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.

Free ATP inhibits thimet oligopeptidase (EC 3.4.24.15) activity, induces autophosphorylation in vitro, and controls oligopeptide degradation in macrophage.

The fate of the proteasome-generated peptides depends upon the cytosolic peptidases whose activities ought to be regulated. One of the most important oligopeptide-degrading and -binding proteins in the cytosol is the thimet oligopeptidase (EC 3.4.24.15), ubiquitously found in mammalian tissues. To date, there is no indication whether thimet oligopeptidase activities are physiologically regulated. Here, we present evidences suggesting that the concentration of unbound ATP in the cytosol regulates the thimet oligopeptidase activities both, in vitro and ex vivo. To perform these studies two oligopeptides were used: a quenched fluorescent peptide, which is susceptible to thimet oligopeptidase degradation, and the ovalbumin257-264 (MHC class I ovalbumin epitope), which displays high affinity to the thimet oligopeptidase without being degraded. We also showed that the thimet oligopeptidase undergoes autophosphorylation by ATP, a modification that does not affect the peptidase activity. The autophosphorylation is abolished in the presence of the thimet oligopeptidase substrates, as well as by the effect of a site directed inhibitor of this enzyme, and by the substitution of Glu474 for Asp at the metallo-peptidase motif. Altogether, the results presented here suggest that Zn2+ at the active center of the thimet oligopeptidase is the target for the ATP binding, leading to the inhibition of the enzyme activity, and inducing autophosphorylation. These effects, which depend upon the concentration of the unbound ATP, may help to explain the fate of the proteasomal-generated oligopeptides in the cytosol.

Activity of Pz-peptidase and endo-oligopeptidase are due to the same enzyme.

During purification of endo-oligopeptidase from rabbit heart, activities cleaving bradykinin, a substrate of endo-oligopeptidase, and Mcc-Pro-Leu-Gly-Pro-D-Lys(Dnp), a substrate of Pz-peptidase, were found in the same fractions. The hydrolysis of both substrates was inhibited by antisera against endo-oligo-peptidase and Pz-peptidase, and reversibly inhibited by 1, 10-phenanthroline. The purified enzyme hydrolysed Dnp-Pro-Leu-Gly-Pro-Trp-D-Lys, another substrate of Pz-peptidase. Purified Pz-peptidase from rabbit muscle degraded bradykinin and was inhibited by an antiserum against endo-oligopeptidase.

Influence of the carboxyl terminus of luteinizing hormone-releasing hormone and bradykinin on hydrolysis by brain endo-oligopeptidases.

A homogeneous preparation of endo-oligopeptidase A from rabbit brain cleaves luteinizing hormone-releasing hormone (less than Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2) at the Tyr-Gly bond only after the removal of Gly-NH2 from the COOH-terminal position of the molecule. The influence of the carboxyl terminus on hydrolysis by brain endo-oligopeptidases was studied using bradykinin as a model substrate. The substitution of the carboxyl group of bradykinin by the amide reduces by 2.5-fold the rate of Phe-Ser bond hydrolysis by endo-oligopeptidase A but has no effect on the rate of hydrolysis of the Pro-Phe bond by endo-oligopeptidase B. On the other hand, the deletion of Phe-Arg from the COOH-terminal portion of bradykinin makes the peptide resistant to hydrolysis by endo-oligopeptidase A whereas it increases by 5-fold the rate of hydrolysis of the Pro-Gly bond by endo-oligopeptidase B.

Thimet oligopeptidase (EC 3.4.24.15), a novel protein on the route of MHC class I antigen presentation.

The initial processing of antigens leading to major histocompatibility complex (MHC) class I antigenic peptides is carried out by the proteasome. However, how the final epitopes are generated and protected from degradation by cytosolic peptidases remains unknown. Coincidentally, peptides associated with the MHC class I molecules range from 8 to 13 amino acid residues, similarly to the optimum substrate size required for the cytosolic thimet oligopeptidase. Here we have investigated the putative intracellular function of thimet oligopeptidase related to antigen presentation. Using a well-characterized antigen-presenting cell system, we were able to demonstrate either inhibition or stimulation of CD8 T cell proliferation and cytotoxicity, manipulating intracellular thimet oligopeptidase levels with its specific inhibitor cFP-Ala-Ala-Tyr-pAb or loading the enzyme itself into the antigen-presenting cells. Our results suggest that thimet oligopeptidase should take an important function in the pathway of antigen presentation via MHC class I through a mechanism yet unknown.

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.

Screening for rabbit brain neuropeptide-metabolizing peptidases. Inhibition of endopeptidase B by bradykinin potentiating peptide 9a (SQ 20881).

Neutral thiol-activated peptidases present in the pH 5-soluble fraction of rabbit brain (separated by step-elution chromatography on diethylaminoethyl cellulose) were screened for the hydrolysis of bradykinin. Lys-bradykinin, Met-Lys-bradykinin, angiotensin I, angiotensin II, substance P, luteinizing hormone-releasing hormone (LH-RH), and neurotensin by bioassay. The column effluent was monitored for bradykinin inactivation and arylamidase activity and combined in six pools on the basis of bradykinin inactivation. The pools were characterized by determining the peptide fragments and amino acids released from bradykinin with an amino acid analyzer. Pools 1 through 3 contained 80% of the kininase activity and essentially all of the endopeptidase A and B activity, whereas pools 4 through 6 accounted for 98% of the recovered arylamidase activity. Bradykinin, angiotensin I, angiotensin II, and substance P were inactivated by all the pools, whereas LH-RH and neurotensin were inactivated by pools 3 and 4, and pools 3, 4, and 5, respectively. These data show that rabbit brain contains peptidases having some selectivity for the inactivation of neuropeptides. Endopeptidase B purified from pool 3 is inhibited by bradykinin-potentiating peptide 9a (BPP9a, SQ 20881) (< Glu-Trp-Pro-Arg-Pro-Gln-Ile-Pro-Pro), a competitive inhibitor of the hydrolysis of bradykinin (Km = 3.5 X 10(-5) M, Ki = 3 X 10(-6) M) which also completely inhibits the inactivation of LH-RH.

Thimet oligopeptidase and the stability of MHC class I epitopes in macrophage cytosol.

In this study we investigated the fate of a class of proteasome-generated oligopeptides, exposing them to the crude cytosol of macrophages or to the purified recombinant thimet oligopeptidase. Among the proteasome products of known sequences are MHC class I epitopes, 13 of which were randomly chosen to be used as putative substrates. Surprisingly, our results clearly showed that the majority of the peptides were poorly or not degraded, either by the purified enzyme or by the crude macrophage cytosol. The peptides, which were resistant to hydrolysis, displayed high affinity for the thimet oligopeptidase as competitive inhibitors. Regardless of the fact that our data do not allow prediction of whether or not a specific peptide would be degraded, it seems very likely that the structural features, which rule out the stability of the MHC class I peptides in the cytosol, may have implications in an optimized repertoire selection for antigen presentation.

Cloning and sequence analysis of a Bothrops jararaca cDNA encoding a precursor of seven bradykinin-potentiating peptides and a C-type natriuretic peptide.

A 1.8-kb cDNA clone was isolated from a Bothrops jararaca venom gland cDNA library that encodes a 256-aa precursor for bradykinin-potentiating peptides (angiotensin-converting enzyme inhibitors) and a C-type natriuretic peptide (CNP). The seven bradykinin-potentiating peptides are aligned tandemly after the hydrophobic signal peptide sequence, followed by a putative intervening sequence and a CNP at the C terminus. Northern blot analysis indicated the predominant expression of a 1.8-kb mRNA in the venom glands as well as in the spleen and the brain. Two lower intensity mRNA bands of 3.5 kb and 5.7 kb also hybridized to the cDNA clone. Radioimmunoassay for the CNP was performed using the antiserum against rat CNP. The presence of CNP immunoreactivity was detected in the low molecular weight fraction of the Bothrops jararaca venom.

Molecular and immunochemical evidences demonstrate that endooligopeptidase A is the predominant cytosolic oligopeptidase of rabbit brain.

Oligopeptidases are tissue endopeptidases that do not attack proteins and are likely to be involved in the maturation and degradation of peptide hormones and neuropeptides. The rabbit brain endooligopeptidase A and the rat testes soluble metallopeptidase (EC 3.4.24.15) are thiol-activated oligopeptidases which are able to generate enkephalin from a number of opioid peptides and to inactivate bradykinin and neurotensin by hydrolyzing the same peptide bonds. A monospecific antibody raised against the purified rabbit brain endooligopeptidase A allowed the identification of a 2. 3 kb cDNA coding for a truncated enzyme of 512 amino acids, displaying the same enzymatic features as endooligopeptidase A. In spite of all efforts, employing several strategies, the full-length cDNA could not be cloned until now. The analysis of the deduced amino acid sequence showed no similarity to the rat testes metalloendopeptidase sequence, except for the presence of the typical metalloprotease consensus sequence [HEXXH]. The antibody raised against recombinant endooligopeptidase A specifically inhibited its own activity and reduced the thiol-activated oligopeptidase activity of rabbit brain cytosol to less than 30%. Analysis of the endooligopeptidase A tissue distribution indicated that this enzyme is mainly expressed in the CNS, whereas the soluble metallo EC 3.4.24.15 is mainly expressed in peripheral tissues.