Biochemical properties of the angiotensin-converting-like enzyme from the leech Theromyzon tessulatum

Assay of angiotensin I-converting enzyme-inhibiting activity based on the detection of 3-hydroxybutyrate with water-soluble tetrazolium salt

A newly synthesized substrate, 3-hydroxybutyrylglycyl-glycyl-glycine (3HB-GGG), was applied to the assay of ACE-inhibiting activity to overcome the smaller selectivity and sensitivity of the conventional method. In this study, an ACE-inhibiting assay was improved by the use of a water-soluble tetrazolium salt, 4-[3-(4-iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3-benzene disulfonate sodium salt (WST-1), for the detection of 3-hydroxybutyrate, derived from 3HB-GGG. The optimized conditions were as follows: 0.333 mM NAD(+), 0.333 mM WST-1, 0.1 mM EDTA, 0.633 U ml(-1) diaphorase, and 0.700 U ml(-1) 3-hydroxybutyrate dehydrogenase. The developed assay was efficiently applicable to evaluate the ACE-inhibiting activity of practical ACE inhibitors.

Angiotensin-converting enzyme-like activity in crab gills and its putative role in degradation of crustacean hyperglycemic hormone

Angiotensin-converting enzyme-like enzyme activity (ACELA) was found in Carcinus maenas using reverse phase high performance liquid chromatography (RP-HPLC) analysis of degradation kinetics of a synthetic substrate (Hippuryl-histidyl-leucine) and a specific inhibitor (captopril). Gills contained the highest ACELA, then brain, muscle, and testis, respectively, while no activity was detected in the following tissues: hepatopancreas, hindgut, hypodermis, heart, and hemolymph. ACELA present in gill membranes exhibited a K(m) of 0.23 mM and V(max) of 7.6 nmol with synthetic substrate. The enzyme activity was dependent on Cl- concentration and was markedly inhibited by captopril, lisinopril, and EDTA. Addition of Zn2+ to membranes previously treated with EDTA restored 89% activity, suggesting that C. maenas ACELA is a Zn2+ metalloenzyme. Gill membranes prepared from premolt crabs showed similar levels of ACELA to those of the intermolt animals. Administration of captopril in vivo lengthened the half life of circulating CHH, while in vitro incubation of gill membranes with captopril reduced CHH. These results suggest that C. maenas ACELA present in gills is likely to be involved in degradation of this neuropeptide.

Characterization of the first non-insect invertebrate functional angiotensin-converting enzyme (ACE): leech TtACE resembles the N-domain of mammalian ACE

Angiotensin-converting enzyme (ACE) is a zinc metallopeptidase that plays a major role in blood homoeostasis and reproduction in mammals. In vertebrates, both transmembrane and soluble ACE, containing one or two homologous active sites, have been characterized. So far, several ACEs from invertebrates have been cloned, but only in insects. They are soluble and display a single active site. Using biochemical procedures, an ACE-like activity was detected in our model, the leech, Theromyzon tessulatum. Annelida is the most distant phylum in which an ACE activity has been observed. To gain more insight into the leech enzyme, we have developed a PCR approach to characterize its mRNA. The approx. 2 kb cDNA has been predicted to encode a 616-amino-acid soluble enzyme containing a single active site, named TtACE (T. tessulatum ACE). Surprisingly, its primary sequence shows greater similarity to vertebrates than to invertebrates. Stable in vitro expression of TtACE in transfected Chinese-hamster ovary cells revealed that the leech enzyme is a functional metalloprotease. As in mammals, this 79 kDa glycosylated enzyme functions as a dipeptidyl carboxypeptidase capable of hydrolysing angiotensin I to angiotensin II. However, a weak chloride inhibitory effect and acetylated N-acetyl-SDKP (Ac SDAcKP) hydrolysis reveal that TtACE activity resembles that of the N-domain of mammalian ACE. In situ hybridization shows that its cellular distribution is restricted to epithelial midgut cells. Although the precise roles and endogenous substrates of TtACE remain to be identified, characterization of this ancestral peptidase will help to clarify its physiological roles in non-insect invertebrate species.

Structure, evolutionary conservation, and functions of angiotensin- and endothelin-converting enzymes

Angiotensin-converting enzyme, a member of the M2 metalloprotease family, and endothelin-converting enzyme, a member of the M13 family, are key components in the regulation of blood pressure and electrolyte balance in mammals. From this point of view, they serve as important drug targets. Recently, the involvement of these enzymes in the development of Alzheimer's disease was discovered. The existence of homologs of these enzymes in invertebrates indicates that these enzyme systems are highly conserved during evolution. Most invertebrates lack a closed circulatory system, which excludes the need for blood pressure regulators. Therefore, these organisms represent excellent targets for gaining new insights and revealing additional physiological roles of these important enzymes. This chapter reviews the structural and functional aspects of ACE and ECE and will particularly focus on these enzyme homologues in invertebrates.

Angiotensin-converting enzyme inhibition studies by natural leech inhibitors by capillary electrophoresis and competition assay

A protocol to follow the processing of angiotensin I into angiotensin II by rabbit angiotensin-converting enzyme (ACE) and its inhibition by a novel natural antagonist, the leech osmoregulator factor (LORF) using capillary zonal electrophoresis is described. The experiment was carried out using the Beckman PACE system and steps were taken to determine (a) the migration profiles of angiotensin and its yielded peptides, (b) the minimal amount of angiotensin II detected, (c) the use of different electrolytes and (d) the concentration of inhibitor. We demonstrated that LORF (IPEPYVWD), a neuropeptide previously found in leech brain, is able to inhibit rabbit ACE with an IC(50) of 19.8 micro m. Interestingly, its cleavage product, IPEP exhibits an IC(50) of 11.5 micro m. A competition assay using p-benzoylglycylglycylglycine and insect ACE established that LORF and IPEP fragments are natural inhibitors for invertebrate ACE. Fifty-four percent of insect ACE activity is inhibited with 50 micro m IPEP and 35% inhibition with LORF (25 mm). Extending the peptide at both N- and C-terminus (GWEIPEPYVWDES) and the cleavage of IPEP in IP abolished the inhibitory activity of both peptides. Immunocytochemical data obtained with antisera raised against LORF and leech ACE showed a colocalization between the enzyme and its inhibitor in the same neurons. These results showed that capillary zonal electrophoresis is a useful technique for following enzymatic processes with small amounts of products and constitutes the first evidence of a natural ACE inhibitor in invertebrates.

Identification of angiotensin I in several vertebrate species: its structural and functional evolution

In order to delineate further the molecular evolution of the renin-angiotensin system in vertebrates, angiotensin I (ANG I) has been isolated after incubation of plasma and kidney extracts of emu (Dromiceus novaehollandiae), axolotl (Ambystoma mexicanum), and sea lamprey (Petromyzon marinus). The identified sequences were [Asp1, Val5, Asn9] ANG I in emu, [Asp1, Val5, His9] ANG I in axolotl, and [Asn1, Val5, Thr9] ANG I in sea lamprey. These results confirmed the previous findings that tetrapods have Asp and fishes including cyclostomes have Asn at the N-terminus, and that the amino acid residue at position 9 of ANG I was highly variable but, those at other positions were well conserved among different species. Since Asp and Asn are convertible during incubation, angiotensinogen sequences were searched in the genome and/or EST database to determine the N-terminal amino acid residue from the gene. The screening detected 12 tetrapod (10 mammalian, one avian, and one amphibian) and seven teleostean angiotensinogen sequences. Among them, all tetrapods have [Asp1] ANG except for Xenopus, and all teleosts have [Asn1] ANG, thereby confirming the above rule. Comparison of the vasopressor activity in the eel revealed that [Asn1] ANG I and II were more potent than [Asp1] peptides, which was opposite to the previous results in mammals and birds, in which [Asp1] ANG I and II were more potent. Collectively, the present results support the general rule that tetrapods have [Asp1] ANG and fishes including cyclostomes have [Asn1] ANG. However, an aquatic anuran (Xenopus) has [Asn1] ANG in its gene despite another aquatic urodele (axolotl) has [Asp1] ANG. From the functional viewpoint, homologous [Asn1] ANG was more potent in fish as is homologous [Asp1] ANG in tetrapods, suggesting that ANG II molecule has undergone co-evolution with its receptor during vertebrate phylogeny.

Elements of angiotensin system are involved in leeches and mollusks immune response modulation

We present immunocytochemical, biochemical and cellular evidences for the presence of a renin-angiotensin system (RAS) in coelomocytes of invertebrates (leech, Theromyzon tessulatum and mollusk Mytilus edulis). Leech coelomocytes are immunoreactive to polyclonal antisera raised against the T. tessulatum angiotensin-converting enzyme (ACE) and leech brain angiotensin II (AII) and a commercial anti-AT1 receptor. Biochemically, renin, ACE- and AT1-like receptor were identified in the leech immune cells. We further demonstrate that leech AII (10(-6) M) alone does not initiate nitric oxide (NO) release in invertebrate immunocytes but does only after pre-exposing the cells to IL-1 (15.9+/-2.6 nM; P<0.005 vs. 1.1 nM when AII is added alone). Similar results were obtained with human leukocytes (14.5+/-2.7 nM; P<0.005 IL-1+AII vs. 0.9 nM when AII is added alone). Then, an immunocytochemical study performed at the structural and ultrastructural levels confirmed the presence in same immune cells all the molecules of the renin-angiotensin system (RAS) in leeches as epitopes to IL-1-like protein and IL-1-like receptor. This is the first report in invertebrates and of a co-action between cytokines like substances and neuropeptides in an immune process and the involvement of the RAS in modulation of the immune response.

The angiotensin system elements in invertebrates

In this review, the different components of the renin-angiotensin system (RAS) in invertebrates are discussed. This system is implicated in osmoregulation, reproduction, memory processes and immune system regulation. As the elements of this hormone-enzymatic system also exist in invertebrates, it appears that the RAS originated very early in evolution.

The neuroendocrine system of annelids

In vertebrates the neuroendocrine system is based on chemical signaling between neural and endocrine structures. Final outcomes may be realized via chemical messengers traveling through circulatory conduits to their specific target sites. This process may rely, in part, on neurosecretion of the signaling molecules. The complexity of this system can be readily visualized when one considers the way in which interactions among classical neurotransmitters, cytokines, growth factors, and neuroendocrine hormones, in combination with autocrine and paracrine communication, can regulate cells and tissues. Apart from the neuroendocrine system there is also neuroimmune communication, consisting of reciprocal signaling between neuroendocrine and immune cells, which use the same molecules to coordinate their activity. Thus, our concept of the neuroendocrine system is constantly growing, despite its complexity, but it may be simply summarized as allowing bidirectional communication between neural and endocrine structures over distances greater than that achieved by synaptic communication. In the light of this, I demonstrate in this review that annelids, which are considered "simple" animals, also possess a neuroendocrine system.

PLGamide characterization and role in osmoregulation in leech brain

Neurons immunoreactive to an antiserum specifically directed against the Prolyl-Leucyl-Glycinamide peptide (PLGamide: Melanocyte Inhibiting Factor: MIF) were detected in the brain of the leech Theromyzon tessulatum. Radioimmunoassay titrations of the PLGamide-like material at different physiological stages of the life cycle indicated a maximal amount at stage 3B, which is correlated to phase of both maximal water uptake and coelomic vitellogenin accumulation. In vivo experiments demonstrate that this cerebral PLGamide-like material is an anti-diuretic factor that would act at stage 3B in order to permit a water uptake leading to water retention allowing coelomic yolk protein accumulation. In brains of the Gnatobdellid leech Hirudo medicinalis and the Pharyngobdellid leech Erpobdella octoculata, anti-PLGamide material was also detected with an amount not differing with the degree of sex maturation of the animals, confirming the link between osmoregulation and ovogenesis in rhynchobdellid leeches. Using a combination of biochemical techniques including high-pressure gel permeation chromatography followed by reversed-phase HPLC on brain extracts and Edman degradation, we demonstrated the presence of an authentic MIF-1 peptide in leech brain. Finally, since in vertebrates MIF-1 belongs to the non-classical opioid peptide family, we studied its binding displacement, in contrast to morphine, on mu-receptors and on nitric oxide (NO) release experiments in leech brain. PLGamide did not bind to mu-alkaloid opioid receptors and did not stimulate NO release.

Invertebrate opioid precursors: evolutionary conservation and the significance of enzymatic processing

Invertebrate tissues contain mammalian-like proenkephalin, prodynorphin, and proopiomelanocortin. Amino acid sequence determination of these opioid gene products reveals the presence of various opioid peptides exhibiting high sequence identity with their mammalian counterparts. These associated peptides are flanked by dibasic amino acid residues, indicating cleavage sites. Together with the presence of various processing enzymes, i.e., neutral endopeptidase 24.11 and angiotensin-converting enzymes, this suggests that opioid precursor processing is also similar to that described in mammals. It is noted that the levels and/or activity of invertebrate neutral endopeptidase 24.11 can be upregulated by signaling molecules shown to perform the same function in mammals, i.e., morphine. Critical to opioid precursor processing are immunocytes that contain the precursors and transport processing enzymes to sites of inflammation, in part, to cleave these peptide precursors, thus liberating immune-stimulating molecules. Furthermore, in response to lipopolysaccharides, Met-enkephalin levels peak immediately and hours after the exposure, revealing a release and induction process. It appears that the opioid precursors and their processing enzymes first evolved in "simple" animals and the have been maintained and embellished during the course of evolution guided by conformational matching.

Mytilus edulis hemolymph contains pro-opiomelanocortin: LPS and morphine stimulate differential processing

Mytilus edulis hemolymph contains mammalian-like proopiomelanocortin (POMC). The 20 kDa protein was purified by high pressure gel permeation chromatography, anti-adrenocorticotropin (ACTH)-affinity column and reverse-phase HPLC. The amino acid sequence determination was by Edman degradation, enzymatic treatments and Western blot analysis. Of the six peptides found in this opioid precursor, methionine-enkephalin, gamma-melanocyte stimulating hormone (MSH), alpha-MSH and ACTH exhibited 100, 80, 85 and 74% sequence identity, respectively, with the mammalian counterparts. beta-Endorphin and gamma-LPH exhibited only 25 and 10% sequence identity. Dibasic amino acid residues were found at the C-terminus of MSH and ACTH, indicating cleavage sites. The alpha-MSH is flanked at the C-terminus by Gly-Lys-Lys, representing an amidation signal. ACTH and CLIP (80% sequence identity) are also C-terminally flanked by dibasic amino acid residues. Furthermore, morphine, in a dose-dependent manner, increased the hemolymph levels of alpha-MSH and ACTH (1-39) in a naloxone and phosphoramidon antagonizable manner, indicating a neutral endopeptidase (24.11; NEP) mediated cleavage. Lipopolysaccharide (10 microg/animal) stimulated the processing of ACTH (1-39) yielding ACTH (1-24) in a cleavage that is independent of NEP, but dependent on aspartyl proteases, demonstrating differential enzymatic cleavage of ACTH (1-39). Taken together, POMC is present in invertebrates and its processing can be altered depending on the signal.

Immunocytochemical distribution of angiotensin I-converting enzyme-like immunoreactivity in the brain and testis of insects

Angiotensin converting enzyme (ACE) is Zn2+ metallopeptidase which plays an important role in blood pressure homeostasis in mammals and other vertebrates. Homologues of ACE involved in the biosynthesis of mammalian peptide hormones have also been identified in the insects, Musca domestica, Drosophila melanogaster and Haematobia irritans exigua. In the pursuit of the biological role of insect ACE, this work focused on the tissue and cellular distribution of ACE in several insect species. The localisation of ACE in the central nervous system and reproductive tissues from a number of insect species suggests that ACE is of physiological importance in these tissues. By means of an antiserum to housefly ACE, we found that ACE-like immunoreactivity was abundantly present in the neuropil areas of the brain of all insects investigated, suggesting a role for ACE in the metabolic inactivation of peptide neurotransmitters. Especially in the fleshfly, Neobellieria bullata neuropile staining is abundant. In the cockroach Leucophaea maderae, immunoreactive staining was abundant in the neuronal perikarya as well as in the neuropilar regions. Staining in neurosecretory cells was also observed in the brains of the lepidopteran species, Bombyx mori and Mamestra brassica. The localisation of ACE in neurosecretory cells is consistent with the role as a processing hormone, involved in the generation of active peptide hormones. ACE was found to be co-localised with peptides of the FXPRLamide family in M. brassica and in B. mori, suggesting a role for the biosynthesis of these hormones. Finally, we found ACE-like immunoreactivity in the testis of Locusta migratoria, N. bullata and Leptinotarsa decemlineata, providing additional evidence for its important role in insect reproduction.

Proline-specific dipeptidyl peptidase from the blue blowfly Calliphora vicina hydrolyzes in vitro the ecdysiostatic peptide trypsin-modulating oostatic factor (Neb-TMOF)

To elucidate the mechanisms of inactivation of the ecdysiostatic peptide trypsin-modulating oostatic factor (Neb-TMOF) in the blue blowfly Calliphora vicina, we investigated its proteolytic degradation. In homogenates and membrane and soluble fractions, this hexapeptide (sequence: NPTNLH) was hydrolyzed into two fragments, NP and TNLH, suggesting the involvement of a proline-specific dipeptidyl peptidase. The dipeptidyl peptidase activity was highest in the late larval stage. It was purified 240-fold from soluble fractions of pupae of mixed age and classified on the basis of several catalytic properties as an invertebrate homologue of mammalian dipeptidyl peptidase IV (EC 3.4.14.5). Fly dipeptidyl peptidase IV has a molecular mass of 200 kDa, showed a pH optimum of 7.5-8.0 with the chromogenic substrate Gly-Pro-4-nitroanilide, and cleaved other chromogenic substrates with penultimate Pro or, with lower activity, Ala. It liberated Xaa-Pro dipeptides from the N-terminus of several bioactive peptides including substance P, neuropeptide Y, and peptide YY but not from bradykinin, indicating that the peptide bond between the two proline residues was resistant to cleavage. Fly dipeptidyl peptidase belongs to the serine class of proteases as the mammalian enzyme does; the fly enzyme, however, is not inhibited by several selective or nonselective inhibitors of its mammalian counterpart. It is suggested that dipeptidyl peptidases exert a regulatory role for the clearance not only of TMOF in files but for other bioactive peptides in various invertebrates.

Biochemical identification and ganglionic localization of leech angiotensin-converting enzymes

We demonstrate the presence of a membrane and soluble form of leech Theromyzon tessulatum angiotensin-converting enzyme (ACE). Four steps in the purification of this enzyme include gel-permeation, captopril-sepharose affinity and anion-exchange chromatography followed by a reverse-phase HPLC. The peptidyl dipeptidases (of approximately 120 and approximately 100 kDa) are glycosylated enzymes hydrolysing the Phe8-His9 bond of angiotensin I, exhibiting the same specific activity and Km whereas the soluble ACE exhibits a higher catalytic efficiency. This hydrolysis is inhibited by the ACE-specific antagonist captopril. Western blot analysis of a polyclonal antiserum raised against the first 11 amino-acid residues of the membrane ACE and the N-terminal sequence of the soluble molecule also demonstrates the presence of two ACE enzymes. Anti-ACE immunocytochemistry also supports the presence of two forms of ACE. This material is found in neurons and glia. We demonstrate for the first time the cellular localization and biochemical characterization of ACEs in the central nervous system of an invertebrate. Thus, the leech brain may represent a simple model for the study of these enzymes.

A renin-like enzyme in the leech Theromyzon tessulatum

We report on the biochemical isolation and characterization of a 32 kDa aspartyl protease from the leech Theromyzon tessulatum. Following a three step purification (gel permeation chromatography, pepstatin A-sepharose affinity column separation followed by reversed-phase HPLC) a renin-like enzyme was purified to homogeneity. The first 124 amino acid residues of the N-terminal part of the purified S-pyridylethylated leech renin exhibits a 26.5-35.5% sequence identity with that of mammals. The 20-81 region of leech renin exhibits a 80% sequence homology with the 175-232 region in mammals. This highly conserved region, which is also found in all aspartic proteases, possesses the aspartyl catalytic residue (D11TGSS). Leech renin hydrolyses at neutral pH and at 37 degrees C the Leu10-Leu11 bond of synthetic porcine angiotensinogen tetradecapeptide yielding the angiotensin I and the Leu11-Val12-Tyr13-Ser14 peptides, with a specific activity of 115 microg AI/min/mg (K[M] 22 microM; K[cat], 2.7). This hydrolysis is inhibited by pepstatin A (IC50: 4.6 microM). Moreover, this enzyme is found on a multiple hormone precursor of 19 kDa which exhibits a specific activity of 850 pmol AI/min/mg of renin. This is the first biochemical characterization of a renin-like enzyme in invertebrates and non-mammalian vertebrates.

Metabolism of enkephalins in head membranes of the leech Theromyzon tessulatum by peptidases: isolation of an enkephalin-degrading aminopeptidase

Metabolism of leucine and methionine enkephalins by enzyme preparations from head parts of the leech Theromyzon tessulatum was investigated. Leech homogenate degraded enkephalins by cleavage of the Tyr1-Gly2 and Gly3-Phe4 bonds. The Tyr1-Gly2-Gly3 was detected as a major metabolite when amastatin (aminopeptidase inhibitor) was present to prevent Tyr1-Gly2 breakdown. Around 50% of enkephalin-degrading activity was isolated in a 20000 x g membrane fraction and was shown to be almost entirely due to an aminopeptidase activity. This enzyme, a homodimer of approx. 70 kDa, has been purified to homogeneity by a combined approach including gel permeation and anion exchange chromatographies followed by reversed-phase HPLC. This enkephalin-degrading aminopeptidase is a typical integral membrane 'zincin' metalloprotein with an apparent k(m) of 30 microM, a specific activity of 12 nmol GGFM min-1 mg protein-1 and a catalytic efficiency (kcat/k(m)) of 46 x 10(6) mol-1 min-1. This enzyme is specifically inhibited by amastatin (IC50 = 0.5 microM), but not by bestatin and actinonin. In leech membranes, the other degrading activities performed at the same time were due to a neuropeptide-endopeptidase (NEP)-like enzyme attack, inhibited by phosphoramidon (IC50 = 0.1 microM) and in the case of the Met-enkephalin by a combined action of an angiotensin-converting-like enzyme, inhibited by captopril (IC50 = 0.2 microM) and the NEP-like enzyme. These two enzymes were previously isolated from head membranes of T. tessulatum and possess towards Met-enkephalin a catalytic efficiency (kcat/k(m)) of, respectively, 12 x 10(6) mol-1 min-1 and 78 x 10(6) mol-1 min-1. These findings constitute the first report in leeches on the nature and the sites of attack of the membrane peptidases involved in the metabolism of enkephalins and also the first biochemical evidence for a novel member of the aminopeptidase family.