Rat brain and kidney metalloendopeptidase: enkephalin heptapeptide conversion to form a cardioactive neuropeptide, Phe-Met-Arg-Phe-amide

Human myeloid plasma membrane glycoprotein CD13 (gp150) is identical to aminopeptidase N

To determine the primary structure of CD13, a 150-kD cell surface glycoprotein originally identified on subsets of normal and malignant human myeloid cells, we isolated the complete sequences encoding the polypeptide in overlapping complementary DNA (cDNA) clones. The authenticity of our cDNA clones was demonstrated by the ability of the coding sequences, subcloned in a retroviral expression vector, to mediate expression of bona fide CD13 molecules at the surface of transfected mouse fibroblasts. The nucleotide sequence predicts a 967 amino acid integral membrane protein with a single, 24 amino acid hydrophobic segment near the amino terminus. Amino-terminal protein sequence analysis of CD13 molecules indicated that the hydrophobic segment is not cleaved, but rather serves as both a signal for membrane insertion and as a stable membrane-spanning segment. The remainder of the molecule consists of a large extracellular carboxyterminal domain, which contains a pentapeptide consensus sequence characteristic of members of the zinc-binding metalloprotease superfamily. Sequence comparisons with known enzymes of this class revealed that CD13 is identical to aminopeptidase N, a membrane-bound glycoprotein thought to be involved in the metabolism of regulatory peptides by diverse cell types, including small intestinal and renal tubular epithelial cells, macrophages, granulocytes, and synaptic membranes prepared from cells of the central nervous system.

Failure of ES 52, a highly potent enkephalinase inhibitor, to affect nociceptive transmission by rat dorsal horn convergent neurones

The effects of ES 52, a highly potent derivative of the enkephalinase (enkephalin-dipeptidylcarboxypeptidase) inhibitor thiorphan, were studied on nociceptive activities of dorsal horn convergent neurones in the anaesthetized rat. Neither the C-fibre component of the responses elicited by supramaximal electrical stimulation of the hindpaw excitatory receptive fields nor diffuse noxious inhibitory controls triggered by immersion of the tail in 46-48 degrees C waterbaths, were affected by ES 52. Thus we conclude that, in our experimental conditions, modulations of the transmission of nociceptive messages at the spinal level are not greatly modified by specifically blocking the degradation of enkephalins. If a major role for enkephalinase (vs aminopeptidase) in the catabolism of enkephalins at the spinal level can be confirmed, then comparison of the present data with our previous results obtained using the opioid antagonist naloxone, might suggest a predominant role for proenkephalin B products (i.e. dynorphins and/or alpha-neo-endorphin) in modulating nociceptive transmission in the spinal cord.

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.

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.

Enkephalin heptapeptide (Tyr-Gly-Gly-Phe-Met-Arg-Phe)-induced bronchospasm in guinea pigs: a non-naloxone reversible phenomenon

Enkephalin heptapeptide (Tyr-Gly-Gly-Phe-Met-Arg-Phe; MEAP ), is an enkephalin-related peptide derived from pre-pro-enkephalin A. Distribution studies have shown that MEAP is localized in the lung; in this organ there is very little Met-enkephalin (ME). To test the direct actions of MEAP in a tissue where it is differentially distributed from ME, MEAP was given intratracheally to anesthetized guinea pigs. MEAP produced a dose-related bronchospasm characterized by a slow onset (30-90 sec) and a duration exceeding 5 min. No significant effects on heart rate or blood pressure were observed concomitant with the pulmonary effects. ME or ethylketocyclazocine, tested at the same doses as MEAP , did not produce a bronchospasm. Moreover, naloxone failed to block MEAP 's effects. Therefore, it appears as if the MEAP -induced bronchospasm does not have an opioid-related mechanism of action. The bronchospasm produced by MEAP was not blocked by an anti-histamine (chlorpheniramine) or by vagotomy. However, inhibition of cyclo-oxygenase by ibuprofen blocked the MEAP -induced bronchospasm, suggesting that an arachidonic acid metabolite was involved in the effect. Blockade of MEAP 's pulmonary effects by an inhibitor of thromboxane synthetase (SQ 80338) and not by a leukotriene antagonist (FPL 55712) implied that MEAP 's mechanism of action may be via bronchospastic thromboxanes. These data are the first to demonstrate a non-opioid, thromboxane-mediated action of an enkephalin-related peptide.

Inhibition of metalloendopeptidases by 2-mercaptoacetyl-dipeptides

A series of 2-mercaptoacetyl-dipeptides, a potential group of metalloendopeptidase inhibitors, has been synthesized by coupling the N-hydroxysuccinimide ester of S-acetyl-2-mercaptoacetic acid with hydrophobic dipeptide methyl ester hydrochlorides, followed by hydrolysis with NaOH in aqueous methanol and acidification with HCl. Thus, the 2-mercaptoacetyl derivatives of L-phenylalanyl-L-leucine, L-leucyl-L-phenylalanine and L-leucyl-D-phenylalanine were prepared. The first two compounds inhibit effectively thermolysin from Bacillus thermoproteolyticus and a metalloendopeptidase isolated from Streptomyces griseus, with Ki values in the micromolar range or below. The third compound inhibits the two enzymes only poorly, showing the stereospecificity of the inhibition process. These inhibitors should provide a useful tool for the study of bacterial and mammalian metalloendopeptidases (or dipeptidyl carboxypeptidases) and for the assessment of their physiological role.

Protease inhibitors implicate metalloendoprotease in synaptic transmission at the mammalian neuromuscular junction

Metalloendoproteases have been implicated in the calcium-dependent exocytosis of histamine from mast cells and in the calcium-dependent fusion of myoblasts. Because metalloendoproteases have also been identified in nervous tissue, we investigated the possibility that these proteases may be involved in neurotransmitter release at mammalian synapses. End-plate potentials were recorded intracellularly from mouse diaphragm/phrenic nerve preparations in vitro. The amplitude of the endplate potentials were reduced by as much as 90% during bath application of phosphoramidon, a specific inhibitor of metalloendoproteases, and by carbobenzoxy-dipeptide-amide synthetic substrates for metalloendoproteases. Only those synthetic dipeptides in which the amino group of the peptide bond was provided by a bulky hydrophobic amino acid, such as phenylalanine or leucine, which are substrates for metalloendoproteases, reduced synaptic transmission. Synthetic substrates in which proline or glycine provided the amino group of the peptide bond, which are not metalloendoprotease substrates, had little or no effect on the amplitude of end-plate potentials. The ability of synthetic substrates to reduce synaptic transmission was also dependent on the amino acid that provided the carboxyl group of the peptide bond, with glycine being more effective than tyrosine or serine. In addition, synthetic dipeptides with free carboxyl or amino termini, which have a low affinity for metalloendoproteases, also had little effect on synaptic transmission. The inhibition of synaptic transmission by phosphoramidon and the synthetic substrates occurred within 2 to 3 min and was completely reversible. Neither phosphoramidon nor the synthetic substrates altered the dose-response characteristics of the postsynaptic membrane to bath-applied carbachol. These results suggest that synaptic transmission requires the activity of a metalloendoprotease in the presynaptic nerve terminal and that proteolysis may be an important step during neurotransmitter exocytosis.

The heptapeptide Met-enkephalin-Arg6, Phe7 is released from rat striatum in vitro by high potassium

We present evidence that the heptapeptide Met-enkephalin-Arg6, Phe7 is released from rat striatal slices following depolarization by a high concentration of KCl. The heptapeptide-immunoreactive material released in the incubation medium, which is shown by HPLC and radioimmunoassay of serial dilutions to represent authentic heptapeptide, is detectable in the incubation media only in the presence of a cocktail of peptidase inhibitors containing thiorphan (0.1 microM), captopril (1 microM) and bestatin (20 microM).