Angiotensin as a model for hormone--receptor interactions

Novel benzimidazole angiotensin receptor blockers with anti-SARS-CoV-2 activity equipotent to that of nirmatrelvir: computational and enzymatic studies

BACKGROUND

Hypertension worsens outcomes in SARS-CoV-2 patients. Sartans, a type of antihypertensive angiotensin receptor blocker-(ARB), reduce COVID-19 morbidity and mortality by targeting angiotensin-converting enzyme-2 (ACE2). This study aimed to evaluate the antiviral and antihypertensive effects of nirmatrelvir, commercial sartans (candesartan, losartan, and losartan carboxylic (Exp3174)), and newly synthesized sartans (benzimidazole-N-biphenyl carboxyl (ACC519C) and benzimidazole-N-biphenyl tetrazole (ACC519T)), compared to nirmatrelvir, the antiviral component of Paxlovid.

RESEARCH DESIGN AND METHODS

Surface plasmon resonance (SPR) and enzymatic studies assessed drug effects on ACE2. Antiviral abilities were tested with SARS-CoV-2-infected Vero E6 cells, and antihypertensive effects were evaluated using angiotensin II-contracted rabbit iliac arteries.

RESULTS

Benzimidazole-based candesartan and ACC519C showed antiviral activity comparable to nirmatrelvir (95% inhibition). Imidazole-based losartan, Exp3174, and ACC519T were less potent (75%-80% and 50%, respectively), with Exp3174 being the least effective. SPR analysis indicated high sartans-ACE2 binding affinity. Candesartan and nirmatrelvir combined had greater inhibitory and cytopathic effects (3.96%) than individually (6.10% and 5.08%). ACE2 enzymatic assays showed varying effects of novel sartans on ACE2. ACC519T significantly reduced angiotensin II-mediated contraction, unlike nirmatrelvir and ACC519T(2).

CONCLUSION

This study reports the discovery of a new class of benzimidazole-based sartans that significantly inhibit SARS-CoV-2, likely due to their interaction with ACE2.

Network-Based Prediction of Side Effects of Repurposed Antihypertensive Sartans against COVID-19 via Proteome and Drug-Target Interactomes

The potential of targeting the Renin-Angiotensin-Aldosterone System (RAAS) as a treatment for the coronavirus disease 2019 (COVID-19) is currently under investigation. One way to combat this disease involves the repurposing of angiotensin receptor blockers (ARBs), which are antihypertensive drugs, because they bind to angiotensin-converting enzyme 2 (ACE2), which in turn interacts with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike protein. However, there has been no in silico analysis of the potential toxicity risks associated with the use of these drugs for the treatment of COVID-19. To address this, a network-based bioinformatics methodology was used to investigate the potential side effects of known Food and Drug Administration (FDA)-approved antihypertensive drugs, Sartans. This involved identifying the human proteins targeted by these drugs, their first neighbors, and any drugs that bind to them using publicly available experimentally supported data, and subsequently constructing proteomes and protein-drug interactomes. This methodology was also applied to Pfizer's Paxlovid, an antiviral drug approved by the FDA for emergency use in mild-to-moderate COVID-19 treatment. The study compares the results for both drug categories and examines the potential for off-target effects, undesirable involvement in various biological processes and diseases, possible drug interactions, and the potential reduction in drug efficiency resulting from proteoform identification.

Computational and Enzymatic Studies of Sartans in SARS-CoV-2 Spike RBD-ACE2 Binding: The Role of Tetrazole and Perspectives as Antihypertensive and COVID-19 Therapeutics

This study is an extension of current research into a novel class of synthetic antihypertensive drugs referred to as "bisartans", which are bis-alkylated imidazole derivatives bearing two symmetric anionic biphenyltetrazoles. Research to date indicates that bisartans are superior to commercially available hypertension drugs, since the former undergo stronger docking to angiotensin-converting enzyme 2 (ACE2). ACE2 is the key receptor involved in SARS-CoV-2 entry, thus initiating COVID-19 infection and in regulating levels of vasoactive peptides such as angiotensin II and beneficial heptapeptides A(1-7) and Alamandine in the renin-angiotensin system (RAS). In previous studies using in vivo rabbit-iliac arterial models, we showed that Na⁺ or K⁺ salts of selected Bisartans initiate a potent dose-response inhibition of vasoconstriction. Furthermore, computational studies revealed that bisartans undergo stable binding to the vital interfacial region between ACE2 and the SARS-CoV-2 "receptor binding domain" (i.e., the viral RBD). Thus, bisartan homologs are expected to interfere with SARS-CoV-2 infection and/or suppress disease expression in humans. The primary goal of this study was to investigate the role of tetrazole in binding and the network of amino acids of SARS-CoV-2 Spike RBD-ACE2 complex involved in interactions with sartans. This study would, furthermore, allow the expansion of the synthetic space to create a diverse suite of new bisartans in conjunction with detailed computational and in vitro antiviral studies. A critical role for tetrazole was uncovered in this study, shedding light on the vital importance of this group in the binding of sartans and bisartans to the ACE2/Spike complex. The in silico data predicting an interaction of tetrazole-containing sartans with ACE2 were experimentally validated by the results of surface plasmon resonance (SPR) analyses performed with a recombinant human ACE2 protein.

Molecular Epidemiology of SARS-CoV-2: The Dominant Role of Arginine in Mutations and Infectivity

Background, Aims, Methods, Results, Conclusions: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a global challenge due to its ability to mutate into variants that spread more rapidly than the wild-type virus. The molecular biology of this virus has been extensively studied and computational methods applied are an example paradigm for novel antiviral drug therapies. The rapid evolution of SARS-CoV-2 in the human population is driven, in part, by mutations in the receptor-binding domain (RBD) of the spike (S-) protein, some of which enable tighter binding to angiotensin-converting enzyme (ACE2). More stable RBD-ACE2 association is coupled with accelerated hydrolysis by proteases, such as furin, trypsin, and the Transmembrane Serine Protease 2 (TMPRSS2) that augment infection rates, while inhibition of the 3-chymotrypsin-like protease (3CL^pro) can prevent the viral replication. Additionally, non-RBD and non-interfacial mutations may assist the S-protein in adopting thermodynamically favorable conformations for stronger binding. This study aimed to report variant distribution of SARS-CoV-2 across European Union (EU)/European Economic Area (EEA) countries and relate mutations with the driving forces that trigger infections. Variants' distribution data for SARS-CoV-2 across EU/EEA countries were mined from the European Centre for Disease Prevention and Control (ECDC) based on the sequence or genotyping data that are deposited in the Global Science Initiative for providing genomic data (GISAID) and The European Surveillance System (TESSy) databases. Docking studies performed with AutoDock VINA revealed stabilizing interactions of putative antiviral drugs, e.g., selected anionic imidazole biphenyl tetrazoles, with the ACE2 receptor in the RBD-ACE2 complex. The driving forces of key mutations for Alpha, Beta, Gamma, Delta, Epsilon, Kappa, Lambda, and Omicron variants, which stabilize the RBD-ACE2 complex, were investigated by computational approaches. Arginine is the critical amino acid in the polybasic furin cleavage sites S1/S2 (681-PRRARS-686) S2' (814-KRS-816). Critical mutations into arginine residues that were found in the delta variant (L452R, P681R) and may be responsible for the increased transmissibility and morbidity are also present in two widely spreading omicron variants, named BA.4.6 and BQ.1, where mutation R346T in the S-protein potentially contributes to neutralization escape. Arginine binders, such as Angiotensin Receptor Blockers (ARBs), could be a class of novel drugs for treating COVID-19.

Actions of Novel Angiotensin Receptor Blocking Drugs, Bisartans, Relevant for COVID-19 Therapy: Biased Agonism at Angiotensin Receptors and the Beneficial Effects of Neprilysin in the Renin Angiotensin System

Angiotensin receptor blockers (ARBs) used in the treatment of hypertension and potentially in SARS-CoV-2 infection exhibit inverse agonist effects at angiotensin AR1 receptors, suggesting the receptor may have evolved to accommodate naturally occurring angiotensin ‘antipeptides’. Screening of the human genome has identified a peptide (EGVYVHPV) encoded by mRNA, complementary to that encoding ANG II itself, which is an inverse agonist. Thus, opposite strands of DNA encode peptides with opposite effects at AR1 receptors. Agonism and inverse agonism at AR1 receptors can be explained by a receptor ‘switching’ between an activated state invoking receptor dimerization/G protein coupling and an inverse agonist state mediated by an alternative/second messenger that is slow to reverse. Both receptor states appear to be driven by the formation of the ANG II charge-relay system involving TyrOH-His/imidazole-Carboxylate (analogous to serine proteases). In this system, tyrosinate species formed are essential for activating AT1 and AT2 receptors. ANGII is also known to bind to the zinc-coordinated metalloprotease angiotensin converting enzyme 2 (ACE2) used by the COVID-19 virus to enter cells. Here we report in silico results demonstrating the binding of a new class of anionic biphenyl-tetrazole sartans (‘Bisartans’) to the active site zinc atom of the endopeptidase Neprilysin (NEP) involved in regulating hypertension, by modulating humoral levels of beneficial vasoactive peptides in the RAS such as vasodilator angiotensin (1–7). In vivo and modeling evidence further suggest Bisartans can inhibit ANG II-induced pulmonary edema and may be useful in combatting SARS-CoV-2 infection by inhibiting ACE2-mediated viral entry to cells.

Diminazene Aceturate Reduces Angiotensin II Constriction and Interacts with the Spike Protein of Severe Acute Respiratory Syndrome Coronavirus 2

Diminazene aceturate (DIZE) is a putative angiotensin-converting enzyme 2 (ACE2) activator and angiotensin type 1 receptor antagonist (AT₁R). Its simple chemical structure possesses a negatively charged triazene segment that is homologous to the tetrazole of angiotensin receptor blockers (ARB), which explains its AT₁R antagonistic activity. Additionally, the activation of ACE2 by DIZE converts the toxic octapeptide angiotensin II (AngII) to the heptapeptides angiotensin 1–7 and alamandine, which promote vasodilation and maintains homeostatic balance. Due to DIZE’s protective cardiovascular and pulmonary effects and its ability to target ACE2 (the predominant receptor utilized by severe acute respiratory syndrome coronavirus 2 to enter host cells), it is a promising treatment for coronavirus 2019 (COVID-19). To determine DIZE’s ability to inhibit AngII constriction, in vitro isometric tension analysis was conducted on rabbit iliac arteries incubated with DIZE or candesartan and constricted with cumulative doses of AngII. In silico docking and ligand interaction studies were performed to investigate potential interactions between DIZE and other ARBs with AT₁R and the spike protein/ACE2 complex. DIZE, similar to the other ARBs investigated, was able to abolish vasoconstriction in response to AngII and exhibited a binding affinity for the spike protein/ACE2 complex (PDB 6LZ6). These results support the potential of DIZE as a treatment for COVID-19.

Understanding the Driving Forces That Trigger Mutations in SARS-CoV-2: Mutational Energetics and the Role of Arginine Blockers in COVID-19 Therapy

SARS-CoV-2 is a global challenge due to its ability to mutate into variants that spread more rapidly than the wild-type virus. Because the molecular biology of this virus has been studied in such great detail, it represents an archetypal paradigm for research into new antiviral drug therapies. The rapid evolution of SARS-CoV-2 in the human population is driven, in part, by mutations in the receptor-binding domain (RBD) of the spike (S-) protein, some of which enable tighter binding to angiotensin-converting enzyme (ACE2). More stable RBD-ACE2 association is coupled with accelerated hydrolysis of furin and 3CLpro cleavage sites that augment infection. Non-RBD and non-interfacial mutations assist the S-protein in adopting thermodynamically favorable conformations for stronger binding. The driving forces of key mutations for Alpha, Beta, Gamma, Delta, Kappa, Lambda and Omicron variants, which stabilize the RBD-ACE2 complex, are investigated by free-energy computational approaches, as well as equilibrium and steered molecular dynamic simulations. Considered also are the structural hydropathy traits of the residues in the interface between SARS-CoV-2 RBD and ACE2 protein. Salt bridges and π-π interactions are critical forces that create stronger complexes between the RBD and ACE2. The trend of mutations is the replacement of non-polar hydrophobic interactions with polar hydrophilic interactions, which enhance binding of RBD with ACE2. However, this is not always the case, as conformational landscapes also contribute to a stronger binding. Arginine, the most polar and hydrophilic among the natural amino acids, is the most aggressive mutant amino acid for stronger binding. Arginine blockers, such as traditional sartans that bear anionic tetrazoles and carboxylates, may be ideal candidate drugs for retarding viral infection by weakening S-protein RBD binding to ACE2 and discouraging hydrolysis of cleavage sites. Based on our computational results it is suggested that a new generation of “supersartans”, called “bisartans”, bearing two anionic biphenyl-tetrazole pharmacophores, are superior to carboxylates in terms of their interactions with viral targets, suggesting their potential as drugs in the treatment of COVID-19. In Brief: This in silico study reviews our understanding of molecular driving forces that trigger mutations in the SARS-CoV-2 virus. It also reports further studies on a new class of “supersartans” referred to herein as “bisartans”, bearing two anionic biphenyltetrazole moieties that show potential in models for blocking critical amino acids of mutants, such as arginine, in the Delta variant. Bisartans may also act at other targets essential for viral infection and replication (i.e., ACE2, furin cleavage site and 3CLpro), rendering them potential new drugs for additional experimentation and translation to human clinical trials.

Discovery of a new generation of angiotensin receptor blocking drugs: receptor mechanisms and in silico binding to enzymes relevant to covid-19

The discovery and facile synthesis of a new class of sartan-like arterial antihypertensive drugs (angiotensin receptor blockers [ARBs]), subsequently referred to as “bisartans” is reported. In vivo results and complementary molecular modelling presented in this communication indicate bisartans may be beneficial for the treatment of not only heart disease, diabetes, renal dysfunction, and related illnesses, but possibly COVID-19. Bisartans are novel bis-alkylated imidazole sartan derivatives bearing dual symmetric anionic biphenyl tetrazole moieties. In silico docking and molecular dynamics studies revealed bisartans exhibited higher binding affinities for the ACE2/spike protein complex (PDB 6LZG) compared to all other known sartans. They also underwent stable docking to the Zn²⁺ domain of the ACE2 catalytic site as well as the critical interfacial region between ACE2 and the SARS-CoV-2 receptor binding domain. Additionally, semi-stable docking of bisartans at the arginine-rich furin-cleavage site of the SARS-CoV-2 spike protein (residues 681–686) required for virus entry into host cells, suggest bisartans may inhibit furin action thereby retarding viral entry into host cells. Bisartan tetrazole groups surpass nitrile, the pharmacophoric “warhead” of PF-07321332, in its ability to disrupt the cysteine charge relay system of 3CLpro. However, despite the apparent targeting of multifunctional sites, bisartans do not inhibit SARS-CoV-2 infection in bioassays as effectively as PF-07321332 (Paxlovid).

Receptor Interactions of Angiotensin II and Angiotensin Receptor Blockers-Relevance to COVID-19

Angiotensin II (Ang II) may contain a charge relay system (CRS) involving Tyr/His/carboxylate, which creates a tyrosinate anion for receptor activation. Energy calculations were carried out to determine the preferred geometry for the CRS in the presence and absence of the Arg guanidino group occupying position 2 of Ang II. These findings suggest that Tyr is preferred over His for bearing the negative charge and that the CRS is stabilized by the guanidino group. Recent crystallography studies provided details of the binding of nonpeptide angiotensin receptor blockers (ARBs) to the Ang II type 1 (AT1) receptor, and these insights were applied to Ang II. A model of binding and receptor activation that explains the surmountable and insurmountable effects of Ang II analogues sarmesin and sarilesin, respectively, was developed and enabled the discovery of a new generation of ARBs called bisartans. Finally, we determined the ability of the bisartan BV6(TFA) to act as a potential ARB, demonstrating similar effects to candesartan, by reducing vasoconstriction of rabbit iliac arteries in response to cumulative doses of Ang II. Recent clinical studies have shown that Ang II receptor blockers have protective effects in hypertensive patients infected with SARS-CoV-2. Therefore, the usage of ARBS to block the AT1 receptor preventing the binding of toxic angiotensin implicated in the storm of cytokines in SARS-CoV-2 is a target treatment and opens new avenues for disease therapy.

From Angiotensin II to Cyclic Peptides and Angiotensin Receptor Blockers (ARBs): Perspectives of ARBs in COVID-19 Therapy

The octapeptide hormone angiotensin II is one of the most studied peptides with the aim of designing and synthesizing non-peptide mimetics for oral administration. To achieve this, cyclizations at different positions within the peptide molecule has been a useful strategy to define the active conformation. These studies on angiotensin II led to the discovery of Sarmesin, a type II angiotensin II antagonist, and the breakthrough non-peptide mimetic Losartan, the first in a series of sartans marketed as a new generation of anti-hypertensive drugs in the 1990s. Angiotensin II receptor blockers (ARBS) and angiotensin I converting enzyme inhibitors (ACEI) were recently reported to protect hypertensive patients infected with SARS-CoV-2. The renin–angiotensin system (RAS) inhibitors reduce excess angiotensin II and increase antagonist heptapeptides alamandine and aspamandine which counterbalance angiotensin II and maintain homeostasis and vasodilation.

A Global Review on Short Peptides: Frontiers and Perspectives

Peptides are fragments of proteins that carry out biological functions. They act as signaling entities via all domains of life and interfere with protein-protein interactions, which are indispensable in bio-processes. Short peptides include fundamental molecular information for a prelude to the symphony of life. They have aroused considerable interest due to their unique features and great promise in innovative bio-therapies. This work focusing on the current state-of-the-art short peptide-based therapeutical developments is the first global review written by researchers from all continents, as a celebration of 100 years of peptide therapeutics since the commencement of insulin therapy in the 1920s. Peptide "drugs" initially played only the role of hormone analogs to balance disorders. Nowadays, they achieve numerous biomedical tasks, can cross membranes, or reach intracellular targets. The role of peptides in bio-processes can hardly be mimicked by other chemical substances. The article is divided into independent sections, which are related to either the progress in short peptide-based theranostics or the problems posing challenge to bio-medicine. In particular, the SWOT analysis of short peptides, their relevance in therapies of diverse diseases, improvements in (bio)synthesis platforms, advanced nano-supramolecular technologies, aptamers, altered peptide ligands and in silico methodologies to overcome peptide limitations, modern smart bio-functional materials, vaccines, and drug/gene-targeted delivery systems are discussed.

Insights into AT1 receptor activation through AngII binding studies

This study investigates the binding of angiotensin II (AngII) to the angiotensin II type 1 receptor (AT1R), taking into consideration several known activation elements that have been observed for G-protein-coupled receptors (GPCRs). In order to determine the crucial interactions of AngII upon binding, several MD simulations were implemented using AngII conformations derived from experimental data (NMR ROEs) and in silico flexible docking methodologies. An additional goal was to simulate the induced activation mechanism and examine the already known structural rearrangements of GPCRs upon activation. Performing MD simulations to the AT1R - AngII - lipids complex, a series of dynamic changes in the topology of AngII and the intracellular part of the receptor were observed. Overall, the present study proposes a complete binding profile of AngII to the AT1R, as well as the key transitional elements of the receptor and the agonist peptide upon activation through NMR and in silico studies.

A Review of the Structural and Functional Features of Olmesartan Medoxomil, An Angiotensin Receptor Blocker

The angiotensin II (A-II) type 1 (AT1) receptor-mediated effects of A-II play a key role in the pathophysiology of hypertension. Effective inhibition of A-II is provided by the latest class of antihypertensive medications, the AT1 receptor blockers (ARBs). These orally available agents were developed around a common imidazole-based structural core. The most recent member of this drug class to be approved by the Food and Drug Administration, olmesartan medoxomil, contains unique features that may explain its clinical efficacy. Key structural elements of olmesartan medoxomil include a hydroxyalkyl substituent at the imidazole 4-position and a hydrolyzable ester group at the imidazole 5-position. Inter- and intramolecular hydrogen bonding involving these groups may contribute to the potentiation of antagonist activity. After oral administration, olmesartan medoxomil is deesterified in the intestinal tract to produce the active metabolite olmesartan, which undergoes no additional metabolic change. The marked antihypertensive efficacy of olmesartan medoxomil may result from a unique pharmacological interaction of the drug with the AT1 receptor, resulting in a potent, long-lasting, dose-dependent blockade of A-II. This review article characterizes the structural features of olmesartan that may be responsible for its clinical efficacy. Inferential pharmacological studies compare and contrast the effects of olmesartan to those of other ARBs in comparable preclinical animal models.

Comparison of the solution structures of angiotensin I & II. Implication for structure-function relationship

Conformational analysis of angiotensin I (AI) and II (AII) peptides has been performed through 2D 1H-NMR spectroscopy in dimethylsulfoxide and 2,2,2-trifluoroethanol/H2O. The solution structural models of AI and AII have been determined in dimethylsulfoxide using NOE distance and 3JHNHalpha coupling constants. Finally, the AI family of models resulting from restrained energy minimization (REM) refinement, exhibits pairwise rmsd values for the family ensemble 0.26 +/- 0.13 A, 1.05 +/- 0.23 A, for backbone and heavy atoms, respectively, and the distance penalty function is calculated at 0.075 +/- 0.006 A2. Comparable results have been afforded for AII ensemble (rmsd values 0.30 +/- 0.22 A, 1.38 +/- 0.48 A for backbone and heavy atoms, respectively; distance penalty function is 0.029 +/- 0.003 A2). The two peptides demonstrate similar N-terminal and different C-terminal conformation as a consequence of the presence/absence of the His9-Leu10 dipeptide, which plays an important role in the different biological function of the two peptides. Other conformational variations focused on the side-chain orientation of aromatic residues, which constitute a biologically relevant hydrophobic core and whose inter-residue contacts are strong in dimethylsulfoxide and are retained even in mixed organic-aqueous media. Detailed analysis of the peptide structural features attempts to elucidate the conformational role of the C-terminal dipeptide to the different binding affinity of AI and AII towards the AT1 receptor and sets the basis for understanding the factors that might govern free- or bound-depended AII structural differentiation.

Receptor interactions of the position 4 side chains of angiotensin II analogues: importance of aromatic ring quadrupole

A triad of interacting groups (TyrOH-His-O2C) in angiotensin II (ANG II) has been postulated to create the tyrosinate anion pharmacophore (tyanophore) responsible for receptor activation/triggering (Biochim. Biophys. Acta 1991, 1065, 21). In the present study we investigated the effects on bioactivity of substituting the Tyr4 residue in [Sar1]ANG II with other anionic or electronegative amino acids, and with a number of aromatic amino acids lacking a hydroxyl group. [Sar1 Nva(delta-OH)4]ANG II, [Sar1 Nva(delta-OCH3)4]ANG II, [Sar1 Met4]ANG II, [Sar1 Gln4]ANG II, [Sar1 Glu4]ANG II and [Sar1 DL-Alg]ANG II had agonist activities in the rat isolated uterus assay of 4, 3, 19, 10, < 0.1 and < 0.1%, respectively, of that of ANG II. [Sar1 Nal4]ANG II, [Sar1 Pal4]ANG II, [Sar1 DL-Phg(4'-F)4]ANG II, [Sar1 Phe(4'-F)4]ANG II, [Sar1 Phe(F5)4]ANG II and [Sar1 His4]ANG II had agonist activities of 4.5, 7, < 0.1, 0.2, 1 and 0.6%, respectively. All peptides investigated were devoid of measurable antagonist activity except [Sar1 Phe(4'-F)4ANG II (pA2 = 7.7). These findings illustrate that anionic or electronegative aliphatic side chains replacing tyrosinate at position 4 can partially activate the angiotensin receptor. For ANG II analogues containing an aromatic amino acid other than Tyr at position 4, ligand binding and agonist activity are not dependent on the electronegativity or dipole moment of the aromatic ring, or on the ability of the 4' ring substituent to accept a proton.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of methylation of the histidine ring of [Sar1]angiotensin II on conformation and biological activity

[Sar1His(1-Me)6]ANG II and [Sar1His(3-Me)6]ANG II were synthesized by the solid-phase method and purified by reversed-phase HPLC. 1H-NMR spectroscopy at 400 MHz demonstrated the presence of two major conformers for both peptides in DMSO, representing cis and trans isomers (ratio 1:3 and 1:4, respectively) due to restricted rotation at the His-Pro bond. The contractile activities of these peptides in the rat isolated uterus assay were < 0.1 and 27% of that of ANG II, respectively. The bioactivities of these analogues were mirrored in their NMR spectra: the inactive analogue [Sar1His(1-Me)6]ANG II showed perturbations of the Sar and His residues which were not present for [Sar1His(3-Me)6]ANG II and the reference agonist [Sar1]ANG II. The high activity of the analogue methylated at His N3 suggests that an ionizable imidazole proton is not an absolute requirement for expression of biological activity by angiotensin ligands, and that the imidazole group in the molecule may function in an ion dipole-based mechanism when an intramolecular proton transfer (charge relay) mechanism is not available. The biological activity of the ligand appears to depend on the degree of proton transfer from Tyr-OH to the imidazole acceptor, wherein complete (formal) proton transfer represents 100% activity.

Specific, high-affinity binding sites for angiotensin II on Mycoplasma hyorhinis

Mycoplasmataceae are known to express various proteins that are similar to those present in mammals. We report a strain of Mycoplasma hyorhinis isolated from opossum kidney cells with specific, high-affinity binding sites for human angiotensin II (Kd = 5.1 +/- 1.9 nM). In contrast, two strains of M. hominis revealed no specific binding. These binding sites resembled mammalian angiotensin II receptors by their high affinity and by their sensitivity to dithiothreitol. However, they are different from mammalian angiotensin II receptors in that they bind angiotensin I with high affinity (Kd = 1.6 +/- 0.29 nM) but not angiotensin III (Kd approximately 330,000 nM). [125I]-angiotensin II binding was not inhibited by angiotensin receptor subtype antagonists DuP 753 and CGP 42112A but it was sensitive to bacitracin and aprotinin. Positions Asp1, Ile5, His6 and Pro7 were essential for binding to M. hyorhinis as deletion of these residues led to a more than 10,000-fold decrease in affinity.

Influence of polyfluorination of the phenylalanine ring of angiotensin II on conformation and biological activity

[Phe(F5)8]angiotensin II was synthesized by the solid phase method and purified by reverse-phase HPLC. In rat uterus and rabbit aorta bioassays the analogue had 10 and 50%, respectively, of the contractile activity of angiotensin II and demonstrated antagonist properties. These findings illustrate that inversion of the Phe8 ring quadrupole moment in angiotensin II decreases agonist activity and invokes antagonist properties. 1H-NMR studies at 400 MHz in DMSO-d6 demonstrated the presence of cis and trans isomers in the ratio 1:3 due to restricted rotation of the His-Pro bond. Downfield shifts of the His C2 and C4 protons in [Phe(F5)]ANG II compared to ANG II suggest that the Phe(F5) residue may be involved in a parallel-plate ring pairing interaction with the imidazole group. However heteronuclear NOE studies, carried out by measuring the proton difference spectrum before and after saturation of the fluorine resonances, showed the absence of any NOE enhancement illustrating that electrostatic influences of the Phe(F5) ring occur at relatively long range.

Importance of the N-terminal domain of the type I angiotension II antagonist [Sar1,Ile8]ANG II for receptor blockade

Analogues of the Type I angiotensin (ANG) antagonist, [Sar1,Ile8]ANG II, in which the N-terminal dipeptide was modified were synthesized by the solid phase method and purified by reversed-phase HPLC. Antagonist potencies (pA2) of the peptides were determined on the rat isolated uterus using ANG II as the agonist. Substitution of the Arg residue occupying position 2 of [Sar1,Ile8]ANG II (pA2 8.1) by Gly, Ala, Nle, Phe, Pro or Sar reduced the antagonist potency to pA2 = 7.0, 6.8, 6.7, 6.8, 5.8 and 5.3, respectively. Deletion of the N-terminal Sar residue in these same peptides gave pA2 = 6.8, 5.7, 5.5, 5.9, 6.1 and 7.5, respectively. The characteristically long duration of action of [Sar1,Ile8] was absent for all of these analogues including (des1, Sar2, Ile8]ANG II. These findings demonstrate that the antagonist potencies of Type I angiotensin antagonists for smooth muscle receptors, and also the long duration of action, are dependent on the location of positive charges within the peptide and on the conformation of the molecule in determining favorable electrostatic interactions with the receptor. A model is proposed in which the two positively charged loci on the angiotensin molecule (N-terminus and Arg) interact with two corresponding anionic binding sites on the smooth muscle receptor. The possibility that the prolonged duration of action of [Sar1, Ile8]ANG II results from binding to a different site on the angiotensin receptor from that occupied by ANG II is discussed in relation to the present findings.

Proton NMR studies of angiotensin II and its analogs in aqueous solution

The 1H nuclear magnetic resonance (NMR) spectra of angiotensin II (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe) and five of its octapeptide analogs as well as angiotensin I (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu) and angiotensin III (Arg-Val-Tyr-Ile-His-Pro-Phe) in aqueous solutions (90% H2O/10% D2O) were completely assigned by two-dimensional COSY and ROESY experiments. All of the peptides give rise to two distinct sets of signals. The minor set accounts for about 5% of the total population below pH 5.5 and increases to 12-20% around pH 7.0. The two sets of signals result from a cis-trans isomerization of the His-Pro peptide bond with the major resonances arising from the trans isomer. One analog in which the Pro is replaced with a D-Pro displays a very different isomerization behavior. The measured coupling constants JNH-alpha CH, the temperature dependence of the amide proton shifts and the relative intensities of the intraresidue and sequential NH-alpha CH ROEs, are all indicative of an extended backbone conformation for ANGII. However, some evidence for the existence of conformers with local structure involving preferred sidechain positions for the Tyr, His, Phe, and the carboxyl group of the Phe was found, particularly in the ROESY and pH-titration experiments. Moreover, pH effects and the unusual amide exchange behavior of the Arg epsilon NH suggests the presence of interactions between the Asp and Arg sidechains of ANGII. At low temperatures the Arg guanidinium NH2 protons were detected as two broad peaks which are related by sizeable exchange peaks in ROESY experiments. This behavior could be useful as a general probe for the study of Arg sidechain mobility and accessibility in other peptides and proteins.

Fluorescence properties of angiotensin II analogues in receptor-simulating environments: relationship between tyrosinate fluorescence lifetime and biological activity

Nanosecond time-resolved decays from excited-state tyrosinate fluorescence of angiotensin II analogues were measured from the emission at 350 nm. Fluorescence lifetimes were determined in several different solvents using N-acetyltyrosinamide as the reference standard. Long-lifetime tyrosinate fluorescence (LTF) of angiotensin II (ANG II) was observed in propylene glycol, trifluoroethanol and isopropanol but not in DMSO or water. The addition of SDS at a concentration sufficient to induce micelle formation in water resulted in LTF for ANG II. LTF for ANG II was longer in propylene glycol (21 ns) than in isopropanol (16 ns) whereas the % conformer(s) producing LTF was higher in isopropanol (79%) than in propylene glycol (19%). For a series of ten angiotensin analogues, LTF values determined in propylene glycol and isopropanol were a reflection of the contractile activities of these analogues in the rat uterus assay. In propylene glycol, with the notable exception of ANG III, biologically active analogues had longer LTFs (13-21 ns) than inactive analogues (0-11 ns). In isopropanol, strong agonists had longer LTFs (13-16 ns) than weak agonists/inactive analogues (0-11 ns). Structure-fluorescence relationships suggest that the primary TyrOH acceptor in ANG II is the His6 imidazole group, and that the C-terminal carboxylate has an essential auxiliary role in generating long-lived tyrosinate fluorescence. The present findings appear to support the proposition that the receptor conformation of ANG II contains a tripartite interaction of Tyr, His and carboxylate groups which is analogous to that found at the active site of serine proteinases, and that the tyrosinate nucleophile may activate angiotensin receptors. Solvents of intermediate polarity such as propylene glycol and isopropanol appear to induce conformations for small peptides such as angiotensin which resemble those present at membrane receptors.

Synthesis and biological activities of angiotensin II and Sarmesin analogues containing cyclohexylalanine

Analogues of angiotensin II with cyclohexylalanine (Cha) at position 4 or 8, and analogues of the competitive (type II) angiotensin antagonist [Sar1,Tyr(Me)4]ANG II (Sarmesin) with Cha at position 8, have been prepared by the solid phase method and purified by reversed-phase HPLC. Analogues of ANG II with Cha at position 8 in which the position 1 residue was substituted with sarcosine (Sar) or amino-isobutyric acid (Aib) or was deleted (Des), were slowly reversing (Type I) antagonists with "pA2" values in the rat isolated uterus assay of approximately 8.5. The additional substitution of Tyr(Me) for Tyr at position 4 of these peptides gave reversible competitive (Type I/II) antagonists with pA2 values of 6.7, 5.8, and less than 5, while substitution of Phe for Tyr gave pA2 values of 7.4, 6.7, and less than 5, respectively. All 19 peptides synthesized in this study had low intrinsic agonist activity in the rat isolated uterus assay except for the type I antagonists [Sar1, Cha8]ANG II (7%), [Aib1, Cha8]ANG II (12%) and [Des1, Cha8]ANG II (20%). These data illustrate that the substitution of Cha at position 8 of ANG II analogues produces potent antagonists; however, Type I antagonists retain significant agonist activity whereas Type I/II antagonists do not. In contrast, substitution of Cha at position 4 in a variety of ANG II analogues resulted in severely diminished biological activity, illustrating that the presence of an aromatic ring quadrupole at position 4 is obligatory for receptor binding and activity.

1H-NMR studies of [Sar1]angiotensin II conformation by nuclear Overhauser effect spectroscopy in the rotating frame (ROESY): clustering of the aromatic rings in dimethylsulfoxide

The conformational properties of the octapeptide [Sar1]ANG II in dimethylsulfoxide-d6 were investigated by rotating frame nuclear Overhauser effect spectroscopy (ROESY). Interresidue ROESY interactions were observed between Tyr ortho and Phe ring protons, between Phe ring and Pro C gamma protons, and also between His C alpha and Pro C delta protons. A weak connectivity was also observed between the Sar N-CH3 protons and a Tyr ortho proton. Intraresidue interactions between alpha and beta protons in Tyr, His and Phe indicated restricted rotation for the side-chains of the three aromatic residues. These findings suggest that [Sar1]ANG II takes up a folded conformation in DMSO in which the three aromatic rings form a cluster. Connectivities between the His C alpha proton and the two Pro C delta protons illustrated a preferred conformation for angiotensin II in DMSO in which the His-Pro bond exists as the trans isomer. The NMR spectroscopic evidence is consistent with the presence of a Tyr charge relay system in the biologically active conformation of angiotensin II and with the postulated role of the Tyr hydroxyl group in angiotensin II for receptor activation.

1H-NMR studies of sarmesin and [des1]sarmesin conformation in dimethylsulfoxide by nuclear Overhauser effect (NOE) enhancement spectroscopy: folding of the N- and C-terminal domains

The conformational properties of the competitive angiotensin II antagonist sarmesin [Sar-Arg-Val-Tyr(Me)-His-Pro-Phe] and its heptapeptide analogue [des1]sarmesin in dimethylsulphoxide-d6 were investigated by nuclear Overhauser effect (NOE) enhancement studies. Assignment of all backbone and side-chain protons was possible by combining information from intraresidue NOE studies with two-dimensional correlated spectroscopy (COSY) studies. Saturation of the His C alpha proton of sarmesin produced essentially the same interresidue NOE enhancement of the two Pro C delta protons, illustrating the presence of the trans His-Pro bond. Saturation of the Sar N-methyl group caused enhancement of one of the His C beta protons, suggesting the presence of a turn in the N-terminal region of the molecule. Saturation of His C2 in sarmesin and [des1]sarmesin enhanced the Tyr(Me) methyl signal. Saturation of the Tyr(Me) methyl protons in [des1]sarmesin produced NOE enhancement of the His C2 and C4 protons, and saturation of the His C2 proton enhanced the Tyr(Me) meta and ortho proton signals. Interresidue interactions between the Tyr(Me) and His protons in sarmesin and [des1]sarmesin illustrate that these two side-chains remain in close proximity even in the absence of the postulated hydrogen bond between Tyr hydroxyl and the His imidazole ring in angiotensin II. The data suggest a preferred conformation for sarmesin in DMSO in which the peptide backbone is S-shaped and similar to that for angiotensin II.

Calculation of the magnitude and orientation of electrostatic interactions between small aromatic rings in peptides and proteins: implications for angiotensin II

Electrostatic interactions between aromatic rings are believed to contribute to the intramolecular structuring and biological function of peptides and proteins. The modes of interaction of benzene with 1) benzene (Phe:Phe), 2) imidazole (Phe:His), and 3) imidazole anion (Phe:His), were calculated using all-electron ab initio wavefunctions. In all cases parallel-plate stackings were found to be purely repulsive, whereas perpendicular-plate geometries were attractive with interaction energies of -0.82 (Phe:Phe), -1.19 (Phe:His) and -3.39 (Phe:His) kcal/mole. These data show that small ring interactions in peptides and proteins will prefer perpendicular-plate geometry. For the proposed His:Phe interaction in angiotensin II, the attraction will be three times greater when the imidazole ring carries a negative charge.