Hemorphins inhibit angiotensin IV binding and interact with aminopeptidase N

Synthesis, molecular docking, electrochemical and fluorimetric analysis of new caffeic and cinnamic acid-conjugated hemorphin derivatives designed as potential anticonvulsant and antinociceptive agents

Based on the pharmacophore model of opioid receptors, our team recently synthesized a series of short-chain hemorphin peptide analogs containing non-natural amino acids. They demonstrated anticonvulsant and antinociceptive activity with low neurotoxicity. In the present study, a series of novel bioconjugates of N-modified hemorphin analogs containing second pharmacophore cinnamic acids (CA) or caffeic (KA) were synthesized by a traditional solid-phase Fmoc chemistry method for peptide synthesis. Electrochemical and fluorimetric analysis, in vivo anticonvulsant and antinociceptive activity in mice were conducted on the compounds. The three CA acid- (H4-CA, H5-CA, and H7-CA) and three KA acid- (H4-KA, H5-KA, and H7-KA) conjugated hemorphin derivatives exhibited potency at the highest doses of 2 µg/5 µl, administered by intracerebroventricular (icv) mode, against seizure spread in the maximal electroshock test (MES) in mice. The KA-conjugated H5-KA derivate, at the lowest dose, was the only compound that suppressed clonic seizures in the subcutaneous pentylenetetrazol (scPTZ) test. Except for the H5-CA, all tested CA acid- and KA acid-conjugated peptide derivates had the potency to increase the latency for clonic seizures in a dose-dependent mode. The activity against the psychomotor seizures in the 6-Hz test was detected only for the H4-CA (0.5 µg) and H4-KA (0.5 µg and 1 µg), respectively. All investigated peptides showed a more pronounced antinociceptive effect in the "intraplantar formalin" test compared to the "hot plate" test. Shorter chain analogs showed a better antinociceptive profile against tonic pain. The data suggest a DOR and KOR-mediated mechanism of action. According to the docking analysis, H7-CA showed a different antinociceptive profile than other investigated peptides. The novel peptide derivates did not exhibit neurotoxicity in the rotarod test. Our findings suggest that conjugated CA and KA morphine peptides can be used to develop novel morphine-related analogs with anticonvulsant and antinociceptive activity.

New N- and C-modified RGD-hemorphins as potential biomedical application on Ti-surface materials: synthesis, characterization and antinociceptive activity

This manuscript presented the synthesis and characterization of two new N- and C-modified analogues of VV-hemorphin-7 containing RGD (Arg-Gly-Asp) residues as potential nociceptive agents and bioactive materials. It has been shown that the addition of one or two RGD sequences to natural VV-hemorphin-7 increases its effect on acute nociception, but the reduction of the inflammatory phase depends on the concentration of the peptide. The structure-property relationship of the new peptide derivatives was highlighted by electrochemical and FT-IR methods of analysis. Because of the proven bone-structural bonds of hydroxyapatite, the simultaneous deposition of peptide/hydroxyapatite on the surface of a titanium surface was investigated. The deposition was performed in a medium of gelatin solution containing dissolved amounts of peptide and hydroxyapatite using ultrasound. SEM-EDS analyzes confirmed the presence of a layer of the studied system.

Synthesis and characterization of new 5,5′-dimethyl- and 5,5′-diphenylhydantoin-conjugated hemorphin derivatives designed as potential anticonvulsant agents

Herein, the synthesis and characterization of some novel N-modified hybrid analogues of hemorphins containing a C-5 substituted hydantoin residue as potential anticonvulsants and for the blockade of sodium channels are presented.

From Angiotensin IV to Small Peptidemimetics Inhibiting Insulin-Regulated Aminopeptidase

It was reported three decades ago that intracerebroventricular injection of angiotensin IV (Ang IV, Val-Tyr-Ile-His-Pro-Phe) improved memory and learning in the rat. There are several explanations for these positive effects of the hexapeptide and related analogues on cognition available in the literature. In 2001, it was proposed that the insulin-regulated aminopeptidase (IRAP) is a main target for Ang IV and that Ang IV serves as an inhibitor of the enzyme. The focus of this review is the efforts to stepwise transform the hexapeptide into more drug-like Ang IV peptidemimetics serving as IRAP inhibitors. Moreover, the discovery of IRAP inhibitors by virtual and substance library screening and direct design applying knowledge of the structure of IRAP and of related enzymes is briefly presented.

Molecular basis of the therapeutic properties of hemorphins

Hemorphins are endogenous peptides, 4-10 amino acids long, belonging to the family of atypical opioid peptides released during the sequential cleavage of hemoglobin protein. Hemorphins have been shown to exhibit diverse therapeutic effects in both human and animal models. However, the precise cellular and molecular mechanisms involved in such effects remain elusive. In this review, we summarize and propose potential mechanisms based on studies that investigated the biological activity of hemorphins of different lengths on multiple therapeutic targets. Special emphasis is given to molecular events related to renin-angiotensin system (RAS), opioid receptors and insulin-regulated aminopeptidase receptor (IRAP). This review provides a comprehensive coverage of the molecular mechanisms that underpin the therapeutic potential of hemorphins. Furthermore, it highlights the role of various hemorphin residues in pathological conditions, which could be explored further for therapeutic purposes.

Potential anticonvulsant activity of novel VV-hemorphin-7 analogues containing unnatural amino acids: synthesis and characterization

Herein, some new analogues of VV-hemorphin-7, modified at position 4 and 7 by the unnatural amino acids followed the structure Val-Val-Tyr-Xxx-Trp-Thr-Yyy-Arg-Phe-NH₂, where Xxx is Ac5c (1-aminocyclopentanecarboxylic acid) or Ac6c (1-aminocyclohexane carboxylic acid) and Yyy is Dap (diaminopropanoic acid) or Dab (diaminobutanoic acid), were synthesized, characterized and investigated for anticonvulsant activity. The new synthetic peptide analogues were prepared by standard solid-phase peptide synthesis-Fmoc chemistry. A single intracerebroventricular (i.c.v.) injection at doses of 5, 10, and 20 µg/10 µl, respectively, was given before evaluation with timed intravenous pentylenetetrazole (ivPTZ) infusion test and 6-Hz psychomotor seizure test in mice. The acute neurological toxicity was determined using the rotarod test. To explain the structure-active properties of the modified peptides, some physicochemical characteristic was obtained. The FT-IR spectra and their second derivatives of the amide I, II, and III bands of the peptides show ß-sheet structure conformation. The calculation of isoelectric points, by potentiometric determination of dissociated constants, is in the range from 9.79 to 10.84. This study, for the first time, also reported on the reduction-oxidative potentials of the guanidine at Arg-moiety on such kind of peptides containing arginine and tyrosine residues in different medium and electrode surface. The VV-hemorphin-7 analogues 4 and 5 were the most active against the ivPTZ test, with the effect comparable to that of peptide 1 used as a positive control. Except compound 8, all other tested peptide analogues were ineffective to raise the threshold for the clonic seizures. The peptide analogue 5 showed 100% protection in the 6-Hz test, while the other seven VV-hemorphin-7 analogues have dose-dependent activity against psychomotor seizures comparable to 1. The novel peptides did not show neurotoxicity in the rotarod test.

Insulin-regulated aminopeptidase inhibitor-mediated increases in dendritic spine density are facilitated by glucose uptake

Ethyl2-acetylamino-7-hydroxy-4-pyridin-3-yl-4H-chromene-3-carboxylate (HFI-419), the benzopyran-based inhibitor of insulin-regulated aminopeptidase (IRAP), has previously been shown to improve spatial working and recognition memory in rodents. However, the mechanism of its cognitive-enhancing effect remains unknown. There is a close correlation between dendritic spine density and learning in vivo and several studies suggest that increases in neuronal glucose uptake and/or alterations to the activity of matrix metalloproteinases (MMPs) may improve memory and increase dendritic spine density. We aimed to identify the potential mechanism by which HFI-419 enhances memory by utilizing rat primary cultures of hippocampal cells. Alterations to dendritic spine density were assessed in the presence of varying concentrations of HFI-419 at different stages of hippocampal cell development. In addition, glucose uptake and changes to spine density were assessed in the presence of indinavir, an inhibitor of the glucose transporter 4 (GLUT₄ ), or the matrix metalloprotease inhibitor CAS 204140-01-2. We confirmed that inhibition of IRAP activity with HFI-419 enhanced spatial working memory in rats, and determined that this enhancement may be driven by GLUT₄ -mediated changes to dendritic spine density. We observed that IRAP inhibition increased dendritic spine density prior to peak dendritic growth in hippocampal neurons, and that spine formation was inhibited when GLUT₄ -mediated glucose uptake was blocked. In addition, during the peak phase of dendritic spine growth, the effect of IRAP inhibition on enhancement of dendritic spine density resulted specifically in an increase in the proportion of mushroom/stubby-like spines, a morphology associated with memory and learning. Moreover, these spines were deemed to be functional based on their expression of the pre-synaptic markers vesicular glutamate transporter 1 and synapsin. Overall, or findings suggest that IRAP inhibitors may facilitate memory by increasing hippocampal dendritic spine density via a GLUT₄ -mediated mechanism. Cover Image for this issue: doi: 10.1111/jnc.14745.

Anticonvulsant evaluation and docking analysis of VV-Hemorphin-5 analogues

VV-Hemorphin-5 is an endogenous opioid peptide of the Hemorphin family with affinity at opioid receptors. A series of C-amide analogues have been synthesized, based on the structure of VV-Hemorphin-5, modified at position 1 and 7 by the un/natural amino acids (Aa8-Val-Val-Tyr-Pro-Trp-Thr-Gln-NH₂ and Val-Val-Tyr-Pro-Trp-Thr-Aa1-NH₂ ) using SPPS, Fmoc-chemistry. The peptide derivatives were evaluated for their anticonvulsant activity in three acute seizure tests in male ICR mice, the maximal electroshock (MES), the 6 Hz psychomotor seizure test, and the timed intravenous pentylenetetrazole (ivPTZ) infusion test. Their neurotoxicity was assessed in the rotarod test. Among the tested peptide analogues, V4 showed anticonvulsant activity in the three seizure tests that was comparable to the VV-Hemorphin-5 (V1) used as a positive control. While V5, V6, and V7 peptide derivatives exhibited anticonvulsant activity in the MES and 6 Hz test, they were inactive (V7) or showed pro-convulsant effect (V5 and V6) in the i.v. PTZ test. At a dose of 10 μg/mouse the peptide V2 was effective against clonic seizures induced by PTZ. Motor coordination was not affected by newly developed analogues of VV-Hemorphin-5. Docking study results suggest that kappa opioid receptor binding could be the mechanism of action of peptide derivatives with anticonvulsant activity. The results suggest that incorporation of nonproteinogenic and/or natural amino acids at position 1 and 7 of the VV-Hemorphin-5 scaffold deserve further evaluation in models of epilepsy and derivatization.

Synthesis, characterization and anticonvulsant activity of new azobenzene-containing VV-hemorphin-5 bio photoswitch

A novel analog of VV-hemorphin-5 containing azobenzene moiety has been synthesized and investigated for anticonvulsant activity in relation to its E → Z photophysical properties activated by long wavelength light at 365 nm. The synthesis was achieved by a modified SPPS by Fmoc-dimerization strategy. The electrochemical behavior before and after UV illumination was investigated using different voltammetric modes. The number of electrons transferred, heterogenic rate constant and diffusion coefficient for E- and Z-isomers were also evaluated. Revealing the governing principles involved in signaling and nerve pulse propagation requires the detailed characterization of the electrical properties of cell membranes. For probing the effect of synthesized azo-peptide on the membrane electrical properties, we measured the specific capacitance of lipid bilayers, representing a basic physical model of biomembranes with their simple reproducibility in laboratory conditions at controlled membrane composition and physicochemical parameters of the surrounding aqueous medium. Our results have shown reduced membrane capacitance in the presence of the azo-peptide, thus providing evidences for possible alterations in the dielectric permittivity of the bilayer. The (Val-Val-Tyr-Pro-Trp-Thr-Gln)₂Azo peptide was explored also in vivo for preliminary anticonvulsant activity by using the 6-Hz seizure test and pentylenetetrazol (PTZ) seizure test in mice. The Z-isomer has exhibited higher potency compared to E-isomer most pronouncedly in the 6 Hz test for psychomotor seizures where the compound had activity at all three tested doses. It was found that the Z-isomer decrease the latency for onset of clonic seizures induced by PTZ. These results demonstrate that the Z-isomer deserves further evaluation in other screening tests for anticonvulsant activity.

Isolation of new ligands for orphan receptor MRGPRX1-hemorphins LVV-H7 and VV-H7

The human MAS-related G protein-coupled receptor X1 (MRGPRX1) is a member of the GPCR family. The receptor is primate specific and expressed in the sensory neurons of dorsal root ganglion and trigeminal ganglion, where it is considered to be involved in the pain perception. The MRGPRX1 has unusual binding mechanism, as it is activated by several different ligands as well as several different fragments of precursor proteins. Thus, we hypothesize that it is activated by several unknown compounds as well since the receptor is still classified as orphan. Here, we describe the isolation of two novel endogenous ligands for the MRGPRX1 from human platelet preparation. The isolated ligands are hemoglobin β-chain fragments, known members of the hemorphin family.

AGHLDDLPGALSAL: A hemoglobin fragment potentially competing with albumin to bind transition metal ions

Protein degradation leads to the formation of endogenous peptides, the biological activity of which is most often unknown. The peptide AGHLDDLPGALSAL, recently isolated from mouse brain homogenates, has been recognized as a fragment of the α-chain of hemoglobin. AGHLDDLPGALSAL has the ability of inhibiting the peripheral hyperalgesic inflammatory responses through the indirect activation of the μ-opioid receptors. A peculiarity of AGHLDDLPGALSAL is the presence, at its N-terminus of a strong binding site for divalent transition metal ions, similar to that characterizing the human albumin and called "ATCUN motif". The consequential metal binding ability of AGHLDDLPGALSAL can be connected to its biological activity. For this reason, we decided to investigate the coordination properties of AGHLDDLPGALSAL towards Cu(II), Ni(II) and Zn(II) ions, reported here for the first time. The results confirm that AGHLDDLPGALSAL is a strong ligand for those metals: it can even compete with albumin under suitable conditions. In vitro assays on the inhibition of Cu(II) toxicity towards different cell lines confirmed that the binding ability of AGHLDDLPGALSAL can imply relevant biological consequences.

Angiotensin IV stimulates high atrial stretch-induced ANP secretion via insulin regulated aminopeptidase

Angiotensin IV (Ang IV) is formed by aminopeptidase N (APN) from angiotensin III (Ang III) by removing the first N-terminal amino acid. Previously, we reported that angiotensin II (Ang II) inhibits atrial natriuretic peptide (ANP) secretion via angiotensin II type 1 receptor (AT1R). In contrast, angiotensin-(1-7) [Ang-(1-7)] and Ang III stimulate ANP secretion via Mas receptor (Mas R) and angiotensin II type 2 receptor (AT2R), respectively. However, it is not known whether there is any relationship between Ang IV and ANP secretion. Therefore, the aim of the present study was to determine the effect of Ang IV on ANP secretion and to find its downstream signaling pathway using in isolated perfused beating atria. Ang IV (0.1, 1 and 10μM) stimulated high atrial stretch-induced ANP secretion and ANP concentration in a dose-dependent manner. The augmented effect of Ang IV (1μM) on high atrial stretch-induced ANP secretion and concentration was attenuated by pretreatment with insulin-regulated aminopeptidase (IRAP) antagonist but not by AT1R or AT2R antagonist. Pretreatment with inhibitors of downstream signaling pathway including phosphatidylinositol 3-kinase (PI3K), protein kinase B (Akt) and mammalian target of rapamycin (mTOR) blocked Ang IV-induced ANP secretion and concentration. Therefore, these results suggest that Ang IV stimulates ANP secretion and concentration via IRAP and PI3K-Akt-mTOR pathway.

Discovery of inhibitors of insulin-regulated aminopeptidase as cognitive enhancers

The hexapeptide angiotensin IV (Ang IV) is a metabolite of angiotensin II (Ang II) and plays a central role in the brain. It was reported more than two decades ago that intracerebroventricular injection of Ang IV improved memory and learning in the rat. Several hypotheses have been put forward to explain the positive effects of Ang IV and related analogues on cognition. It has been proposed that the insulin-regulated aminopeptidase (IRAP) is the main target of Ang IV. This paper discusses progress in the discovery of inhibitors of IRAP as potential enhancers of cognitive functions. Very potent inhibitors of the protease have been synthesised, but pharmacokinetic issues (including problems associated with crossing the blood-brain barrier) remain to be solved. The paper also briefly presents an overview of the status in the discovery of inhibitors of ACE and renin, and of AT1R antagonists and AT2R agonists, in order to enable other discovery processes within the RAS system to be compared. The paper focuses on the relationship between binding affinities/inhibition capacity and the structures of the ligands that interact with the target proteins.

Binding of "AT4 receptor" ligands to insulin regulated aminopeptidase (IRAP) in intact Chinese hamster ovary cells

Insulin regulated aminopeptidase (IRAP) recognises "AT(4)-receptor" ligands like angiotensin IV (Ang IV) and peptidomimetics like AL-11. The metabolic stability and high affinity of [(3)H]AL-11 for catalytically active IRAP allowed its detection in Chinese hamster ovary (CHO-K1) cell membranes in the absence of chelators (Demaegdt et al., 2009). Here, we show that, contrary to [(3)H]Ang IV, [(3)H]AL-11 displays high affinity and specificity for IRAP in intact CHO-K1 cells as well. After binding to IRAP at the surface, [(3)H]AL-11 is effectively internalized by an endocytotic process. Unexpectedly, surface binding and internalization of [(3)H]AL-11 was not affected by pretreating the cells with Ang IV but declined with AL-11. In the latter case surface expression of IRAP even increased. After elimination of simpler explanations, it is proposed that metabolically stable "AT(4)-receptor" ligands undergo semi-continuous cycling between the cell surface and endosomal compartments. The in vivo efficacy of stable and unstable "AT(4)-receptor" ligands could therefore differ.

Blood pressure and renal hemodynamic effects of angiotensin fragments

Angiotensin (Ang) II, the main effector peptide of the renin-Ang system, increases arterial blood pressure through Ang II type 1A (AT(1a)) receptor-dependent arterial vasoconstriction and by decreasing renal salt and water excretion through extrarenal and intrarenal mechanisms. AT(2) receptors are assumed to oppose these responses mediated by AT(1) receptors, thereby attenuating the pressor effects of Ang II. Nevertheless, a possible role of AT(2) receptors in the regulation of renal hemodynamics and sodium homeostasis remains to be unclear. Several other Ang fragments such as Ang III, Ang IV, Ang-(1-7) and Ang A have also been shown to display biological activity. In this review, we focus on the effects of these Ang on blood pressure, renal hemodynamics and sodium water handling, and discuss the receptors involved in these actions.

Selective labeling of IRAP by the tritiated AT(4) receptor ligand [3H]Angiotensin IV and its stable analog [3H]AL-11

'AT(4) receptors' through which Angiotensin IV (Ang IV) improves memory acquisition, were recently identified as insulin regulated aminopeptidase (IRAP). Radioligand binding studies have hitherto been performed with iodinated Ang IV in the presence of divalent cation chelators EDTA and 1,10-phenanthrolin. Hence, they referred to the apo-form of IRAP. Presently, binding of [(3)H]Ang IV and [(3)H]AL-11, a stable Ang IV analog, was compared on Chinese hamster ovary (CHO-K1) and mouse hippocampal (P40H1) cell membranes. With chelators, their high affinity sites showed the same pharmacological profile as for [(125)I]Ang IV binding. Without chelators, only high affinity binding was perceived for [(3)H]AL-11. The same pharmacological profile was recorded in both membrane preparations; it was different from the one in the presence of chelators and corresponded to catalytically active IRAP (despite the concurrent presence of aminopeptidase N (APN) in P40H1 cell membranes). This confirms that the active and apo-forms of IRAP have a distinct pharmacological profile.

Significant lower VVH7-like immunoreactivity serum level in diabetic patients: evidence for independence from metabolic control and three key enzymes in hemorphin metabolism, cathepsin D, ACE and DPP-IV

Low circulating VVH7-like immunoreactivity (VVH7 i.r) level was amazingly observed in human diabetic sera. Here, we examined the impact of diabetes type, clinico-biological features and metabolic control on circulating VVH7 i.r level in this disease. ELISA test was used to measure VVH7 i.r in sera of 120 diabetic patients (type 1 diabetes in 64, type 2 diabetes in 56). Three enzymatic tests were also applied to determine serum cathepsin D (CD), dipeptidyl peptidase IV (DPP-IV) and angiotensin-converting enzyme (ACE) activities. A subgroup of 24 type 1 diabetic patients negative for microalbuminuria and hypertension were submitted to an ambulatory blood pressure monitoring to evaluate the relationship between VVH7 i.r level and blood pressure parameters. The mean serum concentration of VVH7 i.r was drastically reduced in diabetic patients (0.91+/-0.93 micromol/l versus 5.63+/-1.11 micromol/l in controls) (p<0.001). A negative correlation between VVH7 i.r level and daytime diastolic blood pressure existed in type 1 diabetic patients. There was no association of low VVH7 i.r with either type of diabetes or HbA1c level. An increase of cathepsin D activity was found in serum of diabetic patients compared to controls (0.47 U/ml versus 0.15 U/ml, respectively) whereas DPPIV activity was significantly decreased in diabetic sera (50.81 U/ml versus 282.10 U/l respectively). Diminution of VVH7 i.r in sera of diabetic patients was confirmed but still remained unexplained. Relationships between higher systolic blood pressure and decrease of VVH7 i.r reinforce the need to investigate this pathway in this disease to elucidate its role in macro- and micro-angiopathy.

Kinetic studies on aminopeptidase M-mediated degradation of human hemorphin LVV-H7 and its N-terminally truncated products

The human hemorphin LVV-H7 belongs to the class of micro-opiod receptor-binding peptides, which also exhibits significant affinity to insulin-regulated aminopeptidase (IRAP) thereby affecting IRAP inhibition. The inhibitory potency towards IRAP is of pharmaceutical interest for the treatment of Alzheimer's disease. Consecutive N-terminal cleavage of the first two amino acid residues of LVV-H7 affects a drastic increase of the binding affinity (V-H7) but ultimately leads to its complete abolition after cleavage of the next amino acid residue (H7). Therefore, we investigated LVV-H7 truncation by aminopeptidase M (AP-M) identified as a LVV-H7 degrading enzyme potentially regulating hemorphin activity towards IRAP in vivo. Using a selective quantitative multi-component capillary zone electrophoretic method (CZE-UV), we analyzed the AP-M-mediated subsequent proteolysis of the hemorphins LVV-H7 (L32-F41), VV-H7 (V33-F41), and V-H7 (V34-F41) in vitro. Incubations were carried out with synthetic hemorphins applied as single substrates or in combination. Maximum velocities (V(max)), catalytic constants (turnover numbers, kcat), and specific enzyme activities (EA) were calculated. L32 cleavage from LVV-H7 happens more than two-times faster (kcat: 140 min(-1) +/- 9%, EA: 1.0 U/mg +/- 9%) than V33 cleavage from VV-H7 (kcat: 61 min(-1) +/- 10%, EA: 0.43 U/mg +/- 10%) or V32 deletion from V-H7 (kcat: 62 min(-1) +/- 8%, EA: 0.46 U/mg +/- 8%). In contrast, we showed that H7 (Y35-F41) was neither degraded by porcine AP-M nor did it act as an inhibitor for this enzyme. Determined turnover numbers were in the same dimension as those reported for dynorphin degradation. This is the first time that AP-M-mediated truncation of natural underivatized LVV-H7 and its physiological metabolites was analyzed to determine kinetic parameters useful for understanding hemorphin processing and designing hemorphin-derived drug candidates.

The moonlighting enzyme CD13: old and new functions to target

Aminopeptidase N (CD13) is a widely expressed ectoenzyme with functions that do not always depend on its enzymatic activity: an aspect that has been overlooked. Numerous CD13-targeting tools have been developed in the last few years. Several of them are already undergoing clinical trials, and there are promising reports on the effectiveness of others in animal models of disease. However, their efficacy might be obscured by their effects on unrecognized functions of CD13, resulting in unexpected complications. The purpose of this review is (i) to discuss the various functions ascribed to CD13 and the possible mechanisms behind them and (ii) to consider some of the questions that need to be answered to achieve a better understanding of the biological relevance of these functions, a more precise interpretation of the results obtained after their manipulation and a more rational design of CD13-targeting agents.

Ligands to the (IRAP)/AT4 receptor encompassing a 4-hydroxydiphenylmethane scaffold replacing Tyr2

Analogues of the hexapeptide angiotensin IV (Ang IV, Val(1)-Tyr(2)-Ile(3)-His(4)-Pro(5)-Phe(6)) encompassing a 4-hydroxydiphenylmethane scaffold replacing Tyr(2) and a phenylacetic or benzoic acid moiety replacing His(4)-Pro(5)-Phe(6) have been synthesized and evaluated in biological assays. The analogues inhibited the proteolytic activity of cystinyl aminopeptidase (CAP), frequently referred to as the insulin-regulated aminopeptidase (IRAP), and were found less efficient as inhibitors of aminopeptidase N (AP-N). The best Ang IV mimetics in the series were approximately 20 times less potent than Ang IV as IRAP inhibitors. Furthermore, it was found that the ligands at best exhibited a 140 times lower binding affinity to the membrane-bound IRAP/AT4 receptor than Ang IV. Although the best compounds still exert lower activities than Ang IV, it is notable that these compounds comprise only two amino acid residues and are considerably less peptidic in character than the majority of the Ang IV analogues previously reported as IRAP inhibitors in the literature.

Angiotensin receptor subtype mediated physiologies and behaviors: new discoveries and clinical targets

The renin-angiotensin system (RAS) mediates several classic physiologies including body water and electrolyte homeostasis, blood pressure, cyclicity of reproductive hormones and sexual behaviors, and the regulation of pituitary gland hormones. These functions appear to be mediated by the angiotensin II (AngII)/AT(1) receptor subtype system. More recently, the angiotensin IV (AngIV)/AT(4) receptor subtype system has been implicated in cognitive processing, cerebroprotection, local blood flow, stress, anxiety and depression. There is accumulating evidence to suggest an inhibitory influence by AngII acting at the AT(1) subtype, and a facilitory role by AngIV acting at the AT(4) subtype, on neuronal firing rate, long-term potentiation, associative and spatial learning, and memory. This review initially describes the biochemical pathways that permit synthesis and degradation of active angiotensin peptides and three receptor subtypes (AT(1), AT(2) and AT(4)) thus far characterized. There is vigorous debate concerning the identity of the most recently discovered receptor subtype, AT(4). Descriptions of classic and novel physiologies and behaviors controlled by the RAS are presented. This review concludes with a consideration of the emerging therapeutic applications suggested by these newly discovered functions of the RAS.

Aminopeptidase N in arterial hypertension

Aminopeptidase N (APN) or CD13 is a conserved type II integral membrane zinc-dependent metalloprotease in the M1 family of ectoenzymes. APN is abundant in the kidneys and central nervous system. Identified substrates include Angiotensin III (Ang III); neuropeptides, including enkephalins and endorphins; and homones, including kallidan and somatostatin. It is developmentally expressed, a myelomonocytic marker for leukemias, and a receptor for coronovirus. There is evolving support for APN in the regulation of arterial blood pressure and the pathogenesis of hypertension. In rodent strains, intracerebraventricular (i.c.v.) infusions of APN reduces, while inhibitors of APN activity have a pressor effect on blood pressure. Dysregulation of central APN has been linked to the pathogenesis of hypertension in the spontaneously hypertensive rat. There is evidence that renal tubule APN inhibits Na flux and plays a mechanistic role in salt-adaptation. A functional polymorphism of the ANP gene has been identified in the Dahl salt-sensitive rat. Signaling by APN impacting on blood pressure is likely mediated by regulation of the metabolism of Ang III to Ang IV. Whether APN regulates arterial blood pressure in humans or is a therapeutic target for hypertension are subjects for future exploration.

Comparative in vitro degradation of the human hemorphin LVV-H7 in mammalian plasma analysed by capillary zone electrophoresis and mass spectrometry

The human hemorphin LVV-H7 (L32VVYPWTQRF41) is a hemoglobin-beta, -gamma, -delta or -epsilon chain derived cationic decapeptide of the micro-opioid receptor binding family. It exhibits potential pharmacological value relevant, for example, for blood pressure regulation, learning performance and Alzheimer's disease. The regulatory potency is strictly dependent on the length of the amino acid sequence which is sensitive towards proteinases from tissues and plasma. To analyse LVV-H7 in vitro degradation in mammalian plasma, a novel multi-component quantitative capillary zone electrophoretic (CZE) procedure was applied, combined with qualitative metabolite profiling by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). In all types of plasma, LVV-H7 was N-terminally truncated generating four metabolites (M1-M4) with an intact C-terminus: M1 (V33VYPWTQRF41), M2 (V34YPWTQRF41), M3 (Y35PWTQRF41) and M4 (W37TQRF41). In EDTA plasma these degradation products were detected exclusively, whereas in citrate and heparin plasma four further metabolites appeared resulting from additional C-terminal cleavage of the dipeptide R40F41: M5 (L32VVYPWTQ39), M6 (V33VYPWTQ39), M7 (V34YPWTQ39) and M8 (Y35PWTQ39). In the presence of selective proteinase inhibitors aminopeptidase M and angiotensin-converting enzyme (for N- and C-terminal truncation, respectively) were identified as plasma enzymes responsible for hemorphin degradation. Furthermore, striking inter-mammalian species distinctions were detected revealing strongly differing degradation velocities but similar metabolite patterns.

Involvement of insulin-regulated aminopeptidase and/or aminopeptidase N in the angiotensin IV-induced effect on dopamine release in the striatum of the rat

Locally administered angiotensin IV causes a dose-dependent increase of the dopamine levels in the striatum of the rat. The aminopeptidases insulin-regulated aminopeptidase (IRAP) and/or aminopeptidase N (AP-N) are proposed to be involved in this effect since both enzymes are inhibited by angiotensin IV. In agreement with this hypothesis we demonstrate that by using the AP-N selective inhibitor 7B, about 60% of the aminopeptidase activity in striatal membranes could be attributed to AP-N (pK(i)=9.20). Higher concentrations of 7B are capable of inhibiting IRAP as well (pK(i)=7.26). Interestingly, in vivo, inhibition of IRAP or AP-N activity does not appear to be involved in the angiotensin IV-mediated effect in the striatum since 7B itself is not capable to induce dopamine release such as observed with angiotensin IV. However, 7B at a concentration selective for inhibition of AP-N (100 nM) potentiates the angiotensin IV-mediated increase of dopamine, suggesting that inhibition of AP-N lengthens the half-life of angiotensin IV. On the other hand, inhibition of both AP-N and IRAP by perfusion of 500 nM 7B completely abolishes the effect of angiotensin IV. We therefore hypothesize that the effect of angiotensin IV on dopamine release in the striatum is mediated via activation of IRAP and/or AP-N, possibly acting as receptors for angiotensin IV.

Quantification of angiotensin-converting-enzyme-mediated degradation of human chemerin 145-154 in plasma by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry

Chemerin is a chemoattractive protein acting as a ligand for the G-protein-coupled receptor ChemR23/CMKLR1 and plays an important role in the innate and adaptive immunity. Proteolytic processing of its C terminus is essential for receptor binding and physiological activity. Therefore, we investigated the plasma stability of the decapeptide chemerin 145-154 (P(145)-F(154)) corresponding to the C terminus of the physiologically active chemerin variant E(21)-F(154) from human hemofiltrate. For monitoring concentration-time profiles and degradation products we developed a novel matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry procedure using an internal peptide standard (hemorphin LVV-H7) for quantification. The linear range covers 2.5 orders of magnitude in the lower micromolar concentration range (lower limit of quantification 0.312 microg/ml, 0.25 microM) characterized by satisfactory reproducibility (CV < or =9%), accuracy (< or =10%), ruggedness, and recovery (98%). We found that chemerin 145-154 is C-terminally truncated in human citrate plasma by the cleavage of the penultimate dipeptidyl residue. N-terminal truncation was not observed. In contrast to citrate plasma, no degradation was detected in ethylenediammetetraacetate (EDTA) plasma. We identified angiotensin-converting-enzyme (ACE) to be responsible for C-terminal truncation, which could be completely inhibited by EDTA and captopril. These results are relevant to clarify the natural processing of chemerin and the potential involvement of ACE in mediating the immune response.

Angiotensin AT4 receptor ligand interaction with cystinyl aminopeptidase and aminopeptidase N: [125I]Angiotensin IV only binds to the cystinyl aminopeptidase apo-enzyme

Due to its high affinity for [(125)I]Angiotensin IV, cystinyl aminopeptidase (CAP) has recently been assigned as the 'angiotensin AT(4) receptor'. Since the aminopeptidase N (AP-N) activity is also susceptible to inhibition by Angiotensin IV, it might represent an additional target for this peptide. Based on [(125)I]Angiotensin IV binding and catalytic activity measurements, we compared the ligand interaction properties of recombinant human CAP and human AP-N. Both enzymes displayed distinct pharmacological profiles. Although their activity is inhibited by Angiotensin IV and LVV-hemorphin 7, both peptides are more potent CAP-inhibitors. On the other hand, substance P and l-methionine have a higher potency for AP-N. High affinity binding of [(125)I]Angiotensin IV to CAP occurs in the presence of chelators but not to AP-N in either the absence or presence of chelators. These differences were exploited to determine whether CAP and/or AP-N are present in different cell lines (CHO-K1, COS-7, HEK293, SK-N-MC and MDBK). We provide evidence that CAP predominates in these cell lines and that, comparatively, CHO-K1 cells display the highest level of this enzyme.

Validated multi-component CZE-UV procedure for the quantification of human hemorphin LVV-H7 in plasma stability studies

The human hemorphin LVV-H7 is an endogenous cleavage product of the hemoglobin beta, gamma, epsilon or delta chain exhibiting potential pharmaceutical relevance for blood pressure regulation, the treatment of Alzheimer's disease or learning deficiencies. Here we present the development of a multi-component capillary zone electrophoretic method (CZE-UV), allowing the simultaneous quantification of LVV-H7 and four N-terminal degradation products generated in EDTA plasma. Hemorphins in the supernatant of precipitated plasma samples are quantified by external calibration. Validation of the procedure oriented towards international pharmaceutical guidelines and demonstrated excellent linearity (r2 > or = 0.999), good precision (repeatability and reproducibility below 11%), accuracy (-8.4%-4%), ruggedness and an appropriate lower limit of quantification (LLOQ 1.0 microg mL(-1)). This procedure was applied to stability studies of LVV-H7 in human EDTA plasma attended by profiling metabolites using qualitative MALDI-TOF MS analysis. We detected the activity of a soluble plasma form of aminopeptidase M causing successive N-terminal truncation. This is the first time that LVV-H7 degradation as well as its metabolite production have systematically been monitored by a quantitative CZE-UV procedure, underlining the growing importance of such techniques in peptide analysis. In addition, our results give useful hints for future drug development of LVV-H7.

The CNS renin-angiotensin system

The renin-angiotensin system (RAS) is one of the best-studied enzyme-neuropeptide systems in the brain and can serve as a model for the action of peptides on neuronal function in general. It is now well established that the brain has its own intrinsic RAS with all its components present in the central nervous system. The RAS generates a family of bioactive angiotensin peptides with variable biological and neurobiological activities. These include angiotensin-(1-8) [Ang II], angiotensin-(3-8) [Ang IV], and angiotensin-(1-7) [Ang-(1-7)]. These neuroactive forms of angiotensin act through specific receptors. Only Ang II acts through two different high-specific receptors, termed AT1 and AT2. Neuronal AT1 receptors mediate the stimulatory actions of Ang II on blood pressure, water and salt intake, and the secretion of vasopressin. In contrast, neuronal AT2 receptors have been implicated in the stimulation of apoptosis and as being antagonistic to AT1 receptors. Among the many potential effects mediated by stimulation of AT2 are neuronal regeneration after injury and the inhibition of pathological growth. Ang-(1-7) mediates its antihypertensive effects by stimulating the synthesis and release of vasodilator prostaglandins and nitric oxide and by potentiating the hypotensive effects of bradykinin. New data concerning the roles of Ang IV and Ang-(1-7) in cognition also support the existence of complex site-specific interactions between multiple angiotensins and multiple receptors in the mediation of important central functions of the RAS. Thus, the RAS of the brain is involved not only in the regulation of blood pressure, but also in the modulation of multiple additional functions in the brain, including processes of sensory information, learning, and memory, and the regulation of emotional responses.

Renal and macrophage aminopeptidase activities in cyclosporin-treated mice

Cyclosporin, an immunosuppressive drug, is known to affect macrophage and to exert a nephrotoxic effect. Aminopeptidases play important roles for renal and macrophage functions. In this work, we attempt to test the hypothesis that the aminopeptidases participate within macrophage and renal effects induced by cyclosporin. Macrophage and renal aminopeptidase activities of cyclosporin-treated and control mice were evaluated, as well as renal caspase 3 activity, hematocrit, urinary protein and plasma osmolality, creatinine and uric acid concentrations. Cyclosporin treatment increased caspase 3 activity, hematocrit and osmolality, while urinary protein, creatinine and uric acid were unaltered. Soluble and particulate aminopeptidases in resident and elicited macrophages were unaffected by cyclosporin. The treatment with cyclosporin increased neutral, basic, cystyl, prolyl imino and pyroglutamyl soluble aminopeptidase activities in the renal cortex. Acid and basic soluble aminopeptidase activities increased in the renal medulla. Increased levels of particulate form in the cortex were detected for acid and pyroglutamyl aminopeptidase activities. Cyclosporin increased cortical soluble while decreased medullar particulate prolyl dipeptidyl aminopeptidase IV activity. With the exception of prolyl dipeptidyl aminopeptidase IV, particulate aminopeptidase activities returned to levels similar to controls after fifteen days of cyclosporin withdrawal, and soluble aminopeptidase activities did not regress. Our data indicate that the adopted regimen of cyclosporin treatment produced mild renal impairment with consistent changes on the levels of renal but not macrophage aminopeptidase activities. The obtained profiles of macrophage and renal aminopeptidase activities should be considered into the elaboration of new potential strategies for preventing nephrotoxicity during the treatment with cyclosporin.

Hemorphins: substrates and/or inhibitors of dipeptidyl peptidase IV

Hemorphins are endogenous peptides belonging to the family of "atypical" opioid peptides released from sequentially hydrolyzed hemoglobin. In this paper, we report an inhibitory effect of these peptides on dipeptidyl peptidase IV (DPPIV) activity, known to be involved in regulatory functions such as the activation or inactivation of peptides. The structure activity research revealed that hemorphins N-terminus sequence influences nature of the interaction between hemorphins and DPPIV. Kinetic studies conducted with purified DPPIV demonstrated that hemorphin-7 (H7) constitutes a good substrate (K(cat)/K(m) of 137 mM(-1) s(-1)) for this enzyme but could also act as a selective competitive inhibitor by substrate binding site competition. These blood-derived peptides could represent endogenous regulators of this enzyme activity.

In vitro metabolism of LVV-Hemorphin-7 by renal cytosol and purified prolyl endopeptidase

The metabolism of LVVH7, an endogenous peptide obtained by cathepsin D hydrolysis of the beta chain of hemoglobin, was studied, in vitro, in the presence of cytosol of rat kidney and compared with angiotensin IV. High metabolic activity was found against these two peptides (half life time < 2 min) in this subcellular fraction. The main products of LVVH7 metabolism by renal cytosol are VVH7, H7 and LVVH6 suggesting both aminopeptidase and carboxypeptidase activities. The use of PEP inhibitor in kidney cytosol permitted to demonstrate the major role of prolyl endopeptidase (PEP) in LVVH7 degradation. This fact was reinforced by a kinetic study investigated with purified enzyme (Km/Vmax about 238 mM-1 s-1 and close to that observed for angiotensin related peptides).

Structure-activity study of LVV-hemorphin-7: angiotensin AT4 receptor ligand and inhibitor of insulin-regulated aminopeptidase

The decapeptide LVV-hemorphin-7 binds with high affinity to the angiotensin IV (Ang IV) receptor (AT(4) receptor), eliciting a number of physiological effects, including cellular proliferation and memory enhancement. We have recently shown that the AT(4) receptor is identical to insulin-regulated aminopeptidase (IRAP) and that both LVV-hemorphin-7 and Ang IV inhibit the catalytic activity of IRAP. In the current study, a series of alanine-substituted and N- or C-terminally modified analogs of LVV-hemorphin-7 were evaluated for their abilities to compete for (125)I-Ang IV binding in sheep adrenal and cerebellar membranes. Selected analogs were also analyzed for binding to recombinant human IRAP and inhibition of IRAP aminopeptidase activity. C-Terminal deletions of LVV-hemorphin-7 resulted in modest changes in affinity for IRAP, whereas deletion of the first three N-terminal residues abolished binding. Monosubstitutions of Tyr(4) and Trp(6) with alanine resulted in a 10-fold reduction in affinity. Competition binding studies using recombinant human IRAP demonstrated the same rank order of affinity as obtained for the ovine tissues. All LVV-hemorphin-7 analogs tested, except for Leu-Val-Val-Tyr, inhibit the cleavage of the synthetic substrate, leucine beta-naphthylamide, by IRAP, with K(i) values between 56 and 620 nM. We find that the Val(3) residue is crucial for LVV-hemorphin-7 binding to IRAP, whereas the C-terminal domain seems to play a minor role. The current study highlights the minimal residues necessary for binding and inhibition of IRAP and provides a basis to design peptidomimetic analogs for experimental and potentially clinical use.

Identification and functional characterization of hemorphins VV-H-7 and LVV-H-7 as low-affinity agonists for the orphan bombesin receptor subtype 3

1. The human orphan G-protein coupled receptor bombesin receptor subtype 3 (hBRS-3) was screened for peptide ligands by a Ca(2+)mobilization assay resulting in the purification and identification of two specific ligands, the naturally occurring VV-hemorphin-7 (VV-H-7) and LVV-hemorphin-7 (LVV-H-7), from human placental tissue. These peptides were functionally characterized as full agonists with unique specificity albeit low affinity for hBRS-3 compared to other bombesin receptors. 2. VV-H-7 and LVV-H-7 induced a dose-dependent response in hBRS-3 overexpressing CHO cells, as well as in NCI-N417 cells expressing the hBRS-3 endogenously. The affinity of VV-H-7 was higher in NCI-N417 cells compared to overexpressing CHO cells. In detail, the EC(50) values were 45+/-15 microM for VV-H-7 and 183+/-60 microM for LVV-H-7 in CHO cells, and 19+/-6 microM for VV-H-7 and 38+/-18 microM for LVV-H-7 in NCI-N417 cells. Other hemorphins had no effect. Gastrin-releasing peptide (GRP) and neuromedin B (NMB) showed similar EC(50) values of 13-20 microM (GRP) and of 1-2 microM (NMB) on both cell lines. 3. Structure-function analysis revealed that both the N-terminal valine and the C-terminal phenylalanine residues of VV-H-7 are critical for the ligand-receptor interaction. 4. Endogenous hBRS-3 in NCI-N417 activated by VV-H-7 couples to phospholipase C resulting in changes of intracellular calcium, which is initially released from an inositol trisphosphate (IP(3))-sensitive store followed by a capacitive calcium entry from extracellular space. 5. VV-H-7-induced hBRS-3 activation led to phosphorylation of p42/p44-MAP kinase in NCI-N417 cells, but did not stimulate cell proliferation. In contrast, phosphorylation of focal adhesion kinase (p125(FAK)) was not observed.

Cellular targets for angiotensin II fragments: pharmacological and molecular evidence

Although angiotensin II has long been considered to represent the end product of the renin-angiotensin system (RAS), there is accumulating evidence that it encompasses additional effector peptides with diverse functions. In this respect, angiotensin IV (Ang IV) formed by deletion of the two N terminal amino acids, has sparked great interest because of its wide range of physiological effects. Among those, its facilitatory role in memory acquisition and retrieval is of special therapeutic relevance. High affinity binding sites for this peptide have been denoted as AT(4)- receptors and, very recently, they have been proposed to correspond to the membrane-associated OTase/ IRAP aminopeptidase. This offers new opportunities for examining physiological roles of Ang IV in the fields of cognition, cardiovascular and renal metabolism and pathophysiological conditions like diabetes and hypertension. Still new recognition sites may be unveiled for this and other angiotensin fragments. Recognition sites for Ang-(1-7) (deletion of the C terminal amino acid) are still elusive and some of the actions of angiotensin III (deletion of the N terminal amino acid) in the CNS are hard to explain on the basis of their interaction with AT(1)-receptors only. A more thorough cross-talk between in vitro investigations on native and transfected cell lines and in vivo investigations on healthy, diseased and transgenic animals may prove to be essential to further unravel the molecular basis of the physiological actions of these small endogenous angiotensin fragments.

Comparative effects of angiotensin IV and two hemorphins on angiotensin-converting enzyme activity

The role of angiotensin IV (AngIV) in the regulation of angiotensin-converting enzyme (ACE) was studied in vitro. This study demonstrates that this active fragment appeared as a novel endogenous ACE inhibitor. Inhibitory kinetic studies revealed that AngIV acts as a purely competitive inhibitor with a K(i) value of 35 microM. AngIV was found to be quite resistant to ACE hydrolysis opposite to hemorphins which are both ACE inhibitors and substrates. In order to confirm a putative role of AngIV and hemorphins in the Renin-Angiotensin system (RAS) regulation, we studied their influence on AngI conversion. We noticed that 16.7 microM of both peptides decreased more than 50% of AngI conversion to AngII in vitro. The capacity of hemorphins, particularly LVVH-7, and AngIV to inhibit ACE activity here suggests a synergistic relation between these two peptides and the regulation of RAS.

Effects of angiotensin IV and angiotensin-(1-7) on basal and angiotensin II-stimulated cytosolic Ca2+ in mesangial cells

This study analyzed the influence of two main metabolites of angiotensin II, angiotensin IV and angiotensin-(1-7), on basal and angiotensin II-dependent [Ca2+](i) in rat mesangial cells. Angiotensin IV behaved as a weak agonist. Its effects were abolished by angiotensin AT(1) receptor antagonists. Treatment with angiotensin II abolished the effect of a subsequent treatment with angiotensin IV whereas two successive angiotensin IV-dependent [Ca2+](i) peaks were obtained. Angiotensin II increased [Ca2+](i) in a Ca2+-free medium whereas angiotensin IV was inactive. Leucine-valine-valine-hemorphin 7, a hemorphin specific for the angiotensin AT(4) receptor, was devoid of any agonistic or antagonistic effect. In contrast, angiotensin-(1-7), if without influence on basal [Ca2+](i), inhibited angiotensin II- and angiotensin IV-dependent [Ca2+](i) increases. Total inhibition of the angiotensin IV effect was obtained whereas association of angiotensin-(1-7) to 8-(NN-diethylamino)-octyl-3,4,5-trimethoxybenzoate, an inhibitor of inositol phosphate-mediated Ca2+ release, was necessary to suppress the effect of angiotensin II. These results provide evidence that angiotensin II metabolites may participate in the control of [Ca2+](i) in mesangial cells at the initial stage of binding to the angiotensin AT(1) receptors.

Characterization and signaling of the AT(4) receptor in human proximal tubule epithelial (HK-2) cells

(125)I-divalinal-angiotensin IV (metabolically resistant analog of angiotensin IV) was used as a receptor ligand to identify the expression and properties of the angiotensin AT(4) receptor in epithelial HK-2 cells (an immortalized cell line derived from adult human proximal tubules). Saturation binding isotherms revealed that HK-2 cells contain a saturable (125)I-divalinal-angiotensin IV binding site with an affinity of 3 nmol/L and a density of 508 fmol/mg protein. An analysis of ligand specificity showed that only angiotensin AT(4) receptor ligands (angiotensin IV and divalinal-angiotensin IV) competed with both a high- and low-affinity binding site. GTPgammaS and dithiothreitol did not affect (125)I-Ang IV or (125)I-divalinal-Ang IV binding, suggesting that the AT(4) receptor was not G-protein coupled and did not require sulfhydryl bonds for receptor affinity. Activation of the AT(4) receptor caused a complex concentration-dependent rise in [Ca(2+)](i), an elevation in [Na(+)](i), and increased mitogen-activated protein kinase activity. These results suggest that human proximal tubule epithelial cells contain functional AT(4) receptors that are pharmacologically similar to the AT(4) receptor described in more distal segments of the nephron. Furthermore, the AT(4) receptor uses several intracellular signaling pathways to convey information.

Metabolism alters the selectivity of angiotensin-(1-7) receptor ligands for angiotensin receptors

The present study examined whether metabolism of the putative angiotensin-(1-7) receptor agonist and antagonist [angiotensin-(1-7) and D-alanine(7) angiotensin-(1-7), respectively] altered their ability to interact with angiotensin AT(1), AT(2), and AT(4) receptor subtypes. Both angiotensin-(1-7) and D-alanine(7) angiotensin-(1-7) competed with low affinity for (125)I-sarcosine(1), isoleucine(8) angiotensin II binding to AT(1) and AT(2) receptors in rat liver and adrenal medulla membranes, respectively, and competed with low affinity for (125)I-angiotensin IV binding to AT(4) receptors in bovine kidney epithelial cell membranes. In vitro renal metabolism of the angiotensin-(1-7) receptor ligands (incubating peptides with rat cortical tissue homogenates) had minimal influence on low-affinity binding to AT(1) and AT(2) receptors, yet caused a significant and dramatic shift toward high-affinity binding for AT(4) receptors. Low-affinity angiotensin II binding to the AT(4) receptor was also shifted toward high-affinity binding following renal metabolism of the peptide. Conversely, angiotensins with high affinity for the AT(4) receptor (e.g., angiotensin IV) were shifted toward low-affinity binding states following peptide metabolism. Incubation of (125)I-angiotensin-(1-7) with rat cortical tissue generated the high-affinity AT(4) receptor ligand (125)I-angiotensin-(3-7), whereas the renal metabolism of (125)I-angiotensin II generated both (125)I-angiotensin-(3-7) and (125)I-angiotensin IV. These results reveal that renal metabolism of angiotensin-(1-7) receptor ligands and angiotensin II yields products that have high affinity for the AT(4) receptor and could potentially contribute to the biologic actions of the parent peptide in the kidney.