Characterization of a distinct membrane bound dipeptidyl carboxypeptidase inactivating enkephalin in brain

A comparison of the properties and enzymatic activities of three angiotensin processing enzymes: angiotensin converting enzyme, prolyl endopeptidase and neutral endopeptidase 24.11

The discovery of angiotensin-(1-7) [Ang-(1-7)] as a bioactive Ang II fragment of the renin-angiotensin system (RAS) alters the current understanding of the enzymatic components that comprise the RAS cascade. Two neutral endopeptidases, prolyl endopeptidase (E.C. 3.4.21.26) and neutral endopeptidase 24.11 (E.C. 3.4.24.11), are capable of forming Ang-(1-7) from Ang I and have been implicated in the in vivo processing of Ang I. This makes them putative Ang processing enzymes and part of the RAS cascade. This review summarizes the physical characteristics and distribution of angiotensin converting enzyme (E.C. 3.4.15.1), a known Ang I processing enzyme, and compares its features to what is known of prolyl endopeptidase and neutral endopeptidase 24.11.

Hydrolysis of angiotensin I by peptidases in homogenates of rat lung and aorta

The hydrolytic cleavage of angiotensin I has been studied in homogenate preparations of rat lung and aorta using gradient elution HPLC to monitor the formation of peptide products. Fresh crude homogenate preparations produced a rapid breakdown of angiotensin I to largely unidentifiable fragment peptides. Neither His-Leu nor angiotensin II was observed in these preparations even in the presence of captopril (20 microM) and the amino-peptidase inhibitors, puromycin, amastatin and bestatin. However, in freeze-thawed homogenates, angiotensin II and His-Leu were detectable together with the tetrapeptide, angiotensin (1-4). The addition of captopril (20 microM) reduced the amount of angiotensin II produced but did not completely block its formation. Higher concentrations of captopril or the addition of enalaprilat or EDTA did not further reduce the amount of angiotensin II produced. In the presence of captopril a peptide corresponding to des-Leu(10)angiotensin I was formed in relatively large amounts (equivalent to 40% of angiotensin I catabolized). Homogenates purified by concanavalin A affinity chromatography gave a clean hydrolysis of angiotensin I to angiotensin II and His-Leu which was completely blocked by captopril. These results suggest an ACE-like activity in rat lung and aorta that is not sensitive to converting enzyme inhibitors.

Reaction of opioid peptides with neutral endopeptidase ("enkephalinase")

The kinetics of the reactions of nine opioid peptides with the neutral endopeptidase ("enkephalinase") activities of human kidney, rat kidney, and rat brain have been determined. These opioid peptides can be divided into two classes, those that are good inhibitors of Leu5-enkephalin hydrolysis (Ki less than 75 microM) and good substrates for the enzyme, and those that are poor inhibitors (Ki greater than 500 microM) and are not substrates for the enzyme. The former group includes Leu5-enkephalin, Met5-enkephalin, Met5-enkephalin-Arg6-Phe7, beta-lipotropin, and gamma-endorphin, while the nonreactive opioid peptides include alpha-neo-endorphin, beta-neo-endorphin, dynorphin, and beta-endorphin. These results suggest that those peptides containing the Met5-enkephalin sequence are more reactive than those containing the Leu5-enkephalin sequence. The lack of specificity of this neutral endopeptidase indicates that it may function in the degradation of a variety of biologically active peptides.

Inhibitors of an enkephalin degrading membrane-bound metalloendopeptidase: analgesic properties and effects on striatal enkephalin levels

Intraperitoneal administration of N-[1-(R,S)-carboxy-2-phenylethyl-Phe-p-aminobenzoate, synthesized in this laboratory as a potent inhibitor of membrane-bound metalloendopeptidase (EC 3.4.24.11) caused a prolonged but weak analgesic effect on rats as measured by the tail flick test. It also caused a transitory but significant increase in striatal [Leu5]- and [Met5]enkephalin levels 3 h, after administration. Analogs of the inhibitor in which the phenylalanyl residue was replaced by an alanyl or glycyl residue also elicited prolonged analgesic responses although their inhibitory potencies were 75 and more than 1500 times lower respectively. The glycine containing derivative did not alter striatal enkephalin levels 3 h, after administration. The data suggest that inhibition of the metalloendopeptidase decreases the rate of degradation of endogenous enkephalins, however the analgesic properties of the inhibitors do not seem to be related to their inhibitory potencies. Factors other than changes in striatal enkephalin levels may contribute to the analgesic effect of the three N-carboxyphenylethyl derivatives.

Fluorogenic substrates for the enkephalin-degrading neutral endopeptidase (Enkephalinase)

Rat brain neutral endopeptidase ("Enkephalinase") was shown to hydrolyze a series of fluorogenic substrates of the general structure 2-aminobenzoyl-(amino acid)n- leucylalanylglycine -4- nitrobenzylamide . The hydrolysis of these substrates was competitively inhibited by Leu5-enkephalin, demonstrating that these are indeed substrates for the rat brain neutral endopeptidase. Cleavage of the fluorogenic substrates yielded leucylalanylglycine -4- nitrobenzylamide as a common product. In addition, a series of inhibitors previously shown to inhibit thermolysin-like enzymes inhibited the hydrolysis of both Leu5-enkephalin and the synthetic substrates. The results of this study (a) demonstrate that the enkephalin-degrading endopeptidase is similar in specificity to thermolysin, (b) provide a continuous sensitive assay system for the enzyme, and (c) point out the potential use of this substrate class for probing the specificity of the enzyme.

Membrane-bound enzymes and their role in processing of the dynorphins and of the proenkephalin octapeptide Metenkephalin-Arg-Gly-Leu

Synaptosomal membrane (SPM) bound exo- and endopeptidases cleave the dynorphins and Met-enkephalin-Arg-Gly-Leu at several sites to produce shorter fragments; among these are dynorphin 1-8 from 1-17, and Met-enkephalin from Met-enkephalin-Arg-Gly-Leu. The most vulnerable site is the Tyr-Gly bond cleaved by membrane-bound aminopeptidase(s), with the shorter peptides degraded more rapidly than the longer ones. A purified metalloendopeptidase sensitive to phosphoramidon inactivates the shorter peptide sequences at the Gly3-Phe4 bond, and the 1-13 and 1-17 sequences also at the Arg7-Ile8 bond. The kcat/Km ratios for purified metalloendopeptidase were 20-30 times higher for Leu-enkephalin and the proenkephalin octapeptide than for dynorphins 1-8, 1-13, and 1-17. Dynorphins 1-13 and 1-17 may serve as precursors for the widely distributed CNS neuropeptide dynorphin 1-8 since they were cleaved by a separate SPM endopeptidase insensitive to phosphoramidon. SPM monocarboxypeptidase converted dynorphin 1-13 to 1-12 (release of Lys) and dipeptidyl carboxypeptidase converted dynorphin 1-8 to 1-6; enkephalin octapeptide served as a precursor of Met-enkephalin by sequential action (release of Leu and Arg-Gly) of both carboxypeptidases.

Formation of desTyr dynorphins 5-17 by a purified cytosolic aminopeptidase of rat brain

An aminopeptidase purified to homogeneity from cytosol of rat brain cleaved dynorphins having 5-17 residues and selected proenkephalins at the Tyr-Gly bond only to release Tyr and the desTyr fragments. The enzyme protein consisted of a single polypeptide chain of Mr 103,000 and was inhibited by puromycin, bestatin, and chelating reagents to yield Ki in the micromolar range. Hydrolysis of Leu-2-naphthylamide was inhibited by Dyn 1-5 competitively (Ki, 18 microM); the Km for Dyn 1-5, the best substrate of the series, was 63.8 microM (Kcat/Km ratio 580 mM-1 min-1). Rates of N-tyrosyl release decreased with peptide size; the presence of Arg in position 6 led to 50% loss for Dyn 1-6, and the C-terminal extensions of Dyn 1-13 or 1-17 to a 98% loss in activity as compared to the pentapeptide. Rapid degradation of small peptides is consistent with a paracrine (neurotransmitter) role as compared to the postulated precursor or exocrine roles for the dynorphins with 13 residues or more.

Biochemical and immunological properties of a membrane-bound brain metalloendopeptidase: comparison with thermolysin-like kidney neutral metalloendopeptidase

Membrane-bound neutral metalloendopeptidase ("enkephalinase") was purified from rabbit brain and compared with a homogeneous preparation of a similar enzyme (EC 3.4.24.11) isolated from rabbit kidney. The two enzymes had the same pH optimum and the same apparent molecular weight. They showed identical specificity toward several synthetic substrates and cleaved both Met- and Leu-enkephalin at the Gly-Phe bond. Minor, but significant, differences were found between the two enzymes in the inhibitory constants determined for phosphoramidon and the N-[1(R,S)-carboxy-2-phenylethyl] derivatives of phenylalanyl and alanyl-p-aminobenzoate. A guinea pig antiserum obtained against the rabbit kidney enzyme showed strong crossreactivity with the rabbit brain enzyme when tested in an anticatalytic immunoinhibition assay. Ouchterlony immunodiffusion experiments gave a pattern of precipitation consistent with partial identity of the two enzymes. The kidney enzyme, however, seemed to contain antigenic determinants not present on the brain enzyme. The data indicate that the two enzymes are identical with respect to specificity, pH optimum, and molecular weight, but show minor, although significant, differences in interaction with active-site-directed inhibitors and specific antisera.

Determination of Leu-enkephalin degradation by a soluble enzyme preparation from calf-brain striatum using reversed-phase high-performance liquid chromatography

A rapid procedure for the determination of Leu-enkephalin (Tyr-Gly-Gly-Phe-Leu) and its main metabolic degradation products (Tyr, Tyr-Gly-Gly and Tyr-Gly) by reversed-phase high-performance liquid chromatography was developed. The method has good precision, the coefficient of variation determined in the range 6-20 pmole being 1.5-3% (n = 8), and a very low detection limit of ca. 10 fmole for each metabolite. An unexpectedly high percentage of Tyr-Gly production is observed after enzymatic degradation of Leu-enkephalin by a solubilized enzyme preparation of calf-brain striatum.

The N alpha-acetylenkephalin carboxypeptidase activity of N-acetyltyrosine deacetylase from monkey kidney. Purification, characterization and substrate specificity

N alpha-Acetylenkephalin carboxypeptidase was co-purified with N-acetyltyrosine deacetylase from monkey kidney. Almost 90% of the activity from the homogenate was recovered in a high-speed supernatant without the use of detergents. The crucial steps in the purification were Cibacron Blue F3GA--Sepharose chromatography (involving negative and positive binding sequentially) and metal chelate affinity chromatography. The purified enzyme showed three bands on gel electrophoresis under non-denaturing conditions. All the three bands exhibited both N-acetyltyrosine deacetylase and N-acetylenkephalin carboxypeptidase activity, indicating their co-migration, Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis in the presence and absence of 2-mercaptoethanol gave a single protein band of mol.wt. 34 000. The native enzyme was a dimer of mol.wt. 66 000 as observed on Bio-Gel P-300 gel filtration. The carboxypeptidase removed two amino acids from the C-terminal end of either N-acetyl[Met5]- or N-acetyl[Leu5]-enkephalin. Non-acetylated enkephalins were less active as substrates. Peptides with their carboxy end blocked were inactive as substrates. Models suggested for carboxypeptidase A [Hartsuck & Lipscomb (1971) Enzymes 3, 1-56] support the idea that the kidney N-acetylated aromatic amino acid deacetylase or acylase III [Endo (1978) Biochim. Biophys. Acta 523, 207-217] can act as a carboxypeptidase on peptides having hydrophobic amino acids at the C-terminal end.

Degradation of enkephalins: the search for an enkephalinase

The opioid peptides methionine-enkephalin and leucine-enkephalin appear to exert their biological effects through a receptor mediated mechanism. There appears to be three potential mechanisms for enkephalin degradation which could serve to control enkephalin levels in the vicinity of enkephalin receptors. These are, 1) cleavage of the tyrosyl-glycine bond by aminopeptidases, 2) cleavage of the glycl-glycine bond by a dipeptidyl aminopeptidase, and 3) cleavage of the glycyl-phenylalanine bond by a dipeptidyl carboxypeptidase. In this review the biochemical properties of these potential enkephalinases are described, and the evidence for each acting as an enkephalinase is reviewed.

Conversion of Met-enkephalin-Arg6-Phe7 by a purified brain carboxypeptidase (cathepsin A)

A carboxypeptidase A-like enzyme known as cathepsin A was purified from rat brain by extraction with Triton X-100, followed by chromatography on DEAE-Sephadex A-50 and gel-filtration. Purified enzyme was devoid of contamination of tryptic-like enzymes, by dipeptidyl carboxypeptidase (angiotensin converting enzyme) and of enkephalinnases cleaving the Tyr-Gly and Gly-Phe bonds of Met-enkephalin. Incubation of purified enzyme with Met-enkephalin-Arg6-Phe7, a naturally occurring enkephalin surrogate, was accompanied by the release of three products as detected by reverse phase HPLC. Subsequent amino acid analysis identified these as Phe, Met-enkephalin-Arg6, and Met-enkephalin, indicating cleavage at the Arg6-Phe7 and Met5-Phe6 bonds. Breakdown followed a precursor-product-relationship with the hexapeptide appearing as an intermediate and the pentapeptide as the final product. The Km for cleavage of the Arg-Phe site was 0.09 mM. Rates of cleavage of hexa- and heptapeptide accord with those found for synthetic N-protected dipeptide substrates. Cathepsin A does not act as an enkephalinase in the accepted sense, since no breakdown of Met-enkephalin was observed.