Conversion and inactivation of opioid peptides by rabbit brain endo-oligopeptidase A

Neuropeptides and oligopeptidases in schizophrenia

Schizophrenia (SCZ) is a complex psychiatric disorder with severe impact on patient's livelihood. In the last years, the importance of neuropeptides in SCZ and other CNS disorders has been recognized, mainly due to their ability to modulate the signaling of classical monoaminergic neurotransmitters as dopamine. In addition, a class of enzymes coined as oligopeptidases are able to cleave several of these neuropeptides, and their potential implication in SCZ was also demonstrated. Interestingly, these enzymes are able to play roles as modulators of neuropeptidergic systems, and they were also implicated in neurogenesis, neurite outgrowth, neuron migration, and therefore, in neurodevelopment and brain formation. Altered activity of oligopeptidases in SCZ was described only more recently, suggesting their possible utility as biomarkers for mental disorders diagnosis or treatment response. We provide here an updated and comprehensive review on neuropeptides and oligopeptidases involved in mental disorders, aiming to attract the attention of physicians to the potential of targeting this system for improving the therapy and for understanding the neurobiology underlying mental disorders as SCZ.

Cysteinyl proteinases and their selective inactivation

The affinity-labeling of cysteinyl proteinases may now be carried out with a number of peptide-derived reagents with selectivity, particularly for reactions carried out in vitro. These reagents have been described with emphasis on their selectivity for cysteine proteinases and lack of action on serine proteinases, the most likely source of side reactions among proteinases. Perhaps a crucial feature of this selectivity is an enzyme-promoted activation due to initial formation of a hemiketal, which may destabilize the reagent. Prominent among the reagent types that have this class selectivity are the peptidyl diazomethyl ketones, the acyloxymethyl ketones, the peptidylmethyl sulfonium salts, and peptidyl oxides analogous to E-64. The need for specific inhibitors capable of inactivating the target enzyme in intact cells and animals is inevitably pushing the biochemical application of these inhibitors into more complex molecular environments where the possibilities of competing reactions are greatly increased. In dealing with the current state and potential developments for the in vivo use of affinity-labeling reagents of cysteine proteinases, the presently known variety of cysteinyl proteinases had to be considered. Therefore this chapter has, at the same time, attempted to survey these proteinases with respect to specificity and gene family. The continual discovery of new proteinases will increase the complexity of this picture. At present the lysosomal cysteine proteinases cathepsins B and L and the cytoplasmic calcium-dependent proteinases are reasonable goals for a fairly complete metabolic clarification. The ability of investigators to inactivate individual members of this family in vivo, possibly without complications due to concurrent inactivation of serine proteinases by improvements in reagent specificity, is increasing. Among the cysteine proteinases, at least those of the papain super family, hydrophobic interactions in the S2 and S3 subsites are important and some specificity has been achieved by taking advantage of topographical differences among members of this group. Some of this has probably involved surface differences removed from the regions involved in proteolytic action. The emerging cysteine proteinases include some which, in contrast to the papain family, have a pronounced specificity in S1 for the binding of basic side chains, familiar in the trypsin family of serine proteinases. At least a potential conflict with serine proteinases can be avoided by choice of a covalent bonding mechanism. The departing group region, has not been exploited. As a sole contributor to binding, this region may be rather limited as a source of specificity.(ABSTRACT TRUNCATED AT 400 WORDS)

Soluble metalloendopeptidases and neuroendocrine signaling

Peptidases play a vital and often highly specific role in the physiological and pathological generation and termination of peptide hormone signals. The thermolysin-like family of metalloendopeptidases involved in the extracellular processing of neuroendocrine and cardiovascular peptides are of particular significance, reflecting both their specificity for particular peptide substrates and their utility as therapeutic targets. Although the functions of the membrane-bound members of this family, such as angiotensin-converting enzyme and neutral endopeptidase, are well established, a role for the predominantly soluble family members in peptide metabolism is only just emerging. This review will focus on the biochemistry, cell biology, and physiology of the soluble metalloendopeptidases EC 3.4.24.15 (thimet oligopeptidase) and EC 3.4.24.16 (neurolysin), as well as presenting evidence that both peptidases play an important role in such diverse functions as reproduction, nociception, and cardiovascular homeostasis.

Expression of endo-oligopeptidase A in the rat central nervous system: a non-radioactive in situ hybridization study

Endo-oligopeptidase A (EOPA, formerly EC 3.4.22.19), a thiol-activated oligopeptidase, is able to degrade both bradykinin and neurotensin, and also to convert enkephalin-containing peptides into enkephalins. The expression of this enzyme was studied in the rat brain by in situ hybridization using non-radiotopic probes. The distribution of EOPA transcripts included many regions of the rat central nervous system, with higher expression in some regions, such as the hippocampus, cerebellum, and basal nucleus of Meynert. The marked EOPA expression in these areas could be anticipated, since they are rich in neuropeptides that are known to be EOPA substrates in vitro. The data characterize a widespread occurrence of EOPA in the rat brain and reinforce the suggestion of a critical role for EOPA in peptide processing.

Species specificity of thimet oligopeptidase (EC 3.4.24.15)

The recombinant rat testes metallo-endooligopeptidase (EC 3.4.24.15) and the rabbit brain endooligopeptidase A (formerly EC 3.4.22.19) were compared, side-by-side, in view of their striking similarities in both the physicochemical features and the specificities for oligopeptides. Concerning the tissue distribution in rat and rabbit, no relation between the levels of enzyme activity in cytosol and the levels of metallo-endooligopeptidase 24.15 mRNA could be established. The results suggest that the predominant neuropeptide-metabolizing activity attributed to the metallo-endooligopeptidase 24.15 is performed by, at least, two distinct cytosolic enzymes, one predominant in rat testes and the other in rabbit brain and testes, and possibly also in rat brain. Both enzymes are activated by dithiothreitol and irreversibly inhibited by a SH-affinity labeling dynorphin-related compound, but they are not inhibited by EDTA in a concentration dependent manner. Both enzymes exhibit the same specificity toward several bioactive peptides, except for LH-RH and substance P, which are only hydrolysed by the rat testes enzyme. Taken together, these results lead us to conclude that it is unlikely that the recombinant rat testes metallo-endooligopeptidase 24.15 and the rabbit brain endooligopeptidase A are the same molecule although they might belong to the same family of oligopeptidases.

Characterization of an endooligopeptidase A-like protein in PC12 cells: activity modulation by cAMP but not by basic fibroblast growth factor

Endooligopeptidase A is a putative neuropeptide-metabolizing enzyme. It converts small enkephalin-containing peptides into the corresponding enkephalins and inactivates biopeptides such as bradykinin and neurotensin in vitro. We investigated the presence of endooligopeptidase A in PC12 cells. This cell line was derived from a rat pheochromocytoma tumor and resembles fetal chromaffin cell. Depending on the supplements added to the cell culture, this cell line can be differentiated into mature chromaffin cell or sympathetic neuron-like cell. Endooligopeptidase A activity was measured in soluble cellular extracts using a specific fluorogenic substrate QF-ERP7. The PC12 endooligopeptidase A-like activity shared similar but not identical biochemical properties with rabbit brain endooligopeptidase A. Similarly to rabbit brain endooligopeptidase A, the PC12 endooligopeptidase A-like activity was enhanced by DTT, totally inhibited by DTNB and 1-10 Phenanthroline, partially inhibited by cFP-AAF-pAb, and not affected by PMSF. Furthermore, the PC12 endooligopeptidase A-like activity displayed identical elution profile as rabbit brain endooligopeptidase A in gel filtration and anion-exchange chromatography. In addition, an antiserum raised against rabbit brain endooligopeptidase A cross-reacted with a 71 kDa component from PC12 cell extracts in Western blotting and was also able to partially neutralize the PC12 endooligopeptidase A-like activity. Treatment of PC12 cells with basic fibroblast growth factor (bFGF), a neurotrophic factor for this cell line, did not modify the specific activity of this enzyme. However, cAMP analogs decreased the specific activity of the enzyme. These results indicate the presence of an endooligopeptidase A-like activity in PC12 cells which is modulated by cAMP but not by bFGF.

Endo-oligopeptidase A, a putative enkephalin-generating enzyme, in the vertebrate retina

Endo-oligopeptidase A, EC 3.4.22.19, converts small enkephalin-containing peptides into the corresponding enkephalins in vitro. We investigated the presence of endooligopeptidase A in the retina and its possible colocalization with enkephalins in retinal neurons. The specific activity of endo-oligopeptidase. A found in pigeon retinae (30.3 +/- 7.3 mU/mg, mean +/- standard deviation) was four times higher than in rabbit retinae (7.0 +/- 1.1 mU/mg). The enzyme activity was not modified by EDTA, but it was enhanced by dithiothreitol and inhibited by zinc and 5,5'-dithiobis(2-nitrobenzoic acid). Immunohistochemical experiments with a purified antiserum against rabbit endo-oligopeptidase A revealed labeled neurons in both the inner nuclear layer and the ganglion cell layer of pigeon and rabbit retinae. Double-labeling immunofluorescence experiments demonstrated that about 90% of neurons containing endo-oligopeptidase A-like immunoreactivity also contained [Leu5]-enkephalin-like immunoreactivity. These colocalization results may represent an important step toward the demonstration of the possible involvement of endo-oligopeptidase A in enkephalin generation in vivo.

Chicken liver Pz-peptidase, a thiol-dependent metallo-endopeptidase

Pz-peptidase was purified from chicken liver as a protein of Mr 80,000 and pI 5.2. The purified enzyme hydrolysed phenylazobenzyloxycarbonyl-Pro-Leu-Gly-Pro-D-Arg, 2,4-dinitrophenyl-Pro-Leu-Gly-Pro-Trp-D-Lys. 7-methoxycoumarin-3-carboxylyl-Pro-Leu-Gly-Pro-D-(2,4-dinitropheny l)Lys, benzoyl-Gly-Ala-Ala-Phe-p-aminobenzoate, Ac-Ala4 (at the Ala-1-Ala-2 bond) and bradykinin (at the Phe-5-Ser-6 bond). No hydrolysis of proteins was detected. Loss of activity in the presence of EDTA or 1,10-phenanthroline was time-dependent. Metal ions found to restore activity after treatment with EDTA were Zn2+, Mn2+, Ca2+, Co2+ and Cd2+, in decreasing order of effectiveness. Ni2+, Fe2+ and higher concentrations of Zn2+ were inhibitory. Inhibition by N-[1-(RS)-carboxy-3-phenylpropyl]-Ala-Ala-Tyr-p-aminobenzoate and related compounds showed Ki values (down to 5 nM) somewhat lower than those for the rat enzyme. Pz-peptidase was activated by low concentrations of 2-mercaptoethanol and dithiothreitol, but inhibited by higher concentrations. p-Chloromercuribenzoate and some other thiol-blocking reagents were inhibitory. Inactivation by diethyl pyrocarbonate that was reversible by hydroxylamine showed the presence of essential histidine residue(s). We conclude that chicken Pz-peptidase is a metallo-endopeptidase with thiol-dependence. Moreover, the properties of chicken Pz-peptidase agree with those described for mammalian soluble metallo-endopeptidase and endo-oligopeptidase A. consistent with the view that these three types of activity are all attributable to the single enzyme for which the name thimet peptidase has been proposed.

Localization of endo-oligopeptidase (EC 3.4.22.19) in the rat nervous tissue

The subcellular and regional distribution of endo-oligopeptidase (EC 3.4.22.19), an enzyme capable of generating enkephalin by single cleavage from enkephalin-containing peptides, was determined by an enzymatic assay using metorphamide and by immunochemical techniques in the CNS of the rat. The rat CNS contains a membrane-associated form of endo-oligopeptidase, an enzyme predominantly associated with the soluble fraction of brain homogenates. Subcellular fractionation showed that approximately 17% of the total activity of the enzyme is associated with membrane fractions including synaptosomes. Synaptosomal membranes were prepared from neocortex, striatum, hypothalamus, medulla, spinal cord, and cerebellum. The amount of EC 3.4.22.19 activity solubilized by 3-[( 3-cholamidopropyl]dimethylammonio)-1-propanesulfonate from synaptosomal membranes was similar in neocortex, striatum, and hypothalamus, being three- to 10-fold greater than in spinal cord, cerebellum, and medulla. A polyclonal antibody exhibiting high affinity for endo-oligopeptidase was raised in rabbits against the purified rat brain enzyme and used to localize endo-oligopeptidase by Western blotting and by immunoperoxidase techniques. A strong band corresponding to the Mr of EC 3.4.22.19 was found in solubilized proteins obtained from synaptosomal membranes prepared from hypothalamus, neocortex, and striatum when subjected to Western blotting. The immunohistochemical localization of endo-oligopeptidase indicated that the immunoreactivity was confined to gray matter in regions known to be rich in peptide-containing neurons such as the striatum. In the cerebellum, a region poor in peptides, no staining could be detected. The nonuniform distribution of endo-oligopeptidase in rat brain suggests a role in neurotransmitter processing in the CNS.

Endooligopeptidase A activity in rabbit heart: generation of enkephalin from enkephalin containing peptides

Two endopeptidases displaying similar specificities towards peptide hormone substrates but differing in molecular size have been identified in rabbit heart and isolated by a combination of ion-exchange chromatography, gel filtration and preparative gel electrophoresis. These two enzymes share several properties with the previously described rabbit brain endooligopeptidase A. They were shown to produce, by a single peptide bond cleavage, [Met5] enkephalin and [Leu5]enkephalin from small enkephalin containing peptides. They also hydrolyze the Phe5-Ser5 bond of bradykinin and the Arg8-Arg9 bond of neurotensin. Characteristically, the activity of both low and high Mr enzymes is restricted to oligopeptides. Both forms of heart endooligopeptidase A are inhibited by antibodies raised against the brain enzyme. When electrophoresed in SDS-polyacrylamide gel under denaturing conditions, the low Mr heart enzyme shows a major band of Mr = 73,000, comparable in size to the brain enzyme. The SDS-PAGE of the high and low Mr enzymes analyzed by immunoblotting with an antibody raised against low Mr brain endooligopeptidase A, showed a major antigen band corresponding to Mr = 72,000. In addition, immunoblotting has also demonstrated that a monoclonal antibody antitubulin reacts with a polypeptide corresponding to Mr = 50,000 present in the purified high Mr endooligopeptidase A. Both enzymes are activated by dithiothreitol and inhibited by thiol reagents, but are not affected by leupeptin, DFP or EDTA, thus indicating that they should be classified as nonlysosomal cysteinyl-endooligopeptidase A.

Enkephalin is liberated from metorphamide and dynorphin A1-8 by endo-oligopeptidase A, but not by metalloendopeptidase EC 3.4.24.15

It has been previously reported that both the cysteinyl-endo-oligopeptidase A and the metalloendopeptidase EC 3.4.24.15 are able to generate enkephalin from a number of enkephalin-containing peptides, including dynorphin A1-8. The present study shows that only endo-oligopeptidase A is able to generate [Leu5]enkephalin and [Met5]enkephalin from dynorphin A1-8 and from metorphamide respectively. It is also shown that endo-oligopeptidase A neither hydrolyses the specific EC 3.4.24.15 substrate alpha-N-benzoyl-Gly-Ala-Ala-Phe p-aminobenzoate, nor is inhibited by the specific EC 3.4.24.15 inhibitor N-[1(RS)-carboxy-2-phenylethyl]-alpha-Ala-Ala-Phe p-aminobenzoate.

Purification and characterization of bradykinin-hydrolyzing enzyme from 2-day-old rat epidermis

Bradykinin-hydrolyzing enzyme was purified 200-fold from a soluble fraction of cornified cells from 2-day-old rat epidermis. The enzyme has an Mr of 80,000 as identified by SDS polyacrylamide gel electrophoresis and HPLC gel filtration. The isoelectric point of the enzyme is 5.05. The enzyme hydrolyzed Phe5-Ser6 of bradykinin and seven bradykinin-related peptides, and Tyr5-Ser6 of Tyr5-bradykinin. Production of bradykinin fragments, Arg-Pro-Pro-Gly-Phe and Ser-Pro-Phe-Arg, proceeded in a stoichiometric fashion. Km and Vmax values for bradykinin were 33 microM and 22.2 mumol/min per mg, respectively. The enzyme did not hydrolyze azocasein, denatured hemoglobin or synthetic substrates for other epidermal proteinases. The enzyme activity was enhanced by reducing agents and inhibited by sulfhydryl-blocking agents and divalent cations. Diisopropyl fluorophosphate and phenylmethylsulfonyl fluoride had no effects. The enzyme has a pH optimum of 7.0-7.5 and is stable at 4 degrees C for 1 month, but loses activity completely at 60 degrees C for 10 min. The epidermal endopeptidase differs in several properties from endooligopeptidase A purified from brain which hydrolyzes Phe5-Ser6 of bradykinin.

Liberation of enkephalins from enkephalin-containing peptides by brain endo-oligopeptidase A

Endo-oligopeptidase A, highly purified from the cytosol fraction of bovine brain by immunoaffinity chromatography, has been characterised as a thiol endopeptidase. This enzyme, known to hydrolyse the Phe5-Ser6 bond of bradykinin and the Arg8-Arg9 bond of neurotensin has been shown to produce, by a single cleavage, [Leu]enkephalin or [Met]enkephalin from small enkephalin-containing peptides. Enkephalin formation could be inhibited in a concentration dependent manner by the alternative substrate bradykinin. The optimal substrate size was found to be 8-13 amino acids, with enkephalin the only product released from precursors in which this sequence is immediately followed by a pair of basic residues. However, the specificity constants (kcat/Km) obtained for endo-oligopeptidase A hydrolysis of bradykinin, neurotensin and dynorphin B are of the same order. Taken together, these results indicate that the substrate amino acid sequence is not the only factor determining the cleavage site of this enzyme. Finally, endo-oligopeptidase A and metalloendopeptidase EC 3.4.24.15 are two different enzymes. The latter is not able to liberate enkephalins from metorphamide and dynorphin.

Proteolytic enzymes in the post-translational processing of polypeptide hormone precursors

Selective and limited proteolysis is a key step in the post-translational modification of peptide hormone precursors. This process appears to involve a proteolytic machinery including highly specific endoproteases. Some of the enzyme systems possibly involved in the processing of pro-neuropeptides will be described and their mechanism of action discussed. Special emphasis will be on the following: i) the physico-chemical characteristics of proteolytic enzymes which are believed to be involved in the processing of some of these polypeptide hormone precursors; ii) the bio-specificity of these enzymes toward the substrates; iii) the importance of both secondary and tertiary structures of the cleavage domain in recognition by the selective proteases. These properties will be discussed in connection with the possible importance of the maturation enzymes in the in vivo regulation of hormone biosynthesis.

Brain endo-oligopeptidase A, a putative enkephalin converting enzyme

Endo-oligopeptidase A, highly purified from the cytosol fraction of bovine brain by immunoaffinity chromatography, has been characterized as a thiol endopeptidase. This enzyme, known to hydrolyze the Phe5-Ser6 bond of bradykinin and the Arg8-Arg9 bond of neurotensin, has been shown to produce, by a single cleavage, Leu5-enkephalin or Met5-enkephalin from small enkephalin-containing peptides. Enkephalin formation could be inhibited in a concentration-dependent manner by the alternative substrate bradykinin. The optimal substrate size was found to be eight to 13 amino acids, with enkephalin the only product released from precursors in which this sequence is immediately followed by a pair of basic residues. However, the specificity constants (kcat/Km) obtained for endo-oligopeptidase A hydrolysis of bradykinin, neurotensin, and dynorphin B are of the same order, a result indicating that the substrate amino acid sequence is not the only factor determining the cleavage site of this enzyme.