Catalysis of slow C-terminal processing reactions by carboxypeptidase H

Capillary electrophoresis electrospray ionization-mass spectrometry for peptidomics-based processing site determination

Proteolytic cleavage at specific sites is a key event that modulates protein functions in biological processes. These cleavage sites are identified through mass spectrometry-based peptidomics of overlapping peptide sequences. Here, we assessed to what extent a recent capillary electrophoresis (CE) system interfaced with electrospray ionization-mass spectrometry (ESI-MS) contributes to identifying endogenous peptides present in a biological sample. Peptides released by a human endocrine cell line stimulated for secretion was analyzed for uncovering potential processing sites created by proprotein convertases (PCs) that cleave precursors in the secretory pathway. CE-ESI-MS was conducted, in comparison to a standard liquid chromatography (LC)-ESI-MS platform. LC and CE complemented each other in elucidating processing sites that match PC consensus sequences from known substrates. We suggest that the precursors BIGH3, STC1, LFNG, QSOX1 and CYTC are potential substrates for PCs, and that a CE-ESI system would come in handy and garner greater recognition as a robust tool in peptidomics.

Dissecting carboxypeptidase E: properties, functions and pathophysiological roles in disease

Since discovery in 1982, carboxypeptidase E (CPE) has been shown to be involved in the biosynthesis of a wide range of neuropeptides and peptide hormones in endocrine tissues, and in the nervous system. This protein is produced from pro-CPE and exists in soluble and membrane forms. Membrane CPE mediates the targeting of prohormones to the regulated secretory pathway, while soluble CPE acts as an exopeptidase and cleaves C-terminal basic residues from peptide intermediates to generate bioactive peptides. CPE also participates in protein internalization, vesicle transport and regulation of signaling pathways. Therefore, in two types of CPE mutant mice, Cpe^fat/Cpe^fat and Cpe knockout, loss of normal CPE leads to a lot of disorders, including diabetes, hyperproinsulinemia, low bone mineral density and deficits in learning and memory. In addition, the potential roles of CPE and ΔN-CPE, an N-terminal truncated form, in tumorigenesis and diagnosis were also addressed. Herein, we focus on dissecting the pathophysiological roles of CPE in the endocrine and nervous systems, and related diseases.

Peptides derived from the prohormone proNPQ/spexin are potent central modulators of cardiovascular and renal function and nociception

Computational methods have led two groups to predict the endogenous presence of a highly conserved, amidated, 14-aa neuropeptide called either spexin or NPQ. NPQ/spexin is part of a larger prohormone that contains 3 sets of RR residues, suggesting that it could yield more than one bioactive peptide; however, no in vivo activity has been demonstrated for any peptide processed from this precursor. Here we demonstrate biological activity for two peptides present within proNPQ/spexin. NPQ/spexin (NWTPQAMLYLKGAQ-NH(2)) and NPQ 53-70 (FISDQSRRKDLSDRPLPE) have differing renal and cardiovascular effects when administered intracerebroventricularly or intravenously into rats. Intracerebroventricular injection of NPQ/spexin produced a 13 ± 2 mmHg increase in mean arterial pressure, a 38 ± 8 bpm decrease in heart rate, and a profound decrease in urine flow rate. Intracerebroventricular administration of NPQ 53-70 produced a 26 ± 9 bpm decrease in heart rate with no change in mean arterial pressure, and a marked increase in urine flow rate. Intraventricular NPQ/spexin and NPQ 53-70 also produced antinociceptive activity in the warm water tail withdrawal assay in mice (ED(50)<30 and 10 nmol for NPQ/spexin and NPQ 53-70, respectively). We conclude that newly identified peptides derived from the NPQ/spexin precursor contribute to CNS-mediated control of arterial blood pressure and salt and water balance and modulate nociceptive responses.

Neuropeptidomic analysis establishes a major role for prohormone convertase-2 in neuropeptide biosynthesis

Prohormone convertase 2 (PC2) functions in the generation of neuropeptides from their precursors. A quantitative peptidomics approach was used to evaluate the role of PC2 in the processing of peptides in a variety of brain regions. Altogether, 115 neuropeptides or other peptides derived from secretory pathway proteins were identified. These peptides arise from 28 distinct secretory pathway proteins, including proenkephalin, proopiomelanocortin, prodynorphin, protachykinin A and B, procholecystokinin, and many others. Forty one of the peptides found in wild-type (WT) mice were not detectable in any of the brain regions of PC2 knockout mice, and another 24 peptides were present at levels ranging from 20% to 79% of WT levels. Most of the other peptides were not substantially affected by the mutation, with levels ranging from 80% to 120% of WT levels, and only three peptides were found to increase in one or more brain regions of PC2 knockout mice. Taken together, these results are consistent with a broad role for PC2 in neuropeptide processing, but with functional redundancy for many of the cleavages. Comparison of the cleavage sites affected by the absence of PC2 confirms previous suggestions that sequences with a Trp, Tyr, and/or Pro in the P1' or P2' position are preferentially cleaved by PC2 and not by other enzymes present in the secretory pathway.

Evolutionary sequence modeling for discovery of peptide hormones

There are currently a large number of "orphan" G-protein-coupled receptors (GPCRs) whose endogenous ligands (peptide hormones) are unknown. Identification of these peptide hormones is a difficult and important problem. We describe a computational framework that models spatial structure along the genomic sequence simultaneously with the temporal evolutionary path structure across species and show how such models can be used to discover new functional molecules, in particular peptide hormones, via cross-genomic sequence comparisons. The computational framework incorporates a priori high-level knowledge of structural and evolutionary constraints into a hierarchical grammar of evolutionary probabilistic models. This computational method was used for identifying novel prohormones and the processed peptide sites by producing sequence alignments across many species at the functional-element level. Experimental results with an initial implementation of the algorithm were used to identify potential prohormones by comparing the human and non-human proteins in the Swiss-Prot database of known annotated proteins. In this proof of concept, we identified 45 out of 54 prohormones with only 44 false positives. The comparison of known and hypothetical human and mouse proteins resulted in the identification of a novel putative prohormone with at least four potential neuropeptides. Finally, in order to validate the computational methodology, we present the basic molecular biological characterization of the novel putative peptide hormone, including its identification and regional localization in the brain. This species comparison, HMM-based computational approach succeeded in identifying a previously undiscovered neuropeptide from whole genome protein sequences. This novel putative peptide hormone is found in discreet brain regions as well as other organs. The success of this approach will have a great impact on our understanding of GPCRs and associated pathways and help to identify new targets for drug development.

Neuropeptide-processing enzymes: applications for drug discovery

Neuropeptides serve many important roles in communication between cells and are an attractive target for drug discovery. Neuropeptides are produced from precursor proteins by selective cleavages at specific sites, and are then broken down by further cleavages. In general, the biosynthetic cleavages occur within the cell and the degradative cleavages occur postsecretion, although there are exceptions where intracellular processing leads to inactivation, or extracellular processing leads to activation of a particular neuropeptide. A relatively small number of peptidases are responsible for processing the majority of neuropeptides, both inside and outside of the cell. Thus, inhibition of any one enzyme will lead to a broad effect on several different neuropeptides and this makes it unlikely that such inhibitors would be useful therapeutics. However, studies with mutant animals that lack functional peptide-processing enzymes have facilitated the discovery of novel neuropeptides, many of which may be appropriate targets for therapeutics.

Relative quantitation of peptides in wild-type and Cpe(fat/fat) mouse pituitary using stable isotopic tags and mass spectrometry

Cpe(fat/fat) mice have a point mutation in the coding region of the carboxypeptidase E gene that renders the enzyme inactive. As a result, these mice have reduced levels of several neuropeptides and greatly increased levels of the peptide processing intermediates that contain C-terminal basic residues. However, previous studies examined a relatively small number of neuropeptides. In the present study, we used a quantitative peptidomics approach with stable isotopic labels to examine the levels of pituitary peptides in Cpe(fat/fat) mice relative to wild-type mice. Pituitary extracts from mutant and wild type mice were labeled with the stable isotopic label [3-(2,5-dioxopyrrolidin-1-yloxycarbonyl)propyl]trimethylammonium chloride containing nine atoms of hydrogen or deuterium. Then, the two samples were pooled and analyzed by liquid chromatography/mass spectrometry (LC/MS). The relative abundance of peptides was determined from a comparison of the intensities of the heavy and light peaks. Altogether, 72 peptides were detected in the Cpe(fat/fat) and/or wild-type mouse pituitary extracts of which 53 were identified by MS/MS sequencing. Several peptides identified in this analysis represent previously undescribed post-translational processing products of known pituitary prohormones. Of the 72 peptides detected in pituitary, 17 were detected only in the Cpe(fat/fat) mouse extracts; these represent peptide processing intermediates containing C-terminal basic residues. The peptides common to both Cpe(fat/fat) and wild-type mice were generally present at 2-5-fold lower levels in the Cpe(fat/fat) mouse pituitary extracts, although some peptides were present at equal levels and one peptide (acetyl beta-endorphin 1-31) was increased approximately 7-fold in the Cpe(fat/fat) pituitary extracts. In contrast, acetyl beta-endorphin 1-26 was present at approximately 10-fold lower levels in the Cpe(fat/fat) pituitary, compared with wild-type mice. The finding that many peptides are substantially decreased in Cpe(fat/fat) pituitary is consistent with the broad role for carboxypeptidase E in the biosynthesis of numerous neuropeptides.

Identification and characterization of three members of the human metallocarboxypeptidase gene family

Amino acid homology searches of the human genome revealed three members of the metallocarboxypeptidase (metallo-CP) family that had not been described in the literature in addition to the 14 known genes. One of these three, named CPA5, is present in a gene cluster with CPA1, CPA2, and CPA4 on chromosome 7. The cDNA encoding a mouse homolog of human CPA5 was isolated from a testis library and sequenced. The deduced amino acid sequence of human CPA5 has highest amino acid sequence identity (60%) to CPA1. Modeling analysis shows the overall structure to be very similar to that of other members of the A/B subfamily of metallocarboxypeptidases. The active site of CPA5 is predicted to cleave substrates with C-terminal hydrophobic residues, as do CPA1, -2, and -3. Using Northern blot analysis, CPA5 mRNA is detected in testis but not in kidney, liver, brain, or lung. In situ hybridization analysis shows that CPA5 is localized to testis germ cells. Mouse pro-CPA5 protein expressed in Sf9 cells using the baculovirus system was retained in the particulate fraction of the cells and was not secreted into the media. Pro-CPA5 was not enzymatically active toward standard CPA substrates, but after incubation with prohormone convertase 4 the resulting protein was able to cleave furylacryloyl-Gly-Leu, with 3-4-fold greater activity at pH 7.4 than at 5.6. Two additional members of the human CP gene family were also studied. Modeling analysis indicates that both contain the necessary amino acids required for enzymatic activity. The CP on chromosome 8 is predicted to have a CPA-like specificity for C-terminal hydrophobic residues and was named CPA6. The CP on chromosome 2 is predicted to cleave substrates with C-terminal acidic residues and was named CPO.

Carboxypeptidase E, a prohormone sorting receptor, is anchored to secretory granules via a C-terminal transmembrane insertion

Carboxypeptidase E (CPE) is a sorting receptor that directs the prohormone pro-opiomelanocortin (POMC) to the regulated secretory pathway, and is also a prohormone processing enzyme in neuro/endocrine cells. It has been suggested that the 25 C-terminal amino acids are necessary for the binding of CPE to secretory granule membranes, but its orientation in the membrane is not known. In this study, we examined the structure and orientation of the membrane-binding domain at the C-terminus of CPE. In vitro experiments using model membranes demonstrated that the last 22 amino acids of CPE (CP peptide) insert in a shallow orientation into lipid bilayers at low pH. Circular dichroism analysis indicated that the CP peptide adopts a partial alpha-helical configuration at low pH, and helix content increases when it is bound to lipid. Protease protection experiments, immunolabeling, and immunoisolation of intact secretory granules with a C-terminal antibody revealed a cytoplasmic domain in CPE, consistent with a transmembrane orientation of this protein. We conclude that the membrane-binding domain of CPE must adopt an alpha-helical configuration to bind to lipids, and that CPE may require another integral membrane "chaperone" protein to insert through the lipid bilayer in a transmembrane fashion.

Altered expression of the neuropeptide-processing enzyme carboxypeptidase E in the rat brain after global ischemia

Carboxypeptidase E, an exoprotease involved in the processing of bioactive peptides released by a regulated secretory pathway, was identified in a subtractive complementary DNA library derived from an ischemic rat brain by differential screening. In situ hybridization and immunocytochemical analysis showed the presence of carboxypeptidase E messenger RNA and protein in the cerebral cortex, thalamus, striatum, and hippocampus of a healthy rat brain. After 15 minutes of transient global ischemia followed by 8 hours of reperfusion, increased levels of carboxypeptidase E messenger RNA and protein were observed in the hippocampal CA1 and CA3 regions and in the cortex, as detected by Northern and Western blot analyses and in situ hybridization. After extended reperfusion (24 to 72 hours), both carboxypeptidase E messenger RNA and protein levels were decreased. The ischemia-induced changes in carboxypeptidase E expression suggest that this enzyme may play a role in modulating the brain's response to ischemia.

Missense polymorphism in the human carboxypeptidase E gene alters enzymatic activity

Carboxypeptidase E (CPE) is involved in the biosynthesis of peptide hormones and neurotransmitters, including insulin. One of the features of type 2 diabetes mellitus (T2DM) is an elevation in the proinsulin level and/or proinsulin/insulin molar ratio, suggesting that mutations in proinsulin processing enzymes may contribute to the development of T2DM. We scanned CPE for mutations in a collection of Ashkenazi T2DM families and identified five novel single nucleotide polymorphisms (SNPs). An SNP in the 283(rd) codon, c.847C>T, changes arginine to tryptophan (R283W). The residue Arg283 is conserved among CPE orthologs as well as most enzymatically active metallocarboxypeptidases. Of the 272 Ashkenazi T2DM pedigrees screened, we found four families segregating R283W. Within these four families, patients who inherited one copy of this variant had much earlier age of onset for T2DM. The R283W CPE protein cleaves peptide substrates with substantially lower efficiencies and is less stable at elevated temperature. In addition, the R283W CPE variant has a narrower pH optimum and is much less active at pH 6.0-6.5, indicating that the R283W CPE variant would be substantially less active than wild type CPE in the trans-Golgi network and immature secretory vesicles where the enzyme functions in vivo. To summarize, we uncovered a rare non-conservative missense mutation in CPE and demonstrated that the mutant protein has altered enzymatic properties. We predict that this mutant could cause hyperproinsulinism and diabetes in the homozygous state.

Impaired prohormone convertases in Cpe(fat)/Cpe(fat) mice

A spontaneous point mutation in the coding region of the carboxypeptidase E (CPE) gene results in a loss of CPE activity that correlates with the development of late onset obesity (Nagert, J. K., Fricker, L. D., Varlamov, O., Nishina, P. M., Rouille, Y., Steiner, D. F., Carroll, R. J., Paigen, B. J., and Leiter, E. H. (1995) Nat. Genet. 10, 135-142). Examination of the level of neuropeptides in these mice showed a decrease in mature bioactive peptides as a result of a decrease in both carboxypeptidase and prohormone convertase activities. A defect in CPE is not expected to affect endoproteolytic processing. In this report we have addressed the mechanism of this unexpected finding by directly examining the expression of the major precursor processing endoproteases, prohormone convertases PC1 and PC2 in Cpe(fat) mice. We found that the levels of PC1 and PC2 are differentially altered in a number of brain regions and in the pituitary. Since these enzymes have been implicated in the generation of neuroendocrine peptides (dynorphin A-17, beta-endorphin, and alpha- melanocyte-stimulating hormone) involved in the control of feeding behavior and body weight, we compared the levels of these peptides in Cpe(fat) and wild type animals. We found a marked increase in the level of dynorphin A-17, a decrease in the level of alpha-melanocyte-stimulating hormone, and an alteration in the level of C-terminally processed beta-endorphin. These results suggest that the impairment in the level of these and other peptides involved in body weight regulation is mainly due to an alteration in carboxypeptidase and prohormone convertase activities and that this may lead to the development of obesity in these animals.

Decreased expression of carboxypeptidase E protein is correlated to estrogen-induction of rat pituitary tumors

Following the cleavage of peptide precursors by endopeptidases such as the proprotein convertases PC2 and PC3, carboxypeptidase E (CPE) functions to remove basic amino adds from the C-terminus of various pituitary hormones. We investigated the role of CPE in the differential sensitivity between rat strains to estrogen-induced pituitary tumors. Pituitary CPE protein levels were unchanged by diethylstilbestrol (DES) in tumor-resistant Sprague-Dawley (SD) rats. However, in tumor-susceptible Fischer 344 (F344) rats, DES decreased CPE protein levels such that by 7 and 8 weeks of treatment, CPE was barely detectable. One week withdrawal of DES caused an increase in CPE protein levels at 8 weeks. After 2 and 4 weeks of DES treatment, CPE protein levels in F344 rats decreased to 18 and 2.3% of control values, respectively, but no strain difference was observed in the protein levels of proprotein convertase 2 (PC2) or PC3. Additionally, Brown Norway (BN), F344, and F1 hybrid (BN x F344) rats were treated with DES for 10 weeks. The level of pituitary CPE protein was not affected by DES in BN rats whereas F344 rats had only 8% of the level of CPE pituitary protein of BN rats. The pituitaries of F1 rats, which had an intermediate weight response to DES, had an intermediate level of CPE protein (31% that of BN rats). Levels of CPE mRNA were not affected by DES in SD rats while in F344 rats DES tended to decrease levels of CPE mRNA after both 2 and 4 weeks of treatment, although the response was variable. It thus appears that pituitary CPE protein is differentially regulated by DES between tumor-resistant rats and F344 rats primarily at the post-transcriptional level. Furthermore, in F344 rats, levels of CPE protein are inversely correlated to increases in pituitary weight caused by DES treatment.

The C-terminal region of carboxypeptidase E involved in membrane binding is distinct from the region involved with intracellular routing

Carboxypeptidase E (CPE) is involved in the biosynthesis of numerous peptide hormones and neurotransmitters. Previously, the C-terminal region of CPE has been shown to participate in the binding of the protein to membranes and to also contribute to the sorting of CPE into the regulated pathway. In this study, the role of the C-terminal region of CPE was further examined using several approaches. A series of CPE mutants with C-terminal deletions was expressed in the baculovirus system; constructs with a deletion of 14 or 23 residues were expressed at levels comparable to wild-type CPE. In contrast, deletion of 33 or more residues eliminated CPE activity, and the resulting protein was not secreted from the cells. Even though CPE mutants with a deletion of 14 or 23 residues were expressed normally, the resulting protein was mainly soluble, whereas approximately 55% of wild-type CPE was membrane associated. When expressed in AtT-20 cells, CPE with a deletion of 43 C-terminal amino acids was not secreted, whereas CPE with a deletion of 23 residues was secreted via the regulated pathway. Pulse-chase analysis revealed the protein with a deletion of 43 residues to be degraded in a non-acidic intracellular compartment. To investigate whether the C-terminal region of CPE can confer membrane binding and regulated pathway sorting to another protein, portions of the CPE C-terminal region were attached to the C terminus of albumin and the fusion proteins expressed in AtT-20 cells. Of the constructs examined, only the protein containing 51 amino acids of CPE was sorted to the regulated pathway, although with reduced efficiency compared to endogenous CPE. Although the C-terminal 14 amino acids of CPE are sufficient to target albumin to membranes, this fusion protein is not sorted into the regulated pathway. Taken together, these results indicate that the C-terminal 14 amino acids of CPE are important for membrane binding and that membrane binding and sorting require distinct signals.

Dopamine antagonist haloperidol increases carboxypeptidase E mRNA in rat neurointermediate pituitary but not in various other rat tissues

Carboxypeptidase E (CPE) is involved with the biosynthesis of many neuropeptides, including several whose genes are regulated by haloperidol treatments. In this study, we examined whether haloperidol alters CPE mRNA levels in a variety of tissues. Rats were treated for either 1, 3, 7, 14, or 21 days with 2 mg/kg haloperidol, and then Northern blot analysis performed on RNA from neurointermediate pituitary, anterior pituitary, hypothalamus, striatum, cerebellum, and adrenal. The 14 and 21 day treatments produced a significant 90-110% elevation of CPE mRNA in neurointermediate pituitary. However, the levels of CPE mRNA in the other tissues were not significantly influenced by the haloperidol treatments. This finding indicates that CPE is not co-regulated with peptide hormone mRNAs in all tissues.

Regulation of carboxypeptidase E by membrane depolarization in PC12 pheochromocytoma cells: comparison with mRNAs encoding other peptide- and catecholamine-biosynthetic enzymes

PC12 cells, a rat pheochromocytoma cell line, have been found to express carboxypeptidase E (CPE) enzymatic activity and CPE, furin, and peptidylglycine alpha-amidating monooxygenase (PAM) mRNAs. PC12 cells secrete CPE activity in response to depolarization induced by 50 mM KCl. Short-term (1- to 3-h) treatments of PC12 cells with KCl stimulates the secretion of CPE but does not appear to stimulate the synthesis of new CPE protein, based on the measurement of CPE activity and incorporation of [35S]-Met into CPE. Also, CPE mRNA is not altered by 2-h treatments with KCl. In contrast, prolonged treatment (24-48 h) of PC12 cells with 50 mM KCl continues to stimulate the secretion of CPE activity, without altering the cellular level of CPE. Levels of CPE mRNA are significantly elevated after long-term treatment of the cells with KCl, with increases of 35% after 5 h and 55-75% after 24 to 72 h of treatment. The level of PAM mRNA is also elevated approximately 70% after 24 h of stimulation with KCl. In contrast, the mRNA levels of furin and dopamine beta-hydroxylase (DBH) do not change on treatment of PC12 cells with KCl. These findings indicate that long-term depolarization, which leads to a prolonged stimulation of PC12 cells to secrete CPE, also stimulates the synthesis of CPE and PAM but not furin or DBH.

Regulation of carboxypeptidase E. Effect of pH, temperature and Co2+ on kinetic parameters of substrate hydrolysis

Carboxypeptidase E is a member of the carboxypeptidase A and B gene family, with many of the putative active-site and substrate-binding residues conserved between these enzymes. However, the pH optimum of carboxypeptidase E is substantially lower than that of carboxypeptidases A and B. To evaluate whether the difference in the pH optima of these carboxypeptidases reflects fundamental differences in the ionization behaviour of active-site residues, the influence of pH on carboxypeptidase E activity was examined. The V(max) for hydrolysis of dansyl-Phe-Ala-Arg is pH-independent between 5 and 7, but decreases at pH values below 5. The pKa for the group the protonation of which leads to the loss of activity is approximately 4.8, and the slope of the V(max.)/pH profile suggests that only a single ionizable group is involved. In contrast, Km and V(max.)/Km are dramatically influenced by pH over the range 5-7, with multiple ionizable groups detected in this pH range. The pKa of the group the protonation of which decreases the V(max.) of substrate hydrolysis is lower (4.5) for carboxypeptidase E which had been reconstituted with Co2+. The enthalpy of ionization of the group observed in the V(max.) profile for carboxypeptidase E is approx. 28.9 kJ/mol. These results are compatible with the active-site model of the homologous carboxypeptidase A: in this model the ionization of a metal-bound water molecule is responsible for the observed decrease in V(max.).

The posttranslational processing of beta-endorphin in human hypothalamus

beta-Endorphin is posttranslationally processed to six derivatives, which, although structurally similar, produce distinctly different biological effects. beta-Endorphin 1-31 is a potent opioid receptor agonist, but beta-endorphin 1-27 exhibits antagonist properties, and beta-endorphin 1-26 and the alpha-N-acetyl derivatives of all three peptides lack opioid receptor activity. In the present study, we identified the beta-endorphin peptides synthesized in human hypothalamus using cation exchange HPLC. First, we tested whether postmortem changes occur by storing rat hypothalami at 4 degrees C. This demonstrated that relative amounts of the six beta-endorphin forms did not change for up to 24 h, although total beta-endorphin immunoreactivity significantly declined after 6 h. HPLC analysis of human hypothalami revealed that beta-endorphin 1-31 was the principal form, constituting 58.4 +/- 5.4% of total immunoreactivity. Substantial amounts of beta-endorphin 1-27 (13.4 +/- 1.2%) and beta-endorphin 1-26 (13.1 +/- 1.6%) were also present, but alpha-N-acetylated forms were quantitatively minor, each comprising approximately 5% of total beta-endorphin. A similar processing pattern occurred in preoptic and suprachiasmatic areas of the hypothalamus. These results show that, despite differences in primary sequence, beta-endorphin is processed similarly in both rat and human hypothalamus. Opiate-active beta-endorphin 1-31 is the principal form in both species.

Purification of a cysteine endopeptidase which is secreted with bioactive peptides from the epidermal glands of Xenopus laevis

The purification is reported of an endopeptidase, XSCEP1 (Xenopus skin cysteine endopeptidase), present in skin secretions of Xenopus. The procedure involved an initial concentration of the enzyme by batchwise anion-exchange chromatography and ammonium sulphate precipitation. The proteolytic activity, determined with Z-Phe-Arg-Amc (Z, benzyloxycarbonyl; Amc, 7-amidomethylcoumarin) as substrate, was fractionated by gradient ion-exchange chromatography, yielding a major component which was purified to homogeneity by chromatography on an organomercury-agarose column. SDS/PAGE demonstrated the presence of a single protein with a molecular mass of 27 kDa. The purified enzyme, which possessed a pH optimum of 5.5, exhibited the properties of a cysteine endopeptidase; it was activated by dithiothreitol and EDTA and inhibited by the mechanism-based inhibitor trans-epoxysuccinyl-L-leucylamido(4-guanidino)butane. XSCEP1 exhibited a marked preference for substrates with a hydrophobic residue in the P1 position and arginine in the P2 position as opposed to a substrate with arginine residues in both positions. The enzyme was also able to cleave a Val-Arg-Gly sequence in a model substrate, reflecting cleavages undergone by a number of peptides present in Xenopus skin. The results point to a functional role for XSCEP1 as a putative processing enzyme.

Reflex splanchnic nerve stimulation increases levels of carboxypeptidase E mRNA and enzymatic activity in the rat adrenal medulla

Carboxypeptidase E (CPE; EC 3.4.17.10) is a carboxypeptidase B-like enzyme involved with the biosynthesis of numerous peptide hormones and neurotransmitters, including the enkephalins. Reflex splanchnic stimulation of the rat adrenal medulla, which has previously been found to substantially increase enkephalin mRNA and enkephalin peptide levels, was examined for an influence on CPE mRNA and enzymatic activity. Several hours after insulin-induced reflex splanchnic stimulation, the levels of CPE activity in rat adrenal medulla are reduced to 40-60% of control. CPE activity returns to the control level 2 days after the treatment and then continues to increase, reaching approximately 200% of control 1 week after the treatment. The time course of the changes in CPE activity is different from those of the changes in epinephrine levels and the previously reported changes in enkephalin peptide levels. CPE mRNA is also influenced by the insulin shock, with levels increasing to 155% of the control level after 6 h and 170% after 2 days. The time course of the change in CPE mRNA levels is similar to that previously found for proenkephalin mRNA. However, the magnitude of the change is much different: Proenkephalin mRNA has been reported to increase by 1,600%. The changes in CPE mRNA and enzymatic activity are consistent with the proposal that CPE is not a rate-limiting enzyme in the biosynthesis of enkephalin.

Comparison of a spectrophotometric, a fluorometric, and a novel radiometric assay for carboxypeptidase E (EC 3.4.17.10) and other carboxypeptidase B-like enzymes

Carboxypeptidase E (CPE) is a carboxypeptidase B-like enzyme involved in the biosynthesis of numerous peptide hormones and neurotransmitters. A sensitive assay for CPE and other carboxypeptidase B-like enzymes has been developed using 125I-acetyl-Tyr-Ala-Arg (125I-AcYAR) as the substrate. This peptide is poorly soluble in ethyl acetate whereas the product of carboxypeptidase B-like enzymatic activity (125I-AcYA) can be quantitatively extracted with this solvent, allowing the rapid separation of product from substrate. This radiometric assay can detect less than 1 pg of either CPE or carboxypeptidase B. For CPE, the assay with 125I-AcYAR is approximately 1000 times more sensitive than a fluorescent assay using dansyl-Phe-Ala-Arg (dans-FAR), and 6000 times more sensitive than a spectrophotometric assay using hippuryl-Arg (hipp-R). CPE hydrolyzes the three substrates with Kcat values of 16 s-1 for AcYAR, 13 s-1 for dans-FAR, and 8.5 s-1 for hipp-R. The Km values for CPE with AcYAR (28 microM) and dans-FAR (34 microM) are similar, and are much lower than the Km with hipp-R (400 microM). Thus, the primary reason for the increased sensitivity of the 125I-AcYAR assay over the fluorescent assay is not a result of kinetic differences but is due to the detection limit of iodinated product (10(-15) mol), compared to the fluorescent product (5 x 10(-11) mol). Applications of this rapid and sensitive radiometric assay to detect CPE in cultured cells and in subcellular fractions of the pituitary are described.

Cultured astrocytes and neurons synthesize and secrete carboxypeptidase E, a neuropeptide-processing enzyme

Carboxypeptidase E (EC 3.4.17.10) is a carboxypeptidase B-like enzyme associated with the biosynthesis of many peptide hormones and neurotransmitters. Media collected from cultured astrocytes contain a carboxypeptidase E-like activity. Cultured astrocytes secrete approximately 73% of their cellular level of carboxypeptidase E per hour, and secretion is not substantially influenced by 35 mM KCl. In contrast, neurons secrete only 29% of their cellular carboxypeptidase E per hour, but secretion increases to 86% on stimulation with 35 mM KCl. Secretion of carboxypeptidase E activity from both neuronal and astrocyte cultures is relatively selective; neither acid phosphatase or acetylglucosaminidase is secreted in appreciable amounts. Cultured neurons and astrocytes express a carboxypeptidase E mRNA of a similar size. The levels of this mRNA differ in astrocytes cultured from different brain regions, with high levels in striatal, cortical, hippocampal, and hypothalamic astrocytes and low levels in cerebellar astrocytes. The level of carboxypeptidase E mRNA in hypothalamic astrocyte cultures is four- to fivefold higher than the level in hypothalamic neuronal cultures. These results indicate that cultured astrocytes express carboxypeptidase E mRNA and enzymatic activity and thus contain one of the enzymes required in the biosynthesis of many peptide hormones and neurotransmitters.