Identification and characterization of a neutral endopeptidase activity in Aplysia californica

Expression of NEP2, a soluble neprilysin-like endopeptidase, during embryogenesis in Drosophila melanogaster

Members of the neprilysin family of neutral endopeptidases (M13) are typically membrane-bound enzymes known to be involved in the extra-cellular metabolism of signalling peptides and have important roles during mammalian embryogenesis. In this study we show that membranes prepared from embryos of Drosophila melanogaster possess neprilysin-like activity that is inhibited by phosphoramidon and thiorphan, both inhibitors of mammalian neprilysin. Unexpectedly, we also found strong neprilysin-like neutral endopeptidase activity in a soluble embryo fraction, which we identify as NEP2 by Western blot and immunoprecipitation experiments using NEP2 specific antibodies. NEP2 is a soluble secreted member of the neprilysin family that has been shown previously to be expressed in larval and adult Malpighian tubules and in the testes of adult males. In situ hybridization studies reveal expression at stage 10-11 in a pattern similar to that previously described for stellate cell progenitors of the caudal visceral mesoderm. In later stages of embryogenesis, some of these cells appear to migrate into the growing Malpighian tubule. Recombinant NEP2 protein is N-glycosylated and displays optimum endopeptidase activity at neutral pH, consistent with a role as an extracellular peptidase. The recombinant enzyme hydrolyses Drosophila tachykinin peptides (DTK) at peptide bonds N-terminal to hydrophobic residues. DTK2, like Locusta tachykinin-1, was cleaved at the penultimate peptide bond (Gly(7)-Leu(8)), whereas the other Drosophila peptides were cleaved centrally at Xxx-Phe bonds. However, the rates of hydrolysis of the latter substrates were much slower than the hydrolysis rates of DTK2 and Locusta tachykinin-1, suggesting that the interaction of the bulky side-chain of phenylalanine at the S'(1) sub-site is less favorable for peptide bond hydrolysis. The secretion of NEP2 from tissues during embryogenesis suggests a possible developmental role for this endopeptidase in peptide signalling in D. melanogaster.

Drosophila melanogaster NEP2 is a new soluble member of the neprilysin family of endopeptidases with implications for reproduction and renal function

The mammalian neprilysin (NEP) family members are typically type II membrane endopeptidases responsible for the activation/inactivation of neuropeptides and peptide hormones. Differences in substrate specificity and subcellular localization of the seven mammalian NEPs contribute to their functional diversity. The sequencing of the Drosophila melanogaster genome has revealed a large expansion of this gene family, resulting in over 20 fly NEP-like genes, suggesting even greater diversity in structure and function than seen in mammals. We now report that one of these genes (Nep2) codes for a secreted endopeptidase with a highly restricted pattern of expression. D. melanogaster NEP2 is expressed in the specialized stellate cells of the renal tubules and in the cyst cells that surround the elongating spermatid bundles in adult testis, suggesting roles for the peptidase in renal function and in spermatogenesis. D. melanogaster NEP2 was found in vesicle-like structures in the syncytial cytoplasm of the spermatid bundles, suggesting that the protein was acquired by endocytosis of protein secreted from the cyst cells. Expression of NEP2 cDNA in D. melanogaster S2 cells confirmed that the peptidase is secreted and is only weakly inhibited by thiorphan, a potent inhibitor of human NEP. D. melanogaster NEP2 also differs from human NEP in the manner in which the peptidase cleaves the tachykinin, GPSGFYGVR-amide. Molecular modelling suggests that there are important structural differences between D. melanogaster NEP2 and human NEP in the S1' and S2' ligand-binding subsites, which might explain the observed differences in inhibitor and substrate specificities. A soluble isoform of a mouse NEP-like peptidase is strongly expressed in spermatids, suggesting an evolutionarily conserved role for a soluble endopeptidase in spermatogenesis.

Zinc-metalloproteases in insects: ACE and ECE

Research on the angiotensin-converting enzyme (ACE) in insects has substantially advanced during the recent decade. The cloning of this enzyme in many insect species, the determination of the 3D-structure and several molecular and physiological studies have contributed to the characterization of insect ACE as we know it today: a functional enzyme with a putative role in reproduction, development and defense. The discovery of the endothelin-converting enzyme in insects occurred more recently and cloning of the corresponding cDNA has been carried out in only one insect species so far. However, activity studies and analysis of insect genomes indicate that this enzyme is also widely distributed among insect species. Making hypotheses about its putative function would be preliminary, but its wide tissue distribution suggests a major and diverse biological role.

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.

Deamidase inactivates a D-amino acid-containing Aplysia neuropeptide

Membrane-catalyzed degradation of the cardioexcitatory peptide, Asn-D-Trp-Phe-NH(2) (N(d)WF-NH(2)), which was previously isolated from Aplysia, was investigated in relation to its inactivation mechanism. The principal degradation was deamidation of the C-terminal amide, producing biologically inert Asn-D-Trp-Phe-OH (N(d)WF-OH). Among membrane fractions prepared from different organs, the fraction from the ganglia showed the highest specific activity of the deamidation reaction. The deamidase activity was inhibited by Ebelactone B and the serine protease inhibitor, phenylmethanesulfonyl fluoride (PMSF), while the degradation of the synthetic stereoisomer, Asn-Trp-Phe-NH(2) (N(l)WF-NH(2)), was sensitive to the divalent cation-chelating agent, o-phenanthroline, and aminopeptidase inhibitors, amastatin and bestatin. The presence of D-Trp residue in the second position of N(d)WF-NH(2) endows this peptide not only with stereospecific bioactivity but also peptidase stability. The deamidation reaction seems to be the major inactivation mechanism for this peptide.

Soluble recombinant neprilysin induces aggrecanase-mediated cleavage of aggrecan in cartilage explant cultures

Neprilysin (neutral endopeptidase, enkephalinase, CALLA, CD10, NEP) is a regulatory Zn metallopeptidase expressed in the brush border membranes of the kidney and has been found in porcine chondrocytes and rat articular cartilage as well as other cell types and tissues. Although its function in cartilage is not currently known, previous observations of high levels of NEP enzymatic activity in the synovial fluid of arthritic patients and on the chondrocyte membranes of human osteoarthritic cartilage have led to the hypothesis that NEP is involved in the inflammation or degradation pathways in articular cartilage. Our study localized endogenous NEP to the membranes of mature bovine articular chondrocytes in a tissue explant model and demonstrated that the addition of soluble recombinant NEP (sNEP) to the culture medium of bovine cartilage explants leads to the degradation of aggrecan through the action of aggrecanase. A 6-day exposure to sNEP was necessary to initiate the degradation, suggesting that the chondrocytes were responding in a delayed manner to an altered composition of regulatory peptides. This NEP-induced degradation was completely inhibited by the NEP inhibitors thiorphan and phosphoramidon. These results suggest that NEP is present as a transmembrane enzyme on articular chondrocytes where it can cleave regulatory peptides and lead to the induction of aggrecanase.

Vasopeptidase inhibition attenuates the progression of renal injury in subtotal nephrectomized rats

BACKGROUND

Vasopeptidase inhibitors are a new class of cardiovascular compounds that inhibit both angiotensin-converting enzyme (ACE) and neutral endopeptidase (NEP). The aim of the present study was to explore the effects of omapatrilat, a vasopeptidase inhibitor, on renal function and pathology in subtotally nephrectomized (STNx) rats.

METHODS

STNx rats were randomized to four groups and treated for 12 weeks: no treatment (N = 14); omapatrilat at a low dose of 10 mg/kg (L, N = 12) and at a high dose of 40 mg/kg (H, N = 10); or an ACE inhibitor, fosinopril, at a dose of 10 mg/kg (N = 12). Sham-operated rats were used as control animals (N = 12).

RESULTS

Elevated blood pressure in STNx rats (174 +/- 9 mm Hg) was reduced by omapatrilat in a dose-dependent manner (L, 121 +/- 3 mm Hg; H, 110 +/- 3 mm Hg) and by fosinopril (149 +/- 5 mm Hg). Proteinuria in STNx rats (246 +/- 73 mg/day) was reduced by treatment with fosinopril (88 +/- 21 mg/day) and was normalized by treatment with omapatrilat (L, 30 +/- 4 mg/day; H, 20 +/- 2 mg/day vs. control 25 +/- 1 mg/day). Decreased glomerular filtration rates, elevated plasma urea and creatinine and glomerulosclerosis, and tubulointerstitial fibrosis were ameliorated by omapatrilat and fosinopril to a similar degree. Compared with fosinopril, omapatrilat treatment was associated with increased plasma renin activity and decreased renal ACE and NEP binding in a dose-dependent manner.

CONCLUSION

These findings suggest that vasopeptidase inhibition may provide a useful strategy for the treatment of progressive renal disease.

Molecular cloning and biochemical characterization of a new mouse testis soluble-zinc-metallopeptidase of the neprilysin family

Because of their roles in controlling the activity of several bio-active peptides, members of the neprilysin family of zinc metallopeptidases have been identified as putative targets for the design of therapeutic agents. Presently, six members have been reported, these are: neprilysin, endothelin-converting enzyme (ECE)-1 and ECE-2, the Kell blood group protein, PHEX (product of the phosphate-regulating gene with homologies to endopeptidase on the X chromosome) and X-converting enzyme (XCE). In order to identify new members of this important family of peptidases, we designed a reverse transcriptase-PCR strategy based on conserved amino acid sequences of neprilysin, ECE-1 and PHEX. We now report the cloning from mouse testis of a novel neprilysin-like peptidase that we called NL1. NL1 is a glycoprotein that, among the members of the family, shows the strongest sequence identity with neprilysin. However, in contrast with neprilysin and other members of the family which are type II integral membrane proteins, NL1 was secreted when expressed in cultured mammalian cells, likely due to cleavage by a subtilisin-like convertase at a furin-like site located 22 amino acid residues in the C-terminus of the transmembrane domain. The recombinant enzyme exhibited neprilysin-like peptidase activity and was efficiently inhibited by phosphoramidon and thiorphan, two inhibitors of neprilysin. Northern blot analysis and in situ hybridization showed that NL1 mRNA was found predominantly in testis, specifically in round and elongated spermatids. This distribution of NL1 mRNA suggests that it could be involved in sperm formation or other processes related to fertility.

Biological and immunological characterization of multiple GnRH in an opisthobranch mollusk, Aplysia californica

Gonadotropin-releasing hormone (GnRH) is a neurohormone central to the regulation of reproductive functions in vertebrates. Recently, several studies have reported the presence of GnRH immunoreactivity (IR) in a number of mollusks, suggesting that the distribution of GnRH may not be restricted to Phylum Chordata. In the present study, we extend our investigations to an opisthobranch mollusk, Aplysia californica, to characterize the source, chemical nature, and biological activity of molluscan GnRH-related molecules. Specific radioimmunoassays (RIAs) of various tissue extracts of Aplysia revealed that only ovotestis, hemocytes, and hemolymph contained significant amounts of GnRH that crossreacts with antisera raised against tunicate-I (tI) and mammalian (m) GnRH. Further RIAs and extractions revealed that the GnRH-IR in the hemolymph is biochemically and immunologically distinct from the GnRH-IR in the hemocytes and ovotestis. Using reverse-phase high-performance liquid chromatography coupled with RIAs, the GnRH-IR in the hemolymph was resolved into two major peaks. The first peak eluted earlier than most known forms of vertebrate GnRH, and the later peak coeluted with m, lamprey I, chicken II, and tI-GnRH. However, both peaks were broad and may contain a heterogeneous mixture of GnRH-IR. Immunocytochemical study showed that tI-GnRH-IR was present in the connective sheath surrounding the central nervous system, with a strong presence in what appeared to be vascular space, again suggesting the close association between Aplysia GnRH-IR and circulation. Finally, treatment of the neuroendocrine bag cells with chicken II GnRH significantly decreased the duration of the afterdischarge (AD, a characteristic pattern of electrical firing in bag cell neurons) and the number of action potentials fired during an AD, indicating the presence of a corresponding GnRH receptor in the Aplysia central nervous system. Overall, the results demonstrated the presence of multiple forms of GnRH-IR that crossreact with tI- and mGnRH antisera in A. californica and the ability of a vertebrate GnRH to alter Aplysia neural activity. Together, these data suggest that GnRH may be a factor released by the ovotestis and hemocytes into the circulation to alter neural functions. GnRH-IR produced by the latter may serve as a novel mediator of the neural and immune functions in Aplysia.

Cloning and characterization of Aplysia neutral endopeptidase, a metallo-endopeptidase involved in the extracellular metabolism of neuropeptides in Aplysia californica

Cell surface metallo-endopeptidases play important roles in cell communication by controlling the levels of bioactive peptides around peptide receptors. To understand the relative relevance of these enzymes in the CNS, we characterized a metallo-endopeptidase in the CNS of Aplysia californica, whose peptidergic pathways are well described at the molecular, cellular, and physiological levels. The membrane-bound activity cleaved Leu-enkephalin at the Gly3-Phe4 bond with an inhibitor profile similar to that of the mammalian neutral endopeptidase (NEP). This functional homology was supported by the molecular cloning of cDNAs from the CNS, which demonstrated that the Aplysia and mammalian NEPs share all the same amino acids that are essential for the enzymatic activity. The protein is recognized both by specific anti-Aplysia NEP (apNEP) antibodies and by the [125I]-labeled NEP-specific inhibitor RB104, demonstrating that the apNEP gene codes for the RB104-binding protein. In situ hybridization experiments on sections of the ganglia of the CNS revealed that apNEP is expressed in neurons and that the mRNA is present both in the cell bodies and in neurites that travel along the neuropil and peripheral nerves. When incubated in the presence of a specific NEP inhibitor, many neurons of the buccal ganglion showed a greatly prolonged physiological response to stimulation, suggesting that NEP-like metallo-endopeptidases may play a critical role in the regulation of the feeding behavior in Aplysia. One of the putative targets of apNEP in this behavior is the small cardioactive peptide, as suggested by RP-HPLC experiments. More generally, the presence of apNEP in the CNS and periphery may indicate that it could play a major role in the modulation of synaptic transmission in Aplysia and in the metabolism of neuropeptides close to their point of release.

Cloning and characterization of Aplysia neutral endopeptidase, a metallo-endopeptidase involved in the extracellular metabolism of neuropeptides in Aplysia californica

Cell surface metallo-endopeptidases play important roles in cell communication by controlling the levels of bioactive peptides around peptide receptors. To understand the relative relevance of these enzymes in the CNS, we characterized a metallo-endopeptidase in the CNS of Aplysia californica, whose peptidergic pathways are well described at the molecular, cellular, and physiological levels. The membrane-bound activity cleaved Leu-enkephalin at the Gly3-Phe4 bond with an inhibitor profile similar to that of the mammalian neutral endopeptidase (NEP). This functional homology was supported by the molecular cloning of cDNAs from the CNS, which demonstrated that the Aplysia and mammalian NEPs share all the same amino acids that are essential for the enzymatic activity. The protein is recognized both by specific anti-Aplysia NEP (apNEP) antibodies and by the [125I]-labeled NEP-specific inhibitor RB104, demonstrating that the apNEP gene codes for the RB104-binding protein. In situ hybridization experiments on sections of the ganglia of the CNS revealed that apNEP is expressed in neurons and that the mRNA is present both in the cell bodies and in neurites that travel along the neuropil and peripheral nerves. When incubated in the presence of a specific NEP inhibitor, many neurons of the buccal ganglion showed a greatly prolonged physiological response to stimulation, suggesting that NEP-like metallo-endopeptidases may play a critical role in the regulation of the feeding behavior in Aplysia. One of the putative targets of apNEP in this behavior is the small cardioactive peptide, as suggested by RP-HPLC experiments. More generally, the presence of apNEP in the CNS and periphery may indicate that it could play a major role in the modulation of synaptic transmission in Aplysia and in the metabolism of neuropeptides close to their point of release.

Purification and properties of a membrane aminopeptidase from Ascaris suum muscle that degrades neuropeptides AF1 and AF2

We have identified on the membranes of the locomotory muscle of Ascaris suum an amastatin-sensitive aminopeptidase that hydrolyses the bioactive neuropeptides AF1 (KNEFIRF-NH2) and AF2 (KHEYLRF-NH2), by cleavage of the Lys1-Asn2 and Lys1-His2 peptide bonds, respectively. AF2 (1.2 nmol of HEYLRF-NH2 formed min[-1] (mg protein[-1])) was hydrolysed at a faster rate compared to AF1 (0.2 nmol of NEFIRF-NH2 formed min[-1] (mg protein[-1])). AF1 hydrolysis by the aminopeptidase was inhibited by the amastatin (IC50, 9.0 microM), leuhistin (IC50, 1.25 microM) but was insensitive to puromycin, indicating a similarity to mammalian aminopeptidase N. The enzyme was also inhibited by arphamenine B (IC50, 9.0 microM), (2S, 3R)-3-amino-2-hydroxy-4-(4-nitrophenyl)butanoyl-L-leucine (IC50, 8.0 microM), bestatin (IC50, 15.0 microM) and 1 mM 1-10 bis-phenanthroline. The detergent Triton X-100 solubilised enzyme had a pI of 5.0 and after 1000-fold purification by ion-exchange chromatography, appeared to have a Mr of around 240,000 by SDS-PAGE. The purified aminopeptidase had a Km of 534 microM for the hydrolysis of AF1 and cleaved Phe1 from FMRF-NH2, but was unable to hydrolyse DFMRF-NH2 or FDMRF-NH2. The aminopeptidase that we have described in this report might have a role in the extracellular metabolism and inactivation of neuropeptides acting on the locomotory muscle of A. suum.

L5-67 and LUQ-1 peptide precursors of Aplysia californica: distribution and localization of immunoreactivity in the central nervous system and in peripheral tissues

Two genes (L5-67 and LUQ-1) that encode neuropeptide precursors have recently been shown to be expressed in a distinct and non-overlapping manner in the five left upper quadrant (LUQ) cells of the abdominal ganglion of Aplysia (Landry et al. [1992]. J. Neurobiol 23:89-101). By using wholemount immunohistochemistry and radioimmunoassay (RIA), the pattern of expression of these two genes was assessed at the protein level throughout the central nervous system (CNS) and in peripheral tissues of Aplysia californica. The distribution of LUQ-1 precursor-like immunoreactivity was fairly limited, occurring in the ventral LUQ cell (L5) and in a total of approximately 20 additional neurons in the abdominal and cerebral ganglia. L5-67 precursor-like immunoreactive material was more prevalent, appearing in a total of approximately 100 neurons distributed among each of the central ganglia. Identified L5-67-immunoreactive neurons included the four dorsal LUQ cells (L2-4 and L6) and two giant neurons (R2 and LPI1). In one group of cells, the H cluster of the cerebral ganglion, L5-67 immunofluorescence was substantially more intense in larger versus smaller animals, suggesting that this peptide precursor is subject to developmental regulation in certain neurons. Immunoelectron microscopic examination of the subcellular localization of L5-67 immunoreactivity in LUQ cell somata and axons revealed its association with dense-core vesicles (approximately 114 nm in diameter). In the periphery, L5-67-immunoreactive fibers were detected in specific regions of the circulatory system (auricle, ventricle, cristae aorta, anterior aorta) and the reproductive system (genital ganglion, large hermaphroditie duct, small hermaphroditie duct, ovotestis). The kidney and the intestine, two tissues in which considerable secretion and absorption occur, contained material immunoreactive to both L5-67 and LUQ-1 antisera. The localization of the two peptide precursors in these tissues differed substantially, with L5-67 occurring in widely ramifying varicose fibers, whereas LUQ-1 was found in restricted foci of fibers and in small spherical cells that appeared to lack processes. These results support previous findings concerning the heterogeneity of neurotransmitter phenotypes in the LUQ cells. Furthermore, they are indicative of a fairly broad role for the L5-67-derived neuropeptides, and a more limited role for the LUQ-1-derived neuropeptides, in the regulation of the visceral organ systems of Aplysia.

Molecular pharmacology of endothelin converting enzymes

A critical processing step in endothelin biosynthesis is the conversion of the intermediate "big endothelin" to its biologically active product catalysed by endothelin converting enzyme (ECE). In this commentary we discuss critically the cellular location, structure, and activity of the isoforms of ECE. The current evidence supporting a metallopeptidase ECE as the physiological regulator of endothelin production is described. Its sensitivity to inhibition by the fungal metabolite phosphoramidon and subsequent cloning of the enzyme indicate it to be a type II integral membrane protein homologous with neural endopeptidase-24.11 (E-24.11), the major neuropeptide-degrading ectoenzyme in brain and other tissues. Unlike E-24.11, however, ECE exists as a disulphide-linked dimer of subunit M(r) 120-130 kDa and is not inhibited by other E-24.11 inhibitors such as thiorphan. Alternative splicing produces two forms of ECE with distinct N-terminal tails. These isoforms of ECE-1 show similar specificity converting big endothelin-1 (ET-1) to ET-1 but big ET-2 and big ET-3 are converted much less efficiently. This suggests that additional forms of ECE remain to be isolated. Immunocytochemical studies indicate a predominant cell-surface location for ECE-1, like E-24.11. This is consistent with the conversion of exogenous big ET-1 when administered in vivo and the inhibition of this event by phosphoramidon. However, mature ET-1 can be detected in intracellular vesicles in endothelial cells, suggesting that some processing occurs in the constitutive secretory pathway. This may be mediated by ECE-2, a recently cloned member of the E-24.11/ECE family which has an acidic pH optimum. Selective inhibitors of ECE may have therapeutic applications in cardiovascular and renal medicine.

Metabolism of AF1 (KNEFIRF-NH2) in the nematode, Ascaris suum, by aminopeptidase, endopeptidase and deamidase enzymes

We have studied the metabolism and inactivation of AF1 (KNEFIRF-NH2) by membranes prepared from the locomotory muscle of Ascaris suum. FIRF-NH2 and KNEFIRF were identified as three primary degradation products, resulting from the action of an endopeptidase, aminopeptidase and a deamidase, respectively. The endopeptidase resembled mammalian neprilysin (NEP, endopeptidase 24.11) in that the enzyme activity was inhibited by phosphoramidon and thiorphan and that it cleaved AF1 on the amino side of phenylalanine. The aminopeptidase activity was inhibited by amastatin and bestatin but not by puromycin. The deamidation of AF1 was inhibited by phenylmethylsulfonyl fluoride, p-chloromercuricphenylsulfonate and mercuric chloride, indicating that the deamidase enzyme is a serine protease with a requirement for a free thiol group for activity. AF1 (1 microM) induces an increase in tension and an increase in the frequency and amplitude of spontaneous contractions of an A. suum muscle strip. None of the aforementioned AF1 metabolites (2-20 microM) retained biological activity in this bioassay, indicating that the endopeptidase, aminopeptidase and deamidase have the potential to terminate the action of AF1 on locomotory muscle of A. suum.

Identification and properties of a neuropeptide-degrading endopeptidase (neprilysin) of Ascaris suum muscle

We have previously identified in membranes of the locomotory muscle of Ascaris suum a phosphoramidon-sensitive endopeptidase which hydrolyses the neuropeptide AF1 (Lys-Asn-Glu-Phe-Ile-Arg-Phe-NH2) by cleavage of the Glu3-Phe4 bond (Sajid & Isaac, 1994). We have determined the properties of this neuropeptide-degrading enzyme of A. suum muscle using AKH-1 (pGlu-Leu-Asn-Phe-Thr-Pro-Asn-Trp-Gly-Thr-NH2) and [D-Ala2, Leu5]enkephalin as convenient endopeptidase substrates. Phosphoramidon, thiorphan and SQ 28603, potent inhibitors of mammalian neprilysin (neutral endopeptidase, endopeptidase 24.11), inhibited the endopeptidase activity towards AKH-I with IC50 values of 0.13 microM, 22 microM and 6.3 microM, respectively. Two other neprilysin inhibitors (SCH 32615 and SCH 39370) and the bivalent metal ion chelators, EDTA (1 mM) and 1, 10 bis-phenanthroline (1 mM) failed to inhibit the nematode enzyme. The endopeptidase had a neutral pH optimum and a significant proportion (45%) of the enzyme activity partitioned into the detergent-rich phase of Triton X-114, indicating that the enzyme is an integral membrane protein. The muscle enzyme also attacked [D-Ala2, Leu5]enkephalin cleaving the Gly3-Phe4 bond and this hydrolytic activity was inhibited by phosphoramidon and thiorphan (IC50, 0.28 microM and 15.8 microM, respectively) but not by EDTA and 1, 10 bis-phenanthroline. The phosphoramidon-sensitive endopeptidase activity was detected on intact muscle cells prepared by collagenase treatment of the body wall musculature, indicating that endopeptidase is accessible to peptide molecules that interact with the cell surface.

Processing of the L5-67 precursor peptide and characterization of LUQIN in the LUQ neurons of Aplysia californica

Metabolic labeling of the dorsal Left Upper Quadrant (LUQ) cells of the abdominal ganglion of Aplysia californica and RP-HPLC separation of their peptide content allowed us to identify the L5-67 precursor and its processed peptides. Cleavage of the signal peptide occurred between amino acids 23 and 24 of the prepropeptide and generated a propeptide of 89 amino acids. Further processing by endopeptidases at the twin basic residues Lys12-Arg13 of the precursor generated a peptide of 76 amino acids, as well as an amidated decapeptide, LUQIN. The sequence of LUQIN was determined by amino acid sequencing and by its comigration with the synthetic peptide Ala-Pro-Ser-Trp-Arg-Pro-Gln-Gly-Arg-Phe-amide in three different RP-HPLC systems. The amidation of LUQIN was further demonstrated by its resistance to carboxypeptidase A digestion.