC 3.4.15.1). The affinity of ACE to captopril is high and changes slightly with the pH, because the number of protons linked to binding is low. The determination of the enthalpy change at different pH values suggests that the protonated group in the captopril-ACE complex exhibits a heat protonation of approximately -30 kJ/mol. This value agrees with the protonation of an imidazole group. The residues which may become protonated in the complex could be two histidines existing in two active sites, which are joined to the amino acids coordinated to Zn2+. Calorimetric measurements indicate that captopril binds to two sites in the monomer of ACE, this binding being enthalpically unfavorable and being dominated by a large positive entropy change. Thus, binding is favored by both electrostatic and hydrophobic interactions. The temperature dependence of the free energy of binding deltaG degrees is weak because of the enthalpy-entropy compensation caused by a large heat capacity change, deltaCp =-4.3+/-0.1 kJ/K/mol of monomeric ACE. The strong favorable binding entropy and the negative deltaCp indicate both a large contribution to binding due to hydrophobic effects, which seem to originate from dehydration of the ligand-protein interface, and slight conformational changes in the vicinity of the active sites.

Codominant interference, antieffectors, and multitarget drugs

The insufficient selectivity of drugs is a bane of present-day therapies. This problem is significant for antibacterial drugs, difficult for antivirals, and utterly unsolved for anticancer drugs, which remain ineffective against major cancers, and in addition cause severe side effects. The problem may be solved if a therapeutic agent could have a multitarget, combinatorial selectivity, killing, or otherwise modifying, a cell if and only if it contains a predetermined set of molecular targets and lacks another predetermined set of targets. An earlier design of multitarget drugs [Varshavsky, A. (1995) Proc. Natl. Acad. Sci. USA 92, 3663-3667] was confined to macromolecular reagents such as proteins, with the attendant difficulties of intracellular delivery and immunogenicity. I now propose a solution to the problem of drug selectivity that is applicable to small (</=1 kDa) drugs. Two ideas, codominant interference and antieffectors, should allow a therapeutic regimen to possess combinatorial selectivity, in which the number of positively and negatively sensed macromolecular targets can be two, three, or more. The nature of the effector and interference moieties in a multitarget drug determines its use: selective killing of cancer cells or, for example, the inhibition of a neurotransmitter-inactivating enzyme in a specific subset of the enzyme-containing cells. The in vivo effects of such drugs would be analogous to the outcomes of the Boolean operations "and," "or," and combinations thereof. I discuss the logic and applications of the antieffector and interference/codominance concepts, and the attendant problem of pharmacokinetics.

Calorimetric analysis of lisinopril binding to angiotensin I-converting enzyme

Isothermal titration microcalorimetry has been used to measure changes in enthalpy and heat capacity for binding of lisinopril to the angiotensin I-converting enzyme (ACE; EC 3.4.15.1) and to its apoenzyme at pH 7.5 over a temperature range of 15-30 degrees C. Calorimetric measurements indicate that lisinopril binds to two sites in the monomer of both holo- and apo-ACE. Binding of lisinopril to both systems is enthalpically unfavorable and, thus, is dominated by a large positive entropy change. The enthalpy change of binding is strongly temperature-dependent for both holo- and apo-ACE, arising from a large heat capacity change of binding equal to -2.4 +/- 0.2 kJ/K/mol of monomeric holo-ACE) and to -1.9 +/- 0.2 kJ/K/mol of monomeric apo-ACE), respectively. The negative values of deltaCp for both systems are consistent with burial of a large non-polar surface area upon binding. Although the binding of lisinopril to holo- and apo-ACE is favored by entropy changes, this is more positive for the holoenzyme. Thus, the interaction between Zn2+ and lisinopril results in a higher affinity of the holoenzyme for this drug due to a more favorable entropic contribution.

Purification and assay methods for angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE; EN 3.4.15.1) is a peptidyl dipeptide hydrolase that removes the carboxyl terminal His-Leu from angiotensin I to produce the octapeptide angiotensin II. In addition, ACE inactivates bradykinin, a vasodilator peptide/mediator of inflammation, as well as substance P, enkephalins and endorphins. Because of the importance of ACE and its active site-directed inhibitors in the pathogenesis and treatment of cardiovascular disorders such as hypertension and heart failure, ACE purification and assay are of clinical and commercial, as well as scientific interest. This review summarizes the historical development of ACE purification and assay methods and presents some innovative high-performance liquid chromatography-based techniques developed in our own laboratory for high yield and efficient purification and sensitive and specific assay of ACE.

Variation in angiotensin-converting enzyme (ACE) inhibitor affinity at two binding sites on rat pulmonary ACE: influence on bradykinin hydrolysis

1. ACE from rat lung and testis was characterized by radioligand binding studies using [125I]-Ro 31-8472, the radioiodinated hydroxy derivative of the potent ACE inhibitor cilazaprilat. 2. Analysis of the displacement of [125I]-Ro 31-8472 from ACE by ACE inhibitors of different structure by the LIGAND program was best fitted by a two binding site model for lung ACE and a one binding site model for testis ACE. 3. There was marked variation in ACE inhibitor binding affinity at the two binding sites of lung ACE across the panel of ACE inhibitors studied (equilibrium dissociation constant; Kd; pmol/L) for site one vs site two: cilazaprilat 40 +/- 3 vs 430 +/- 92*; lisinopril 25 +/- 1 vs 848 +/- 107**; and quinaprilat 4 +/- 1 vs 1869 +/- 720; *P less than 0.05; **P less than 0.005, t-test, n = 3). Reduction in binding affinity at site two of lung ACE was related to an increase in ACE inhibitor side chain length or complexity of carboxyl terminal moiety. ACE inhibitor binding affinity at the testis ACE binding site resembled site one of lung ACE. 4. Inhibition of bradykinin hydrolysis by lung ACE in the presence of increasing concentrations of cilazaprilat or quinaprilat was similar (F = 0.64; P greater than 0.05), suggesting that bradykinin cleavage predominates at ACE active site one. 5. The differences in ACE inhibitor affinity at the two ACE active sites has implications in physiological substrate selectivity, and may influence the pharmacodynamic effects of different ACE inhibitors.

Angiotensin converting enzyme: implications from molecular biology for its physiological functions

1. The two isozymes of human angiotensin converting enzyme (ACE; EC 3.4.15.1) have recently been cloned and sequenced. 2. The larger, endothelial isozyme has two highly similar internal domains each bearing a putative catalytic site. In contrast the smaller, testicular isozyme has a single catalytic site corresponding to the C-terminal domain of endothelial ACE and represents the ancestral, non-duplicated form of the gene. 3. Both isozymes are anchored in the plasma membrane by a single hydrophobic transmembrane polypeptide located near the C-terminus, and both are extensively N-glycosylated. 4. The testicular isozyme may also be O-glycosylated. 5. The soluble form of ACE in plasma, seminal fluid and other body fluids appears to be derived from the membrane-bound endothelial isozyme by a post-translational modification. 6. ACE has a complex substrate specificity with peptidyl tripeptidase or endopeptidase action on certain peptides, as well as the classical peptidyl dipeptidase activity. 7. Numerous potent inhibitors of the enzyme have been developed and used successfully in the treatment of hypertension, but some of the observed side effects may be due to inhibition of other zinc metalloenzymes. 8. Both endothelial and testicular ACE are highly conserved between species, indicative of the essential role(s) of the enzyme in blood pressure regulation and other physiological processes.