Angiotensin I-converting enzyme inhibitors are allosteric enhancers of kinin B1 and B2 receptor function

The beneficial effects of angiotensin I-converting enzyme (ACE) inhibitors go beyond the inhibition of ACE to decrease angiotensin (Ang) II or increase kinin levels. ACE inhibitors also affect kinin B1 and B2 receptor (B1R and B2R) signaling, which may underlie some of their therapeutic usefulness. They can indirectly potentiate the actions of bradykinin (BK) and ACE-resistant BK analogs on B2Rs to elevate arachidonic acid and NO release in laboratory experiments. Studies indicate that ACE inhibitors and some Ang metabolites increase B2R functions as allosteric enhancers by inducing a conformational change in ACE. This is transmitted to B2Rs via heterodimerization with ACE on the plasma membrane of cells. ACE inhibitors are also agonists of the B1R, at a Zn-binding sequence on the second extracellular loop that differs from the orthosteric binding site of the des-Arg-kinin peptide ligands. Thus, ACE inhibitors act as direct allosteric B1R agonists. When ACE inhibitors enhance B2R and B1R signaling, they augment NO production. Enhancement of B2R signaling activates endothelial NO synthase, yielding a short burst of NO; activation of B1Rs results in a prolonged high output of NO by inducible NO synthase. These actions, outside inhibiting peptide hydrolysis, may contribute to the pleiotropic therapeutic effects of ACE inhibitors in various cardiovascular disorders.

The lung in relation to vasoactive polypeptides

Among the metabolic functions of the lungs are the formation, release, activation and inactivation of biologically active peptides. The following peptides may be present or formed in normal lung: vasoactive intestinal peptide or a peptide closely related to it, a spasmogenic peptide not yet fully identified, bradykinin, substance P, a bombesin-like peptide (especially in fetal and neonatal lung), and eosinophil-chemotactic peptides. These peptides are found in special neuroendocrine cells, in neurons, or in mast cells. Normal lung also inactivates bradykinin and activates angiotensin; both processes are catalysed by the same enzyme (kininase II or angiotensin-converting enzyme), located in pulmonary vascular endothelium. Pulmonary tumours and certain non-tumorous lesions can produce and release a variety of peptide hormones that are not normally generated by the lung in substantial amounts. This 'ectopic' secretion of hormones may be detectable only by sensitive assays or may result in specific clinical syndromes.

Structure and function of human plasma carboxypeptidase N, the anaphylatoxin inactivator

Human carboxypeptidase N (CPN) was discovered in the early 1960s as a plasma enzyme that inactivates bradykinin and was identified 8 years later as the major "anaphylatoxin inactivator" of blood. CPN plays an important role in protecting the body from excessive buildup of potentially deleterious peptides that normally act as local autocrine or paracrine hormones. This review summarizes the structure, enzymatic properties and function of this important human enzyme, including insights gained by the recent elucidation of the crystal structure of the CPN catalytic subunit and structural modeling of the non-catalytic regulatory 83 kDa subunit. We also discuss its physiological role in cleaving substrates such as kinins, anaphylatoxins, creatine kinase, plasminogen receptors, hemoglobin and stromal cell-derived factor-1alpha (SDF-1alpha).

Crystal Structure of the Human Carboxypeptidase N (Kininase I) Catalytic Domain

Human carboxypeptidase N (CPN), a member of the CPN/E subfamily of "regulatory" metallo-carboxypeptidases, is an extracellular glycoprotein synthesized in the liver and secreted into the blood, where it controls the activity of vasoactive peptide hormones, growth factors and cytokines by specifically removing C-terminal basic residues. Normally, CPN circulates in blood plasma as a hetero-tetramer consisting of two 83 kDa (CPN2) domains each flanked by a 48 to 55 kDa catalytic (CPN1) domain. We have prepared and crystallized the recombinant C-terminally truncated catalytic domain of human CPN1, and have determined and refined its 2.1 A crystal structure. The structural analysis reveals that CPN1 has a pear-like shape, consisting of a 319 residue N-terminal catalytic domain and an abutting, cylindrically shaped 79 residue C-terminal beta-sandwich transthyretin (TT) domain, more resembling CPD-2 than CPM. Like these other CPN/E members, two surface loops surrounding the active-site groove restrict access to the catalytic center, offering an explanation for why some larger protein carboxypeptidase inhibitors do not inhibit CPN. Modeling of the Pro-Phe-Arg C-terminal end of the natural substrate bradykinin into the active site shows that the S1' pocket of CPN1 might better accommodate P1'-Lys than Arg residues, in agreement with CPN's preference for cleaving off C-terminal Lys residues. Three Thr residues at the distal TT edge of CPN1 are O-linked to N-acetyl glucosamine sugars; equivalent sites in the membrane-anchored CPM are occupied by basic residues probably involved in membrane interaction. In tetrameric CPN, each CPN1 subunit might interact with the central leucine-rich repeat tandem of the cognate CPN2 subunit via a unique hydrophobic surface patch wrapping around the catalytic domain-TT interface, exposing the two active centers.

Human ACE and bradykinin B2 receptors form a complex at the plasma membrane

To investigate how angiotensin I-converting enzyme (ACE) inhibitors enhance the actions of bradykinin (BK) on B2 receptors independent of blocking BK inactivation, we expressed human somatic ACE and B2 receptors in CHO cells. Bradykinin and its ACE-resistant analog were the receptor agonists. B2 fused with green fluorescent protein (GFP) and ACE were coprecipitated with antisera to GFP or ACE shown in Western blots. Immunohistochemistry of fixed cells localized ACE by red color and B2-GFP by green. Yellow on plasma membranes of coexpressing cells also indicated enzyme-receptor complex formation. Using ACE-fused cyan fluorescent protein donor and B2-fused yellow fluorescent protein (YFP) acceptor, we registered fluorescence resonance energy transfer (FRET) by the enhanced fluorescence of donor on acceptor photobleaching, establishing close (within 10 nm) positions of B2 receptors and ACE. Bradykinin stimulation cointernalized ACE and B2 receptors. We expressed ACE fused to N terminus of B2 receptors, anchoring only receptors to plasma membranes. Here, in contrast to cells, where both ACE and B2 receptors are separately anchored, ACE inhibitors neither enhance activation of chimeric B2 nor resensitize desensitized B2 receptors. Heterodimer formation between ACE and B2 receptors can be a mechanism for ACE inhibitors to augment kinin activity at cellular level.

Kinin- and angiotensin-converting enzyme (ACE) inhibitor-mediated nitric oxide production in endothelial cells

Carboxypeptidase cleavage of the C-terminal Arg of kinins generates specific agonists of the B1 receptor. Activation of B1 receptors produces nitric oxide via eNOS in bovine endothelial cells and iNOS in cytokine-stimulated human endothelial cells. Angiotensin-converting enzyme (ACE) inhibitors are direct agonists of B1 receptors in endothelial cells, although they release NO via a different signaling pathway than peptide ligands in bovine cells. This brief review discusses carboxypeptidase M as a required processing enzyme for generating B1 agonists, how ACE inhibitors and peptide ligands stimulate NO production and the evidence for, as well as some consequences of, the direct activation of B1 receptors by ACE inhibitors.

Angiotensin I-converting enzyme inhibitors block protein kinase C epsilon by activating bradykinin B1 receptors in human endothelial cells

Angiotensin I-converting enzyme (ACE) inhibitors are widely used to treat patients with cardiovascular and kidney diseases, but inhibition of ACE alone does not fully explain the beneficial effects. We reported that ACE inhibitors directly activate bradykinin B1 receptor at the canonical Zn2+ binding site, leading to prolonged nitric oxide (NO) production in endothelial cells. Protein kinase C (PKC) epsilon, a novel PKC isoform, is up-regulated in myocardium after infarction, suggesting a role in the development of cardiac dysfunction. In cytokine-treated human lung microvascular endothelial cells, B1 receptor activation by ACE inhibitors (enalaprilat, quinaprilat) or peptide ligands (des-Arg10-Lys1-bradykinin, des-Arg9-bradykinin) inhibited PKC epsilon with an IC50 = 7 x 10(-9) M. Despite the reported differences in binding affinity to receptor, the two peptide ligands were equally active, even when inhibitor blocked the cleavage of Lys(1), thus the conversion by aminopeptidase. The synthetic undecapeptide (LLPHEAWHFAR) representing the binding site for ACE inhibitors on human B(1) receptors reduced PKC epsilon inhibition by enalaprilat but not by peptide agonist. A combination of inducible and endothelial NO synthase inhibitors, 1400W [N-(3(aminomethyl) benzyl) acetamidine dihydrochloride] and N omega-nitro-L-arginine (2 microM), significantly reduced inhibition by enalaprilat (100 nM), whereas the NO donor (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl) amino]diazen-1-ium-1,2-diolate (100 microM) inhibited PKC epsilon activity just as the B1 ligands did. In conclusion, NO generated by B1 receptor activation inhibits PKC epsilon.

Kallikrein activates bradykinin B2 receptors in absence of kininogen

Kallikreins cleave plasma kininogens to release the bioactive peptides bradykinin (BK) or kallidin (Lys-BK). These peptides then activate widely disseminated B2 receptors with consequences that may be either noxious or beneficial. We used cultured cells to show that kallikrein can bypass kinin release to activate BK B2 receptors directly. To exclude intermediate kinin release or kininogen uptake from the cultured medium, we cultured and maintained cells in medium entirely free of animal proteins. We compared the responses of stably transfected Chinese hamster ovary (CHO) cells that express human B2 receptors (CHO B2) and cells that coexpress angiotensin I-converting enzyme (ACE) as well (CHO AB). We found that BK (1 nM or more) and tissue kallikrein (1-10 nM) both significantly increased release of arachidonic acid beyond unstimulated baseline level. An enzyme-linked immunoassay for kinin established that kallikrein did not release a kinin from CHO cells. We confirmed the absence of kininogen mRNA with RT-PCR to rule out kininogen synthesis by CHO cells. We next tested an ACE inhibitor for enhanced BK receptor activation in the absence of kinin release and synthesized an ACE-resistant BK analog as a control for these experiments. Enalaprilat (1 microM) potentiated kallikrein (100 nM) in CHO AB cells but was ineffective in CHO B2 cells that do not bear ACE. We concluded that kallikrein activated B2 receptors without releasing a kinin. Furthermore, inhibition of ACE enhanced the receptor activation by kallikrein, an action that may contribute to the manifold therapeutic effects of ACE inhibitors.

Hydrolysis of angiotensin peptides by human angiotensin I-converting enzyme and the resensitization of B2 kinin receptors

We measured the cleavage of angiotensin I (Ang I) metabolites by angiotensin I-converting enzyme (ACE) in cultured cells and examined how they augment actions of bradykinin B2 receptor agonists. Monolayers of Chinese hamster ovary cells transfected to stably express human ACE and bradykinin B2 receptors coupled to green fluorescent protein (B2GFP) or to express only coupled B2GFP receptors. We used 2 ACE-resistant bradykinin analogues to activate the B2 receptors. We used high-performance liquid chromatography to analyze the peptides cleaved by ACE on cell monolayers and found that Ang 1-9 was hydrolyzed 18x slower than Ang I and &30% slower than Ang 1-7. Ang 1-7 was cleaved to Ang 1-5. Although micromol/L concentrations of slowly cleaved substrates Ang 1-7 and Ang 1-9 inhibit ACE, they resensitize the desensitized B2GFP receptors in nmol/L concentration, independent of ACE inhibition. This is reflected by release of arachidonic acid through a mechanism involving cross-talk between ACE and B2 receptors. When ACE was not expressed, the Ang 1-9, Ang 1-7 peptides were inactive. Inhibitors of protein kinase C-alpha, phosphatases and Tyr-kinase blocked this resensitization activity, but not basal B2 activation by bradykinin. Ang 1-9 and Ang 1-7 enhance bradykinin activity, probably by acting as endogenous allosteric modifiers of the ACE and B2 receptor complex. Consequently, when ACE inhibitors block conversion of Ang I, other enzymes can still release Ang I metabolites to enhance the efficacy of ACE inhibitors.

Positive cooperativity between the thrombin and bradykinin B2 receptors enhances arachidonic acid release

Bradykinin (BK) or kallikreins activate B2 receptors (R) that couple Galpha(i) and Galpha(q) proteins to release arachidonic acid (AA) and elevate intracellular Ca2+ concentration ([Ca2+]i). Thrombin cleaves the protease-activated-receptor-1 (PAR1) that couples Galpha(i), Galpha(q), and Galpha(12/13) proteins. In Chinese hamster ovary cells stably transfected with human B2R, thrombin liberated little AA, but it significantly potentiated AA release by B2R agonists. We explored mechanisms of cooperativity between constitutively expressed PAR1 and B2R. We also examined human endothelial cells expressing both Rs constitutively. The PAR1 agonist hexapeptide (TRAP) was as effective as thrombin. Inhibitors of components of Galpha(i), Galpha(q), and Galpha(12/13) signaling pathways, and a protein kinase C (PKC)-alpha inhibitor, Gö-6976, blocked potentiation, while phorbol, an activator, enhanced it. Several inhibitors, including a RhoA kinase inhibitor, a [Ca2+]i antagonist, and an inositol-(1,3,4)-trisphosphate R antagonist, reduced mobilization of [Ca2+]i by thrombin and blocked potentiation of AA release by B2R agonists. Because either a nonselective inhibitor (isotetrandrine) of phospholipase A2 (PLA2) or a Ca2+-dependent PLA2 inhibitor abolished potentiation of AA release by thrombin, while a Ca2+-independent PLA2 inhibitor did not, we concluded that the mechanism involves Ca2+-dependent PLA2 activation. Both thrombin and TRAP modified activation and phosphorylation of the B2R induced by BK. In lower concentrations they enhanced it, while higher concentrations inhibited phosphorylation and diminished B2R activation. Protection of the NH2-terminal Ser1-Phe2 bond of TRAP by an aminopeptidase inhibitor made this peptide much more active than the unprotected agonist. Thus PAR1 activation enhances AA release by B2R agonists through signal transduction pathway.

Kinin B1 receptors stimulate nitric oxide production in endothelial cells: signaling pathways activated by angiotensin I-converting enzyme inhibitors and peptide ligands

We reported previously a novel mode of action of angiotensin I-converting enzyme (kininase II; ACE) inhibitors mediated through the direct activation of bradykinin B(1) receptor, independent of endogenous kinins or ACE (J Biol Chem 277:16847-16852, 2002). We aimed to further clarify the mechanism of activation of B(1) receptor, which leads to prolonged nitric oxide (NO) release. The ACE inhibitor enalaprilat and the peptide ligand desArg(10)-kallidin (in nanomolar concentrations) release NO by activating endothelial NO synthase (eNOS) in bovine and inducible NO synthase (iNOS) in stimulated human endothelial cells. The peptide and the ACE inhibitor ligands activate eNOS by facilitating different signaling pathways. DesArg(10)-kallidin enhances inositol-phosphate generation and elevates [Ca(2+)](i) by first augmenting intracellular release and then the influx of extracellular Ca(2+). In contrast, enalaprilat stimulates only the influx of extracellular Ca(2+) through rare earth-sensitive channels, and its effect is blocked by cholera toxin or protein kinase C inhibitors. In addition, unlike desArg(10)-kallidin, enalaprilat can also release NO independent of Ca(2+) in bovine endothelial cells. The inflammatory cytokines interleukin-1beta and interferon-gamma induce both B(1) receptor and iNOS in human endothelial cells. In contrast to eNOS, B(1) ligands activate iNOS similarly. Both desArg(10)-kallidin and ACE inhibitors enhance arginine uptake and release NO independent of [Ca(2+)](i) elevation. This is the first report on the direct activation of B(1) receptor by ACE inhibitors in human endothelial cells. This interaction leads to prolonged NO release and possibly contributes to the documented benefits of the use of ACE inhibitors.

Angiotensin converting enzyme (ACE) and neprilysin hydrolyze neuropeptides: a brief history, the beginning and follow-ups to early studies

Our investigations started when synthetic bradykinin became available and we could characterize two enzymes that cleaved it: kininase I or plasma carboxypeptidase N and kininase II, a peptidyl dipeptide hydrolase that we later found to be identical with the angiotensin I converting enzyme (ACE). When we noticed that ACE can cleave peptides without a free C-terminal carboxyl group (e.g., with a C-terminal nitrobenzylamine), we investigated inactivation of substance P, which has a C-terminal Met(11)-NH(2). The studies were extended to the hydrolysis of the neuropeptide, neurotensin and to compare hydrolysis of the same peptides by neprilysin (neutral endopeptidase 24.11, CD10, NEP). Our publication in 1984 dealt with ACE and NEP purified to homogeneity from human kidney. NEP cleaved substance P (SP) at Gln(6)-Phe(7), Phe(7)[see text]-Phe(8), and Gly(9)-Leu(10) and neurotensin (NT) at Pro(10)-Tyr(11) and Tyr(11)-Ile(12). Purified ACE also rapidly inactivated SP as measured in bioassay. HPLC analysis showed that ACE cleaved SP at Phe(8)-Gly(9) and Gly(9)-Leu(10) to release C-terminal tri- and dipeptide (ratio = 4:1). The hydrolysis was Cl(-) dependent and inhibited by captopril. ACE released only dipeptide from SP free acid. ACE hydrolyzed NT at Tyr(11)-Ile(12) to release Ile(12)-Leu(13). Then peptide substrates were used to inhibit ACE hydrolyzing Fa-Phe-Gly-Gly and NEP cleaving Leu(5)-enkephalin. The K(i) values in microM were as follows: for ACE, bradykinin = 0.4, angiotensin I = 4, SP = 25, SP free acid = 2, NT = 14, and Met(5)-enkephalin = 450, and for NEP, bradykinin = 162, angiotensin I = 36, SP = 190, NT = 39, Met(5)-enkephalin = 22. These studies showed that ACE and NEP, two enzymes widely distributed in the body, are involved in the metabolism of SP and NT. Below we briefly survey how NEP and ACE in two decades have gained the reputation as very important factors in health and disease. This is due to the discovery of more endogenous substrates of the enzymes and to the very broad and beneficial therapeutic applications of ACE inhibitors.

Kallikreins when activating bradykinin B2 receptor induce its redistribution on plasma membrane

The bradykinin (BK) B2 receptor (R) is directly activated by kallikreins and other serine proteases independent of BK release. Both the Galpha(i) and Galpha(q) proteins are involved, shown by the release of arachidonic acid and [Ca2+]i elevation. Site-directed mutagenesis of the receptor and the lack of heterogeneous desensitization of the human B2R by the BK and kallikrein emphasize among others the differences between activation by the proteases and the peptide. To characterize further the mechanism thereby kallikreins activate and desensitize the B2R we investigated the distribution of the human B2R tagged with the green fluorescent protein (B2-GFP(Ct)) on the plasma membrane of stably transfected Chinese hamster ovary (CHO) cells. We visualized the movement of B2-GFP(Ct) R with confocal fluorescence microscopy after activation by BK or a by serine protease. Continued exposure of the cells to BK led to B2R internalization within 15-20 min. Porcine pancreatic and human recombinant tissue kallikreins induced a rapid definite redistribution of receptors on the plasma membrane within 5 min, prior to internalization. These effects of kallikrein were blocked by the B2R antagonist HOE 140 and by the kallikrein inhibitor, aprotinin. The B2R was also activated by endoproteinases LysC and ArgC and trypsin, but these enzymes did not induce redistribution, only internalization. In control experiments, kallikrein had no effect on cells transfected to stably express the angiotensin-converting enzyme-green fluorescent protein (GFP). Thus, kallikreins when activating the BK B2R also trigger its redistribution on plasma membrane.

Activation of bradykinin B1 receptor by ACE inhibitors

ACE or kininase II inhibitors are very important, widely used therapeutic agents for the treatment of a variety of diseases. Although they inhibit ACE, thus, angiotensin II release and bradykinin (BK) inactivation, this inhibition alone does not suffice to explain their successful application in medical practice. Enalaprilat and other ACE inhibitors at nanomolar concentrations activate the BK B1 receptor directly in the absence of ACE and the peptide ligands, des-Arg-kinins. The inhibitors activate at the Zn-binding pentameric consensus sequence HEXXH (195 -199) of B1, a motif also present in the active centers of ACE but absent from the BK B2 receptor. ACE inhibitors, when activating the B1 receptor, elevate intracellular calcium [Ca2+]i and release NO from cultured cells. Activation by ACE inhibitor was abolished by Ca-EDTA, a B1 receptor antagonist, by a synthetic undecapeptide representing the 192-202 sequence in the B1 receptor, and by site-directed mutagenesis of H195 to A. With the exception of the B1 receptor blocker, these agents and the mutation did not affect the actions of the peptide ligand des-Arg10-Lys1-BK. Ischemia and inflammatory cytokines induce B1 receptors and elevate its expression. Direct activation of the B1 receptor by ACE inhibitors can contribute to their therapeutic efficacy, for example, by releasing NO in vascular beds, or to some of their side effects.

Products of angiotensin I hydrolysis by human cardiac enzymes potentiate bradykinin

Some beneficial effects of ACE inhibitors are attributed to potentiation of bradykinin's actions exerted through its B2 receptor. We investigated them on cultured cells transfected or constitutively expressing both ACE and B2 receptor. The potentiation of bradykinin was indirect and attributed to a crosstalk induced between enzyme and receptor via ACE, a heterodimer formation. While looking for endogenous activators, we investigated the split products of angiotensin I (Ang) Ang 1-9 and 1-7, peptides released by enzymes of human atria and ventricle. Ang 1-9 was liberated by a cathepsin A-type enzyme, Ang 1-7 by a different metallopeptidase-protease. Cathepsin A's presence in heart tissue was shown by deamidating enkephalinamide substrate, by immunoprecipitation and by immunohistochemistry. In immunohistochemistry, cathepsin A was detected in myocytes of atrial tissue. Ang 1-9 and Ang 1-7 potentiated the effect of an ACE-resistant bradykinin analogue on the B2 receptor in transfected cells expressing human ACE and B2, and in human endothelial cells. Ang 1-9 and 1-7 augmented arachidonic acid and NO release by bradykinin. NO liberation by bradykinin from endothelial cells was potentiated at 10nmol/L concentration by Ang 1-9 and Ang 1-7; at higher concentrations, Ang 1-9 was significantly more active. Both peptides had little activity in absence of bradykinin or ACE. Ang 1-9 and 1-7 potentiated bradykinin action on its B2 receptor at much lower concentrations than their IC50 values with ACE. They probably induce conformational changes in the ACE/B2 receptor complex via interaction with ACE.

The kinin system: suggestions to broaden some prevailing concepts

The existence and importance of the kallikrein-kinin-kininase system, especially in the circulation, has taken over three-quarters of a century to be established. Finding the multiple components derived from renin-angiotensin and their functions stretched over a century [Erdös EG. Perspectives on the early history of angiotensin-converting enzyme-recent follow-ups. In: Giles TD, editor. Angiotensin-converting enzyme (ACE): clinical and experimental insights. Fort Lee: Health Care Communications; 2001, p. 3-16]. Although the discoveries were made independently, it was shown in 1970 that the angiotensin I-converting enzyme (ACE) is identical with kininase II, previously discovered by us, thus, a single protein can regulate either the activation or inactivation of the two peptide products. It followed that inhibitors of ACE can affect both processes [Bhoola KD, Figueroa CD, Worthy K. Bioregulation of kinins: kallikreins, kininogens, and kininases. Pharmacol Rev 1992;44:1-80]. After being engaged for a long time in characterizing the metabolism of various bio-active peptides, we, as well as others, noticed that the effect of ACE inhibitors go beyond simply blocking angiotensin (Ang) II release and bradykinin (BK) inactivation by the enzyme (Kaplan AP, Joseph K, Silverberg M. Pathways for bradykinin formation and inflammatory disease. J Allergy Clin Immunol 2002; 109(2):195-209, Yamada K, Erd6s EG. Kallikrein and prekallikrein of the isolated basolateral membrane of rat kidney. Kidney Int 1982;22:331-7]. It also became apparent to us that in the complex multistep reactions needed to activate the kallikrein-kinin system, there should be some shortcuts-shunts-to accelerate and simplify important processes. Thus, some basic tenets developed after decades of intensive laboratory investigations-and by now generally accepted-can be challenged. For example, it should be considered that the activities of BK and Lys BK (kallidin) can be substantially different, and that sequentially linked reactions, starting with prokallikrein activation and leading to kinin release from kininogen and inhibition of kininases, may be only one way to activate kinin receptors. A summary of some suggested alterations on prevailing concepts is given below.

Novel mode of action of angiotensin I converting enzyme inhibitors: direct activation of bradykinin B1 receptor

Angiotensin I converting enzyme (kininase II; ACE) inhibitors are important therapeutic agents widely used for treatment in cardiovascular and renal diseases. They inhibit angiotensin II release and bradykinin inactivation; these actions do not explain completely the clinical benefits. We found that enalaprilat and other ACE inhibitors in nanomolar concentrations activate human bradykinin B(1) receptors directly in the absence of ACE and the B(1) agonist des-Arg(10)-Lys(1)-bradykinin. These inhibitors activate at the Zn(2+)-binding consensus sequence HEXXH (195-199) in B(1), which is present also in ACE but not in the B(2) receptor. Activation elevates [Ca(2+)](i) and releases NO from endothelial or transfected cells expressing the B(1) receptor but is blocked by Ca-EDTA, a B(1) receptor antagonist, the synthetic undecapeptide sequence (192-202) of B(1), and the mutagenesis of His(195) to Ala(195). Except for the B(1) antagonist, these agents and manipulations did not block activation by a peptide ligand. Thus, Zn(2+) is essential for B(1) receptor activation by ACE inhibitors at the zinc-binding consensus sequence. Ischemia or cytokines induce abundant B(1) receptor expression. B(1) receptor activation by ACE inhibitors, a novel mode of action reported here first, can contribute to their therapeutic effects by releasing NO in the heart and to some side effects.

Angiotensin 1-9 and 1-7 release in human heart: role of cathepsin A

Human heart tissue enzymes cleave angiotensin (Ang) I to release Ang 1-9, Ang II, or Ang 1-7. In atrial homogenate preparations, cathepsin A (deamidase) is responsible for 65% of the liberated Ang 1-9. Ang 1-7 was released (88% to 100%) by a metallopeptidase, as established with peptidase inhibitors. Ang II was liberated to about equal degrees by ACE and chymase-type enzymes. Cathepsin A's presence in heart tissue was also proven because it deamidated enkephalinamide substrate by immunoprecipitation of cathepsin A with antiserum to human recombinant enzyme and by immunohistochemistry. In immunohistochemistry, cathepsin A was detected in myocytes of atrial tissue. The products of Ang I cleavage, Ang 1-9 and Ang 1-7, potentiated the effect of an ACE-resistant bradykinin analog and enhanced kinin effect on the B(2) receptor in Chinese hamster ovary cells transfected to express human ACE and B(2) (CHO/AB), and in human pulmonary arterial endothelial cells. Ang 1-9 and 1-7 augmented arachidonic acid and nitric oxide (NO) release by kinin. Direct assay of NO liberation by bradykinin from endothelial cells was potentiated at 10 nmol/L concentration, 2.4-fold (Ang 1-9) and 2.1-fold (Ang 1-7); in higher concentrations, Ang 1-9 was significantly more active than Ang 1-7. Both peptides had traces of activity in the absence of bradykinin. Ang 1-9 and Ang 1-7 potentiated bradykinin action on the B(2) receptor by raising arachidonic acid and NO release at much lower concentrations than their 50% inhibition concentrations (IC(50)s) with ACE. They probably induce conformational changes in the ACE/B(2) receptor complex via interaction with ACE.

Neprilysin inhibitors potentiate effects of bradykinin on b2 receptor

Some beneficial effects of angiotensin-I--converting enzyme (ACE, kininase II) inhibitor therapy are attributed to enhancing the activity of bradykinin on its B(2) receptor. Independent of inhibition of bradykinin hydrolysis, ACE inhibitors enhance the action of bradykinin on its B(2) receptor by inducing crosstalk between ACE and the receptor. We investigated whether inhibitors of another kininase II-type enzyme, neprilysin (neutral endopeptidase 24.11; NEP), could augment bradykinin effects unrelated to blocking its breakdown using a NEP-resistant bradykinin analog as ligand. We used transfected Chinese hamster ovary (CHO) cells stably expressing human B(2) receptor and NEP (CHO/NEP-B(2)) or only B(2) (CHO/B(2)) as control and human pulmonary fibroblasts (IMR90), expressing B(2), but more NEP than ACE. NEP inhibitor phosphoramidon (100 nmol/L), or omapatrilat, which inhibits both NEP and ACE, did not potentiate bradykinin in CHO/B(2) cells. In IMR90 cells, 10 nmol/L bradykinin elevated [Ca(2+)](i) and desensitized the receptor. Adding either 100 nmol/L omapatrilat or phosphoramidon resensitized the receptor to the ligand, which was abolished by receptor blocker HOE 140. Arachidonic acid release by bradykinin from CHO/NEP-B(2) cells was also augmented by 100 nmol/L phosphoramidon or omapatrilat about 3-fold, and again, the inhibitors resensitized the desensitized B(2) receptor. The inhibitors did not potentiate bradykinin when soluble rNEP was added to the medium of CHO/B(2) cells. Similar to ACE, NEP inhibitors potentiated bradykinin independent of inhibiting inactivation. Consequently, omapatrilat could augment bradykinin effects on B(2), when either ACE or NEP is expressed close to receptor on cell membrane.

Kinins, receptors, kininases and inhibitors--where did they lead us?

Based on studies presented here and other published experiments performed with surviving tissue preparations, with transfected cells and with cells that constitutively express the human angiotensin I converting enzyme ACE and B2 receptors, we concluded the following: ACE inhibitors and other endogenous peptides that react with the active site of ACE potentiate the effect of bradykinin and its ACE resistant peptide congeners on the B2 receptor. They also resensitize receptors which had been desensitized by the agonist. ACE and bradykinin receptors have to be sterically close, possibly forming a heterodimer, for the ACE inhibitors to induce an allosteric modification on the receptor. When ACE inhibitors augment bradykinin effects, they reduce the phosphorylation of the B2 receptor. The primary actions of bradykinin on the receptor are not affected by protein kinase C or phosphatase inhibitors, but the potentiation of bradykinin or the resensitization of the receptor by ACE inhibitors are abolished by the same inhibitors. The results with protein kinase C and phosphatase inhibitors indicate that another intermediate protein may be involved in the processes of signaling induced by ACE inhibitors, and that ACE inhibitors affect the signal transduction pathway triggered by bradykinin on the B2 receptor.

Human bradykinin B(2) receptor is activated by kallikrein and other serine proteases

Bradykinin (BK) and kallidin (Lys-BK), liberated from kininogens by kallikreins, are ligands of the BK B(2) receptor. We investigated whether kallikreins, besides releasing peptide agonist, could also activate the receptor directly. We studied the effect of porcine and human recombinant tissue kallikrein and plasma kallikrein on [Ca(2+)](i) mobilization and [(3)H]arachidonic acid release from cultured cells stably transfected to express human BK B(2) receptor (CHO/B(2), MDCK/B(2), HEK/B(2)), and endothelial cells were used as control cells. As with BK, the actions of kallikrein were blocked by the B(2) antagonist, HOE 140. Kallikrein was inactive on cells lacking B(2) receptor. Kallikrein and BK desensitized the receptor homologously but there was no cross-desensitization. Furthermore, 50 nM human cathepsin G and 50 nM trypsin also activated the receptor; this also was blocked by HOE 140. Experiments excluded a putative kinin release by proteases. [(3)H]AA release by BK was reduced by 40% by added kininase I (carboxypeptidase M); however, receptor activation by tissue kallikrein, trypsin, or cathepsin G was not affected. Prokallikrein and inhibited kallikrein were inactive, suggesting cleavage of a peptide bond in the receptor. Kallikreins were active on mutated B(2) receptor missing the 19 N-terminal amino acids, suggesting a type of activation different from that of thrombin receptor. Paradoxically, tissue kallikreins decreased the [(3)H]BK binding to the receptor with a low K(D) (3 nM) and inhibited it 78%. Thus, kallikreins and some other proteases activate human BK B(2) receptor directly, independent of BK release. The BK B(2) receptor may belong to a new group of serine protease-activated receptors.

Potentiation of the effects of bradykinin on its receptor in the isolated guinea pig ileum

Angiotensin I-converting enzyme (ACE/kininase II) inhibitors potentiated guinea pig ileum's isotonic contractions to bradykinin (BK) and its analogues, shifting the BK dose-response curve to the left. ACE inhibitors added at the peak of the contraction immediately enhanced it further (343 +/- 40%), although the ileum inactivated BK slowly (t(1/2) = 12-16 min). Chymotrypsin and cathepsin G also augmented the activity of BK up to three- or four-fold, but in a manner slower than that of ACE inhibitors. The BK B(2) receptor blocker HOE 140 inhibited all effects. Histamine and angiotensin II were not potentiated. ACE inhibitors potentiate BK independent of blocking its inactivation by inducing crosstalk between ACE and the BK B(2) receptor; proteases activate the receptor by different mechanism.

Protein kinase C and phosphatase inhibitors block the ability of angiotensin I-converting enzyme inhibitors to resensitize the receptor to bradykinin without altering the primary effects of bradykinin

Angiotensin I-converting enzyme (kininase II) inhibitors (ACEis) are very widely used to treat cardiac conditions and nephropathies, but some of their beneficial activities cannot be attributed to enzyme inhibition alone. We investigated the effects of ACEis on the human bradykinin (BK) B(2) receptor expressed in Chinese hamster ovary cells transfected with the cDNA of human receptor and ACE, and on human pulmonary endothelial cells that constitutively express both proteins. BK and its ACE-resistant peptide analog activated the B(2) receptor to release arachidonic acid and elevate [Ca(2+)](i) and subsequently desensitized it. The release of arachidonic by BK was independent of extracellular Ca(2+). BK enhanced phosphorylation of the immunoprecipitated B(2) receptor but enalaprilat significantly reduced it. ACEi resensitized the receptor by initiating a cross talk between the receptor and ACE. Protein kinase C and phosphatase inhibitors distinguished the signaling by the receptor when activated first by BK from BK acting on the resensitized receptor. Treatment of cells with 1 microM calphostin, 100 nM staurosporine, 100 nM calyculin, or 500 nM okadaic acid did not affect either one of the primary actions of BK on the receptor. Protein kinase C or phosphatase inhibitors, however, blocked the effects of BK on the receptor resensitized by enalaprilat or ramiprilat. The experiments clearly differentiate the primary activation of the receptor by BK from activation of the resensitized receptor after ACEi treatment. The existence of an intermediate component involved in the action of ACEis to enhance release of vasoactive mediators by BK is suggested.

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.

Replacement of the transmembrane anchor in angiotensin I-converting enzyme (ACE) with a glycosylphosphatidylinositol tail affects activation of the B2 bradykinin receptor by ACE inhibitors

To investigate further the relationship of angiotensin I-converting enzyme (ACE) inhibitors to activation of the B(2) bradykinin (BK) receptor, we transfected Chinese hamster ovary cells to stably express the human receptor and either wild-type ACE (WT-ACE), an ACE construct with most of the cytosolic portion deleted (Cyt-del-ACE), or ACE with a glycosylphosphatidylinositol (GPI) anchor replacing the transmembrane and cytosolic domains (GPI-ACE). BK or its ACE-resistant analogue were the agonists. All activities (arachidonic acid release and calcium mobilization) were blocked by the B(2) antagonist HOE 140. B(2) was desensitized by repeated administration of BK but resensitized to agonist by ACE inhibitors in the cells expressing both B(2) and either WT-ACE or Cyt-del-ACE. In GPI-ACE expressing cells, the B(2) receptor was still activated by the agonists, but ACE inhibitors did not resensitize. Pretreatment with filipin returned the sensitivity to inhibitors. In immunocytochemistry, GPI-ACE showed patchy, uneven distribution on the plasma membrane that was restored by filipin. Thus, ACE inhibitors were inactive as long as GPI-ACE was sequestered in cholesterol-rich membrane domains. WT-ACE and B(2) receptor in Chinese hamster ovary cells co-immunoprecipitated with antibody to receptor, suggesting an interaction on the cell membrane. ACE inhibitors augment BK effects on receptors indirectly only when enzyme and receptor molecules are sterically close, possibly forming a heterodimer.

Potentiation of Bradykinin Actions by ACE Inhibitors

Angiotensin I-converting enzyme (kininase II; ACE) inhibitors, antibodies to ACE and slowly cleaved substrates of ACE potentiate the effect of bradykinin and its analogs on their B2 receptors independently of blocking peptide metabolism. ACE inhibitors also resensitized the receptors desensitized by the ligand (tachyphylaxis). The studies were performed on isolated organs and cells co-transfected with the receptor and the enzyme or constitutively expressing them. This enhancement of the effect of B2 ligands is attributed to a crosstalk between the enzyme and the receptor, and not to a direct action on the receptors. It might reflect some of the local activities of ACE inhibitors.

Enhancement of bradykinin and resensitization of its B2 receptor

We studied the enhancement of the effects of bradykinin B2 receptor agonists by agents that react with active centers of angiotensin-converting enzyme (ACE) independent of enzymatic inactivation. The potentiation and the desensitization and resensitization of B2 receptor were assessed by measuring [3H]arachidonic acid release and [Ca2+]i mobilization in Chinese hamster ovary cells transfected to express human ACE and B2 receptor, or in endothelial cells with constitutively expressed ACE and receptor. Administration of bradykinin or its ACE-resistant analogue desensitized the receptor, but it was resensitized (arachidonic acid release or [Ca2+]i mobilization) by agents such as enalaprilat (1 micromol/L). Enalaprilat was inactive in the absence of ACE expression. La3+ (100 micromol/L) inhibited the apparent resensitization, probably by blocking the entry of extracellular calcium. Enalaprilat resensitized the receptor via ACE to release arachidonic acid by bradykinin at a lower concentration (5 nmol/L) than required to mobilize [Ca2+]i (1 micromol/L). Monoclonal antibodies inhibiting the ACE N-domain active center and polyclonal antiserum potentiated bradykinin. The snake venom peptide BPP5a and metabolites of angiotensin and bradykinin (angiotensin-[1-9], angiotensin-[1-7], bradykinin-[1-8]; 1 micromol/L) enhanced arachidonic acid release by bradykinin. Angiotensin-(1-9) and -(1-7) also resensitized the receptor. Enalaprilat potentiated the bradykinin effect in cells expressing a mutant ACE with a single N-domain active site. Agents that reacted with a single active site, on the N-domain or on the C-domain, potentiated bradykinin not by blocking its inactivation but by inducing crosstalk between ACE and the receptor. Enalaprilat enhanced signaling via ACE by Galphai in lower concentration than by Galphaq-coupled receptor.

N-domain-specific substrate and C-domain inhibitors of angiotensin-converting enzyme: angiotensin-(1-7) and keto-ACE

We used the isolated N- and C-domains of the angiotensin 1-converting enzyme (N-ACE and C-ACE; ACE; kininase II) to investigate the hydrolysis of the active 1-7 derivative of angiotensin (Ang) II and inhibition by 5-S-5-benzamido-4-oxo-6-phenylhexanoyl-L-proline (keto-ACE). Ang-(1-7) is both a substrate and an inhibitor; it is cleaved by N-ACE at approximately one half the rate of bradykinin but negligibly by C-ACE. It inhibits C-ACE, however, at an order of magnitude lower concentration than N-ACE; the IC50 of C-ACE with 100 micromol/L Ang I substrate was 1.2 micromol/L and the Ki was 0.13. While searching for a specific inhibitor of a single active site of ACE, we found that keto-ACE inhibited bradykinin and Ang I hydrolysis by C-ACE in approximately a 38- to 47-times lower concentration than by N-ACE; IC50 values with C-ACE were 0.5 and 0.04 micromol/L. Furthermore, we investigated how Ang-(1-7) acts via bradykinin and the involvement of its B2 receptor. Ang-(1-7) was ineffective directly on the human bradykinin B2 receptor transfected and expressed in Chinese hamster ovary cells. However, Ang-(1-7) potentiated arachidonic acid release by an ACE-resistant bradykinin analogue (1 micromol/L), acting on the B2 receptor when the cells were cotransfected with cDNAs of both B2 receptor and ACE and the proteins were expressed on the plasma membrane of Chinese hamster ovary cells. Thus like other ACE inhibitors, Ang-(1-7) can potentiate the actions of a ligand of the B2 receptor indirectly by binding to the active site of ACE and independent of blocking ligand hydrolysis. This potentiation of kinins at the receptor level can explain some of the well-documented kininlike actions of Ang-(1-7).

Cellular carboxypeptidases

This article focuses on four human carboxypeptidases (CPs): two metallo-CPs and two serine CPs. The metallo-CPs are members of the so-called B-type regulatory CP family, as they cleave only the C-terminal basic amino acids Arg or Lys. The plasma membrane-bound CPM and the mainly, but not exclusively, intracellular CPD are surveyed from this group of enzymes. These enzymes can regulate peptide hormone activity at the cell surface and possibly intracellularly after receptor-mediated endocytosis and may also participate in peptide hormone processing. The serine CPs, as their name indicates, contain a serine residue in the active center essential for catalytic activity that reacts with organophosphorus inhibitors. Prolylcarboxypeptidase (PRCP) (angiotensinase C) and deamidase (cathepsin A, lysosomal protective protein) are discussed here. These two enzymes are highly concentrated in lysosomes; however, they may also be active extracellularly after their release from lysosomes in soluble form or in a plasma membrane-bound complex. Whereas deamidase cleaves a variety of peptides with C-terminal or penultimate hydrophobic residues (e.g. substance P, angiotensin I, bradykinin, endothelin, fMet-Leu-Phe). PRCP cleaves only peptides with a penultimate Pro residue (e.g. des-Arg9-bradykinin, angiotensin II). These enzymes may also be involved in terminating signal transduction by inactivating peptide ligands after receptor endocytosis.

Potentiation of the actions of bradykinin by angiotensin I-converting enzyme inhibitors. The role of expressed human bradykinin B2 receptors and angiotensin I-converting enzyme in CHO cells

Part of the beneficial effects of angiotensin I-converting enzyme (ACE) inhibitors are due to augmenting the actions of bradykinin (BK). We studied this effect of enalaprilat on the binding of [3H]BK to Chinese hamster ovary (CHO) cells stably transfected to express the human BK B2 receptor alone (CHO-3B) or in combination with ACE (CHO-15AB). In CHO-15AB cells, enalaprilat (1 mumol/L) increased the total number of low-affinity [3H]BK binding sites on the cells at 37 degrees C, but not at 4 degrees C, from 18.4 +/- 4.3 to 40.3 +/- 11.9 fmol/10(6) cells (P < .05; Kd, 2.3 +/- 0.8 and 5.9 +/- 1.3 nmol/L; n = 4). Enalaprilat preserved a portion of the receptors in high-affinity conformation (Kd, 0.17 +/- 0.08 nmol/L; 8.1 +/- 0.9 fmol/10(6) cells). Enalaprilat decreased the IC50 of [Hyp3-Tyr(Me)8]BK, the BK analogue more resistant to ACE, from 3.2 +/- 0.8 to 0.41 +/- 0.16 nmol/L (P < .05, n = 3). The biphasic displacement curve of the binding of [3H]BK also suggested the presence of high-affinity BK binding sites. Enalaprilat (5 nmol to 1 mumol/L) potentiated the release of [3H]arachidonic acid and the liberation of inositol 1,4,5-trisphosphate (IP3) induced by BK and [Hyp3-Tyr(Me)8]BK. Moreover, enalaprilat (1 mumol/L) completely and immediately restored the response of the B2 receptor, desensitized by the agonist (1 mumol/L [Hyp3-Tyr(Me)8]BK); this effect was blocked by the antagonist, HOE 140. Finally, enalaprilat, but not the prodrug enalapril, decreased internalization of the receptor from 70 +/- 9% to 45 +/- 9% (P < .05, n = 7). In CHO-3B cells, enalaprilat was ineffective. ACE inhibitors in the presence of both the B2 receptor and ACE enhance BK binding, protect high-affinity receptors, block receptor desensitization, and decrease internalization, thereby potentiating BK beyond blocking its hydrolysis.

Detection of neutral endopeptidase 24.11 (neprilysin) in human hepatocellular carcinomas by immunocytochemistry

BACKGROUND

We have reported previously that neutral endopeptidase 24.11 (neprilysin; NEP; CALLA, CD10) activity was very high in rat hepatomas and a cultured human hepatocarcinoma cell line (SK-HEP1).

MATERIALS AND METHODS

While continuing these studies, we detected the presence of NEP in SK-HEP 1 cells by immunocytochemistry and in paraffin-embedded human hepatocellular carcinomas as well. IgG purified from polyclonal antisera to human NEP was employed as a source of antibody.

RESULTS

SK-HEP 1 cells gave a strong positive reaction to the IgG fraction of the antisera. In control studies, where IgG was preabsorbed with recombinant NEP, the results were negative. Of the 18 hepatocellular carcinomas tested, NEP was expressed in 14 (78%) malignant tumors, while adjacent liver tissue did not show the presence of NEP.

CONCLUSIONS

It is suggested that, because none of the known hepatocellular carcinoma markers are highly specific, the detection of NEP in these malignant cells can be an additional useful diagnostic tool.

Angiotensin I-converting enzyme inhibitors potentiate bradykinin's inotropic effects independently of blocking its inactivation

The positive inotropic effects of bradykinin (BK) and 2 analogs resistant to angiotensin I-converting enzyme (ACE) were potentiated on isolated guinea pig atrial preparations by enalaprilat. The stable BK analogs, dextran-BK and [Hyp3-Tyr(Me)8]-BK, were as active as BK. Pretreatment for 5 min with enalaprilat augmented the maximal positive inotropic effect of [Hyp3-Tyr(Me)8]-BK 2.8-fold, from 19% to 53% and that of BK from 28% to 42% over baseline; inotropic responses to dextran-BK (1 microM) were similarly increased. The activity of atrial ACE, a zinc-requiring enzyme, was completely inhibited by 8-hydroxyquinoline-5-sulfonic acid (QSA, 10 mM), which raised the maximal inotropic effect of BK to 39% above baseline. This value rose to 67% when in addition to QSA, 1 microM enalaprilat was added; enalaprilat thus, potentiated the effects of BK independently of enzyme inhibition. The positive inotropic effects to BK and its analogs decline with time in the presence of these agonists. After 10 min of exposure, the response to 1 microM [Hyp3-Tyr(Me)8]-BK decreased to about half, and after 20 min, to 0. Enalaprilat, when present in the tissue bath, prevented the decline in inotropy; even after tachyphylaxis occurred, it reversed this decrease in activity when added. The effects of 1 microM [Hyp3-Tyr(Me)8]-BK, in the absence or presence of enalaprilat, were abolished by the BK B2 receptor antagonist icatibant (0.75 microM). The results indicate that ACE inhibitors, by potentiating the BK effects and blocking BK B2-receptor desensitization, may contribute to the beneficial cardiac effects of BK independently of blocking its inactivation.

Differences in the hydrolysis of enkephalin congeners by the two domains of angiotensin converting enzyme

The hydrolysis of enkephalin (Enk) congeners by the isolated N- (N-ACE) and C-domain of angiotensin I converting enzyme (ACE) and by the two-domain somatic ACE was investigated. Both Leu5- and Met5-Enk were cleaved faster by the C-domain than by N-ACE; rates with somatic ACE were 1600 and 2500 nmol/min/nmol enzyme with both active sites being involved. Substitution of Gly2 by D-Ala2 reduced the rate to 1/3rd to 1/7th of that of the Enks. N-ACE cleaved Met5-Enk-Arg6-Phe7 faster than the C-domain, probably with the highest turnover number of any naturally occurring ACE substrate (7600 min(-1)). This heptapeptide is also hydrolyzed in the absence of Cl-, but the activation by Cl- is unique; Cl- enhances the hydrolysis of the heptapeptide by N-ACE but inhibits it by the C-domain, yielding about a 5-fold difference in the turnover number at physiological pH. This difference may result in the predominant role of the N-domain in converting Met5-Enk-Arg6-Phe7 to Enk in vivo.

Single-domain angiotensin I converting enzyme (kininase II): characterization and properties

Somatic angiotensin I converting enzyme (ACE; kininase II) has two active sites, in two (N and C) domains. We studied the active centers with separate N-domain ACE (N-ACE), testicular C-domain ACE (germinal ACE) and, as control, renal somatic ACE. Germinal ACE cleaved the nonapeptide bradykinin about two times faster than N-ACE in 20 mM Cl-. Bradykinin1-7 was hydrolyzed further to bradykinin1-5 by N-ACE four times faster in the absence of Cl-, but at 300 mM Cl- the C-domain hydrolyzed it twice as fast. The hematopoietic system regulatory peptide acetyl-Ser-Asp-Lys-Pro was split to two dipeptides by N-ACE, depending on the chloride concentration, 8 to 24 times faster than by germinal ACE; at 100 mM Cl-, the Kcat with N-ACE was eight times higher. One millimolar 1-fluoro-2,4-dinitrobenzene inhibited germinal ACE 96% but it inhibited N-ACE by only 31%. [3H]Ramiprilat was displaced by other unlabeled ACE inhibitors to establish their relative affinities. Captopril had the lowest IC50 (0.5 nM) with N-ACE and the highest IC50 (8.3 nM) with the germinal ACE. The IC50 values of ramiprilat and quinaprilat were about the same with both active sites. The association and dissociation constants of [3H]ramiprilat indicated faster association with and faster dissociation from N-ACE than from germinal ACE. After exposure to alkali or moderate heat, somatic ACE was cleaved by plasmin and kallikrein, releasing N-ACE and apparently inactivating the C-domain. These studies affirm the differences in the activity, stability and inhibition of the two active sites of ACE.

Removal of Arg141 from the alpha chain of human hemoglobin by carboxypeptidases N and M

Both human plasma carboxypeptidase N (CPN) and membrane-bound carboxypeptidase M (CPM) released the C-terminal arginine (alpha-Arg141) of the alpha chain of human adult hemoglobin. An arginase contamination present in the hemoglobin preparation, which converted the released arginine to ornithine, was removed by gel filtration. CPM was about 20 times more efficient than CPN or its active subunit in hydrolyzing oxyhemoglobin and cleaved oxyhemoglobin twice as fast as deoxyhemoglobin. The hydrolysis of the peptide bond of alpha-Arg141 accelerated the dissociation rate of the tetramer deoxy-des-alpha-Arg141 hemoglobin to dimers 2500-fold over that of deoxyhemoglobin, as measured by haptoglobin binding. Moreover, the dissociation of the deoxy-des-alpha-Arg141 hemoglobin tetramer to dimers was not affected by 2,3-diphosphoglyceric acid. Des-alpha-Arg141 hemoglobin had a higher oxygen affinity (P50, 5.51 mm Hg; control, 19.94 mm Hg [P50 is the partial pressure of oxygen that gives 50% of the saturation of hemoglobin]) and a lower apparent cooperativity (Hill coefficient: n, 1.02; control, 2.24) than unhydrolyzed hemoglobin. After hemoglobin was incubated in human plasma, its oxygen-binding parameters, the P50, and the Hill coefficient decreased drastically due to cleavage by CPN. In the perfused rat heart, des-alpha-Arg141 hemoglobin was a more effective coronary vasoconstrictor than hemoglobin, possibly because it dissociated to dimers in the coronary vascular bed. A covalently cross-linked hemoglobin was less active than native hemoglobin. The coronary vasoconstriction was caused by multiple factors, including interference with vasodilation by nitric oxide and eicosanoids. Thus, the hydrolysis of hemoglobin by CPM and CPN demonstrated the contribution of the alpha-Arg141 residue to sustaining the tetrameric structure of hemoglobin and its normal oxygen affinity and vasoactivity.

Kininase II-type enzymes. Their putative role in muscle energy metabolism

Because of the importance of bradykinin in improving heart function in some conditions or in enhancing glucose uptake by skeletal muscle, we investigated kininases in these tissues. In P3 fraction of the heart and skeletal muscles, angiotensin I-converting enzyme (ACE) and neutral endopeptidase 24.11 (NEP) are the major kininases, as determined first with specific substrates and second with bradykinin. ACE activity was highest in guinea pig heart (2.7 +/- 0.07 mumol.h-1.mg protein-1) but decreased in other species in this order: dog atrium, rat heart, dog ventricle, and human atrium. The specific activity of NEP was lower: 0.45 mumol.h-1.mg protein-1 in cultured neonatal cardiac myocytes and varying between 0.12 and 0.05 mumol.h-1.mg protein-1 in human, dog, rat, and guinea pig heart. In the skeletal muscle P3, ACE was most active in guinea pig and rat (1.2 and 1.1 mumol.h-1.mg protein-1, respectively) but less so in dog (0.09 mumol.h-1.mg protein-1). NEP activity was higher in dog P3 (0.28 mumol.h-1.mg protein-1) but lower in rat and guinea pig (0.19 and 0.1 mumol.h-1.mg protein-1, respectively). Continuous density gradient centrifugation enriched NEP activity in dog and rat (from 0.3 to 1.0 and 0.49 mumol.h-1.mg protein-1, respectively). Immunoprecipitation with antiserum to purified NEP proved the specificity of the rat enzyme. Bradykinin (0.1 mmol/l) was inactivated in the presence and absence of inhibitors by rat skeletal muscle NEP, as measured by high-performance liquid chromatography. Here, 36% of the activity was caused by NEP and 19% by ACE. In radioimmunoassay (bradykinin 10 nmol/l), 46 and 55% of kininase in rat and dog skeletal muscle P3, respectively, was due to ACE; 36 and 28%, respectively, was due to NEP. Aside from these enzymes, an aminopeptidase in rat P3 also inactivates bradykinin. Thus, in conclusion, heart and skeletal muscle membranes contain kininase II-type enzymes, but their activity depends on the species.

Expression of rat kallikrein and epithelial polarity in transfected Madin-Darby canine kidney cells

Many properties of urinary kallikrein are well characterized, but the intracellular processing of prokallikrein and release by kidney cells have yet to be clarified. We report here on the synthesis of prokallikrein in Madin-Darby canine kidney (MDCK) cells transfected with rat submaxillary gland kallikrein cDNA and on its activation by MDCK cells and by an enriched liver Golgi membrane preparation. Transfected MDCK cells secreted only prokallikrein at both the apical and basolateral sides in about a 4:1 ratio, but cells transfected with kallikrein cDNA in reverse orientation or untreated cells released only traces of the enzyme. Prokallikrein, in culture medium or in homogenized MDCK cells, was fully activated by trypsin but activated only to 44% by thermolysin. Prokallikrein was synthesized and released into the medium at a high rate: the enzyme secreted by 5 x 10(6) cells in 24 hours cleaved 46 nmol/min D-Val-Leu-Arg-7-amino-4-methylcoumarin and liberated 63 ng/min bradykinin after activation. Immunocytology indicated the association of prokallikrein with the Golgi apparatus in the transfected cells. Antiserum to rat urinary kallikrein detected a single band in a Western blot of conditioned medium and also immunoprecipitated the enzyme. Aprotinin inhibited activated prokallikrein. Although MDCK cells released prokallikrein, their homogenates activated prokallikrein at both pH 5.5 and 7.5. Prokallikrein was also activated by a highly enriched liver Golgi membrane fraction and by an endoplasmic reticulum preparation, but the Golgi preparation was 38-fold more active. The activation was blocked significantly by inhibitors of serine proteases and less by cysteine protease inhibitors.(ABSTRACT TRUNCATED AT 250 WORDS)

Carboxypeptidase M activity is increased in bronchoalveolar lavage in human lung disease

Carboxypeptidase M (CPM) cleaves the C-terminal arginine and lysine of peptides; it is expressed in the lung, especially on the plasma membrane of alveolar type I cells. Here, we report on CPM in human bronchoalveolar lavage (BAL) collected from 69 patients and analyzed for activity, cell number and type, and protein level. Seventy-six percent of CPM activity, measured at pH 7.5 with 5-dimethylamino-naphthalene-1-sulfonyl-alanyl-arginine (Dansyl-Ala-Arg) substrate, was immunoprecipitated with polyclonal antibody to purified human enzyme. In patients without active lung disease, CPM activity in BAL was 7.69 (+/- 2.12) nmol/h/mg protein, but in patients with acute pneumonia, it was 29.25 (+/- 4.06) (p < 0.01). In patients with Pneumocystis carinii pneumonia, CPM activity was elevated to 26.00 (+/- 4.85) (p < 0.01) and in patients with lung cancer, to 30.95 (+/- 4.12) (p < 0.01). The activity was not associated with the cellular elements of BAL. The highest specific activity was in the large aggregate fraction of surfactant, which also contained the highest concentration of phosphorus. Transmission electron microscopy of this fraction revealed the presence of typical lamellar bodies and tubular myelin structures. The high CPM activity may stem from its induction and release in acute lung disease. In addition, CPM may be a marker of infection with certain pathogens and an indicator of type I cell injury in parenchymal lung diseases.

Plasma membrane-bound and lysosomal peptidases in human alveolar macrophages

Alveolar macrophages protect the lungs against noxious agents. Proteases and peptidases are essential for this defense and many metabolic activities. Human alveolar macrophages were evaluated for the presence of six important peptidases. Deamidase, a serine peptidase identical with the lysosomal protective protein and possibly with cathepsin A, had high specific activity in alveolar macrophages and is also present in cultured mouse J774A.1 and human U937 cells, used for the sake of comparison. In fractionated J774A cells, most of the deamidase activity was in the lysosomal fraction and in the final supernatant. Deamidase in human alveolar macrophages, obtained by bronchoalveolar lavage from 23 patients, cleaved dansyl-Phe-Leu-Arg at a rate of 2.26 mumol/h/mg protein and hydrolyzed the chemotactic peptide N-f-Met-Leu-Phe even faster, at a rate of 53.1 mumol/h/mg protein, the highest activity for this enzyme with any of the cells we tested. Rabbit antiserum, elicited with the recombinant partial sequence of the enzyme, immunoprecipitated 77-88% of the macrophage deamidase. In immunocytochemistry, this antiserum localized deamidase within the human macrophages. The enzyme was inhibited by diisopropylfluorophosphate (DFP; 1 mM) and by ebelactone B (10 microM), noncompetitively. The mRNA of deamidase was detected in mouse macrophages by Northern blot; the two protein chains of deamidase were shown in human macrophages by Western blot. In addition, two other serine peptidases were also highly active in macrophages: dipeptidyl peptidase IV (1.38 mumol/h/mg protein) and prolylcarboxypeptidase (0.72 mumol/h/mg protein). The activity of plasma membrane zinc metallopeptidases, neutral endopeptidase 24.11 and carboxypeptidase M, in contrast, was low or absent (angiotensin I converting enzyme; kininase II).(ABSTRACT TRUNCATED AT 250 WORDS)

Naturally occurring active N-domain of human angiotensin I-converting enzyme

Angiotensin I-converting enzyme (ACE, kininase II) is a single-chain protein containing two active site domains (named N- and C-domains according to position in the chain). ACE is bound to plasma membranes by its C-terminal hydrophobic transmembrane anchor. Ileal fluid, rich in ACE activity, obtained from patients after surgical colectomy was used as the source. Column chromatography, including modified affinity chromatography on lisinopril-Sepharose, yielded homogeneous ACE after only a 45-fold purification. N-terminal sequencing of ileal ACE and partial sequencing of CNBr fragments revealed the presence of an intact N terminus but only a single N-domain active site, ending between residues 443 and 559. Thus, ileal-fluid ACE is a unique enzyme differing from the widely distributed two-domain somatic enzyme or the single C-domain testicular (germinal) ACE. The molecular mass of ileal ACE is 108 kDa and when deglycosylated, the molecular mass is 68 kDa, indicating extensive glycosylation (37% by weight). In agreement with the results reported with recombinant variants of ACE, the ileal enzyme is less Cl(-)-dependent than somatic ACE; release of the C-terminal dipeptide from a peptide substrate was optimal in only 10 mM Cl-. In addition to hydrolyzing at the C-terminal end of peptides, ileal ACE efficiently cleaved the protected N-terminal tripeptide from the luteinizing hormone-releasing hormone and its congener 6-31 times faster, depending on the Cl- concentration, than the C-domain in recombinant testicular ACE. Thus we have isolated an active human ACE consisting of a single N-domain. We suggest that there is a bridge section of about 100 amino acids between the active N- and C-domains of somatic ACE where it may be proteolytically cleaved to liberate the active N-domain. These findings have potential relevance and importance in the therapeutic application of ACE inhibitors.

Importance of sympathetic innervation in the positive inotropic effects of bradykinin and ramiprilat

Isolated rat left atria or right ventricular strips were electrically stimulated at a constant frequency. The amplitude of twitch contractions, thus elicited, rose as a function of stimulation intensity because of increases in the evoked release of sympathetic catecholamines. Bradykinin had no effect on contractile force in preparations paced at a minimal intensity (threshold). By contrast, bradykinin (1 nmol/L to 1 mumol/L) markedly increased twitch contractile force when the preparations were paced at a high intensity (two to three times threshold). The EC50 for the positive inotropic action of bradykinin averaged 42 nmol/L. Ramiprilat (1 mumol/L), an angiotensin I-converting enzyme/kinase II inhibitor, shifted the EC50 for bradykinin to approximately 2 nmol/L. Ramiprilat (1 mumol/L) per se also produced a modest positive inotropic effect. The effects of bradykinin and/or ramiprilate were inhibited by HOE 140 (300 nmol/L), a bradykinin B2-receptor antagonist. Propranolol (1 mumol/L), a beta-adrenoceptor blocker, abolished the effects of bradykinin. After the destruction of sympathetic nerve endings by use of 6-hydroxydopamine, bradykinin no longer exerted a positive inotropic action. Cocaine (10 micrograms/mL), an inhibitor of catecholamine reuptake, potentiated the effect of bradykinin. Bradykinin did not affect the positive inotropic response to tyramine (10 mumol/L), whereas cocaine blocked it. Furthermore, bradykinin did not modify the dose-response curves for added norepinephrine. omega-Conotoxin (100 nmol/L) inhibited the positive inotropic effect of intensified stimulation and bradykinin potentiation. Bradykinin is suggested to facilitate the evoked release of sympathetic catecholamines and thereby cause a positive inotropic effect.

Increased expression of neprilysin (neutral endopeptidase 24.11) in rat and human hepatocellular carcinomas

BACKGROUND

Neprilysin (EC 3.4.24.11) (NEP), a membrane metallopeptidase, is identical with common acute lymphoblastic leukemia antigen or cluster differentiation antigen 10. This antigen is present in blast cells in acute lymphoblastic leukemias and is implicated in differentiation of B lymphocytes. NEP cleaves a variety of peptides including bradykinin, substance P, bombesin, enkephalins, and atrial natriuretic peptide. We investigated its expression in several variants of rat hepatomas and a human hepatocellular carcinoma cell line. Normal rat and human livers were used as controls.

EXPERIMENTAL DESIGN

The expression of NEP (common acute lymphoblastic leukemia antigen) was determined with: (a) enzyme assays; (b) high performance liquid chromatography analysis of bradykinin metabolism; (c) immunoprecipitation; and (d) mRNA characterization.

RESULTS

NEP activity increased by 2 to 3 orders of magnitude in all rat hepatomas and in the human SK-HEP1 cell line, compared with normal tissues. Antiserum against rat NEP precipitated 93% of endopeptidase activity in rat hepatomas, whereas monoclonal antibody to common acute lymphoblastic leukemia antigen immunoprecipitated 99% of that in human hepatocarcinoma cells. Solubilized rat hepatoma membranes cleaved bradykinin to a hepta- and dipeptide; the reaction was inhibited by an NEP inhibitor. Activity of three other membrane peptidases did not increase in rat hepatomas. Northern hybridization revealed the presence of NEP mRNA in rat hepatoma, but not in normal liver. Reverse transcriptase-polymerase chain reaction showed that hepatomas have higher amounts of NEP mRNA than normal liver of the same strain.

CONCLUSIONS

Rat hepatomas and a human hepatocarcinoma cell line express high amounts of NEP, in contrast to normal rat and human livers, which have very little. The increase in NEP activity could be due to increased transcription by tumor cells and may signal malignant transformation of liver cells.

Sequencing and cloning of human prolylcarboxypeptidase (angiotensinase C). Similarity to both serine carboxypeptidase and prolylendopeptidase families

Prolylcarboxypeptidase, a lysosomal serine carboxypeptidase, cleaves COOH-terminal amino acids linked to proline, as in angiotensin II and III and [des-Arg9] bradykinin. About 25% of the enzyme protein was sequenced, and the complete sequence was deduced from its human kidney cDNA. The cDNA insert contained an open reading frame of 1488 base pairs coding for a protein of 496 residues. The authentic NH2-terminal sequence matched the deduced protein sequence starting with residue 46, suggesting the presence of both a signal and propeptide. The mature enzyme (451 residues) has a calculated M(r) = 51,043, whereas the M(r) of the purified glycoprotein is 58,000, indicating 12% carbohydrate. The overall sequence identity with serine peptidases is low (10-18%), but sequences around residues of the putative catalytic triad (Ser134, Asp333, His411) are similar (30-67%) to both the serine carboxypeptidases (e.g. deamidase or lysosomal protective protein, yeast carboxypeptidase Y, and KEX1 gene product) and the prolylendopeptidase family. Thus, prolylcarboxypeptidase links these two families, suggesting an evolutionary relationship. It is inhibited (Ki = 2.6 x 10(-7) M) by benzyloxycarbonyl-Pro-prolinal, a specific inhibitor of prolylendopeptidase, another angiotensin metabolizing enzyme. Prolylcarboxypeptidase contains serine or threonine residues repeated as the 26th residue 7 out of 9 times, with identical or similar amino acids in other positions in the repeats. The KEX1 gene product contains a similar motif, with serine or threonine as every 27th residue. The importance of prolylcarboxypeptidase is strongly suggested by its presence in various organs and cells and by the substrates it cleaves.