Neurokinin B is hydrolysed by synaptic membranes and by endopeptidase-24.11 (enkephalinase) but not by angiotensin converting enzyme

The major site of hydrolysis was the Gly8-Leu9 bond. Angiotensin converting enzyme (peptidyl dipeptidase A, EC 3.4.15.1) from pig kidney hydrolysed substance P releasing the C-terminal tripeptide Gly-Leu-MetNH2 but failed to hydrolyse neurokinin B. Pig brain striatal synaptic membranes hydrolysed neurokinin B producing a similar pattern of products as did endopeptidase-24.11. Substantial inhibition of this activity was achieved with the selective inhibitor phosphoramidon. A combination of phosphoramidon and bestatin abolished the hydrolysis of neurokinin B by synaptic membranes. Thus, a bestatin-sensitive aminopeptidase may play a role in the synaptic metabolism of neurokinin B in addition to endopeptidase-24.11. This aminopeptidase appears to be distinct from aminopeptidase N (EC 3.4.11.2).     

The metabolism of neuropeptides. The hydrolysis of peptides, including enkephalins, tachykinins and their analogues, by endopeptidase-24.11.

Endopeptidase-24.11 (EC 3.4.24.11), purified to homogeneity from pig kidney, was shown to hydrolyse a wide range of neuropeptides, including enkephalins, tachykinins, bradykinin, neurotensin, luliberin and cholecystokinin. The sites of hydrolysis of peptides were identified, indicating that the primary specificity is consistent with hydrolysis occurring at bonds involving the amino group of hydrophobic amino acid residues. Of the substrates tested, the amidated peptide substance P is hydrolysed the most efficiently (Km = 31.9 microM; kcat. = 5062 min-1). A free alpha-carboxy group at the C-terminus of a peptide substrate is therefore not essential for efficient hydrolysis by the endopeptidase. A large variation in kcat./Km values was observed among the peptide substrates studied, a finding that reflects a significant influence of amino acid residues, remote from the scissile bond, on the efficiency of hydrolysis. These subsite interactions between peptide substrate and enzyme thus confer some degree of functional specificity on the endopeptidase. The inhibition of endopeptidase-24.11 by several compounds was compared with that of pig kidney peptidyldipeptidase A (EC 3.4.15.1). Of the inhibitors examined, only N-[1(R,S)-carboxy-2-phenylethyl]-Phe-p-aminobenzoate inhibited endopeptidase-24.11 but not peptidyldipeptidase. Captopril (D-3-mercapto-2-methylpropanoyl-L-proline), Teprotide (pGlu-Trp-Pro-Arg-Pro-Gln-Ile-Pro-Pro) and MK422 [N-[(S)-1-carboxy-3-phenylpropyl]-L-Ala-L-Pro] were highly selective as inhibitors of peptidyldipeptidase. Although not wholly specific, phosphoramidon was a more potent inhibitor of endopeptidase-24.11 than were any of the synthetic compounds tested.     

Pyridinedicarboxylates, the first mechanism-derived inhibitors for prolyl 4-hydroxylase, selectively suppress cellular hydroxyprolyl biosynthesis. Decrease in interstitial collagen and Clq secretion in cell culture.

Peptidyl-dipeptidase A (angiotensin converting enzyme; ACE, EC 3.4.15.1), has been purified from pig kidney and striatum by affinity chromatography employing the selective inhibitor lisinopril as ligand. The inclusion of a 2.8 nm spacer arm improved the yield of the enzyme compared with the 1.4 nm spacer arm described in previous work. Two forms of striatal ACE (Mr 180,000 and 170,000), but only a single form of kidney ACE (Mr 180,000), were isolated by this procedure. Both forms of striatal ACE were recognized by a polyclonal antibody to kidney ACE. No significant differences in substrate specificity or inhibitor sensitivity between kidney and striatal ACE could be detected. In particular, the amidated neuropeptide, substance P, was hydrolysed identically by both preparations and no significant hydrolysis of the related tachykinin peptides neurokinin A and neurokinin B could be detected. After chemical or enzymic deglycosylation, kidney and both forms of striatal ACE migrated identically on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis with an apparent Mr of 150,000. We suggest that the two detectable forms of ACE in pig brain are not isoenzymes but are the result of differential glycosylation in different cell types in the brain. It appears that ACE, unlike endopeptidase-24.11, does not have the general capacity to hydrolyse and inactivate the tachykinin peptides at a significant rate in brain.     

The metabolism of neuropeptides. Endopeptidase-24.11 in human synaptic membrane preparations hydrolyses substance P.

Synaptic membrane preparations from human striatum and human diencephalon were shown to contain a phosphoramidon-sensitive metalloendopeptidase that appeared identical with endopeptidase-24.11. The activity of endopeptidase-24.11 was determined with an enzymic assay employing [D-Ala2,Leu5]enkephalin as substrate, and its distribution in human brain was similar to that in pig brain, with the striatum containing the highest levels. The choroid plexus and pons also contained substantial activity. A good correlation (r = 0.97) was obtained for the distribution of the endopeptidase in pig brain and pituitary by the enzymic assay and by an immunoradiometric assay specific for pig endopeptidase-24.11. Synaptic membrane preparations from human striatum and diencephalon hydrolysed substance P at the same sites as did preparations of pig striatal synaptic membranes, and hydrolysis was substantially abolished by phosphoramidon. These results suggest that endopeptidase-24.11 is the principal enzyme hydrolysing substance P in human synaptic membrane preparations.     

The hydrolysis of alpha-human atrial natriuretic peptide by pig kidney microvillar membranes is initiated by endopeptidase-24.11.

alpha-Human atrial natriuretic peptide, a 28-amino-acid-residue peptide, was rapidly hydrolysed by pig kidney microvillar membranes in vitro, with a t1/2 of 8 min, comparable with the rate observed with angiotensins II and III. The products of hydrolysis were analysed by h.p.l.c., the pattern obtained with membranes being similar to that with purified endopeptidase-24.11 (EC 3.4.24.11). No hydrolysis by peptidyl dipeptidase A (angiotensin I converting enzyme, EC 3.4.15.1) was observed. The contribution of the various microvillar membrane peptidases was assessed by including specific inhibitors. Phosphoramidon, an inhibitor of endopeptidase-24.11, caused 80-100% suppression of the products. Captopril and amastatin (inhibitors of peptidyl dipeptidase A and aminopeptidases respectively) had no significant effect. Hydrolysis at an undefined site within the disulphide-linked ring occurred rapidly, followed by hydrolysis at other sites, including the Ser25--Phe26 bond.     

The hydrolysis of brain and atrial natriuretic peptides by porcine choroid plexus is attributable to endopeptidase-24.11.

The hydrolysis of the porcine 26-residue brain natriuretic peptide (BNP-26) and its counterpart human 28-residue atrial natriuretic peptide (alpha-hANP) by pig membrane preparations and purified membrane peptidases was studied. When the two peptides were incubated with choroid plexus membranes, the products being analysed by h.p.l.c., alpha-hANP was degraded twice as fast as BNP. The h.p.l.c. profiles of alpha-hANP hydrolysis, in short incubations with choroid plexus membranes, yielded alpha hANP' as the main product, this having been previously shown to be the result of hydrolysis at the Cys7-Phe8 bond. In short incubations this cleavage was inhibited 84% by 1 microM-phosphoramidon, a specific inhibitor of endopeptidase-24.11. BNP-26 was hydrolysed by choroid plexus membranes, kidney microvillar membranes and purified endopeptidase-24.11 in a manner that yielded identical h.p.l.c. profiles. In the presence of phosphoramidon, hydrolysis by the choroid plexus membranes was 94% inhibited. Captopril had no effect and, indeed, no hydrolysis of BNP-26 by peptidyl dipeptidase A (angiotensin-converting enzyme) was observed even after prolonged incubation with the purified enzyme. The stepwise hydrolysis of BNP-26 by endopeptidase-24.11 was investigated by sequencing the peptides produced during incubation. The initial product resulted from hydrolysis at Ser14-Leu15, thereby opening the ring. This product (BNP') was short-lived; further degradation involved hydrolysis at Ile12-Gly13, Arg8-Leu9, Gly17-Leu18, Val22-Leu23, Arg11-Ile12 and Cys4-Phe5. Thus endopeptidase-24.11 is the principal enzyme in renal microvillar and choroid plexus membranes hydrolysing BNP-26 and alpha-hANP.     

Metabolic stability of the LHRH antagonist antide to cell-surface peptidases

The susceptibility to hydrolysis of LHRH and the decapeptide analogue Antide has been compared. The hydrolysis of LHRH by pig kidney brush border membranes is inhibited by phosphoramidon (I50 = 5.6 nM) implicating endopeptidase-24.11 in the initiation of hydrolysis. Under conditions in which LHRH is fully degraded by brush border membranes, Antide was completely resistant to hydrolysis. Similar results were obtained with purified preparations of both endopeptidase-24.11 and angiotensin converting enzyme. These data confirm that the remarkable duration of action of Antide is due principally to its stability to hydrolysis by cell-surface peptidases.     

Purification of endopeptidase-24.11 (''enkephalinase'') from pig brain by immunoadsorbent chromatography.

Membrane preparations from striatum of pig brain contain endopeptidase activity towards iodoinsulin B-chain. Only 50% of the hydrolysis of insulin B-chain is inhibitable by phosphoramidon, and DEAE-cellulose chromatography can resolve the phosphoramidon-sensitive and -insensitive activities. The former activity (now designated 'endopeptidase-24.11') is responsible for hydrolysis of [D-Ala2,Leu5]enkephalin and is identical with an enzyme in brain that has previously been referred to as 'enkephalinase'. Pig striatal endopeptidase-24.11 has now been purified to homogeneity in a single step by immunoadsorbent chromatography using a monoclonal antibody. The overall purification was 23 000-fold, with a yield of 30%. The brain enzyme appears to be identical with kidney endopeptidase-24.11 in amino acid composition as well as by immunological and kinetic criteria. However, it differs slightly in apparent subunit size (Mr = 87 000), attributable to differences in glycosylation.     

A monoclonal antibody to kidney endopeptidase-24.11. Its application in immunoadsorbent purification of the enzyme and immunofluorescent microscopy of kidney and intestine.

Hybridoma methodology has been used to produce a monoclonal antibody, GK 7C2, that binds specifically to microvillar endopeptidase-24.11 (EC 3.4.24.11). The antibody (an immunoglobulin G) was generated by fusion of mouse plasmacytoma cells with splenocytes from a Balb/c mouse immunized with pig kidney microvillar membranes. The identity of the antigen recognized by GK 7C2 was established by immuno-precipitation from detergent-solubilized pig kidney microvilli. The protein had an apparent Mr of 90 000 and contained endopeptidase activity sensitive to phosphoramidon. The identity was confirmed by immunoadsorbent purification of endopeptidase-24.11 by a column to which GK 7C2 had been attached. The endopeptidase, purified in a yield of 40%, was electrophoretically homogeneous and of specific activity comparable with that purified by other means. Fluorescence microscopy established that GK 7C2 bound specifically to the luminal membranes of kidney tubules and the intestinal mucosa. Thus endopeptidase-24.11 is located in the brush-border membranes of both cell types.     

The metabolism of neuropeptides. Hydrolysis of peptides by the phosphoramidon-insensitive rat kidney enzyme ''endopeptidase-2'' and by rat microvillar membranes.

Endopeptidase-2, the second endopeptidase in rat kidney brush border [Kenny & Ingram (1987) Biochem. J. 245, 515-524] has been further characterized in regard to its specificity and its contribution to the hydrolysis of peptides by microvillar membrane preparations. The peptide products were identified, after incubating luliberin, substance P, bradykinin and angiotensins I, II and III with the purified enzyme. The bonds hydrolysed were those involving a hydrophobic amino acid residue, but this residue could be located at either the P1 or P1' site. Luliberin was hydrolysed faster than other peptides tested, followed by substance P and bradykinin. Human alpha-atrial natriuretic peptide and the angiotensins were only slowly attacked. Oxytocin and [Arg8]vasopressin were not hydrolysed. No peptide fragments were detected on prolonged incubation with insulin, cytochrome c, ovalbumin and serum albumin. In comparison with pig endopeptidase-24.11 the rates for the susceptible peptides were, with the exception of luliberin, much lower for endopeptidase-2. Indeed, for bradykinin and substance P the ratio kcat./Km was two orders of magnitude lower. Since both endopeptidases are present in rat kidney microvilli, an assessment was made of the relative contributions to the hydrolysis of luliberin, bradykinin and substance P. Only for the first named was endopeptidase-2 the dominant enzyme; for bradykinin it made an equal, and for substance P a minor, contribution.     

Catabolism of gastrin releasing peptide and substance P by gastric membrane-bound peptidases

The catabolism of two gastric neuropeptides, the C-terminal decapeptide of gastrin releasing peptide-27 (GRP10) and substance P (SP), by membrane-bound peptidases of the porcine gastric corpus and by porcine endopeptidase-24.11 ("enkephalinase") has been investigated. GRP10 was catabolized by gastric muscle peptidases (specific activity 1.8 nmol min-1 mg-1 protein) by hydrolysis of the His8-Leu9 bond and catabolism was inhibited by phosphoramidon (I50 approx. 10(-8) M), a specific inhibitor of endopeptidase-24.11. The same bond in GRP10 was cleaved by purified endopeptidase-24.11, and hydrolysis was equally sensitive to inhibition by phosphoramidon. SP was catabolized by gastric muscle peptidases (specific activity 1.7 nmol min-1 mg-1 protein) by hydrolysis of the Gln6-Phe7, Phe7-Phe8 and Gly9-Leu10 bonds, which is identical to the cleavage of SP by purified endopeptidase-24.11. The C-terminal cleavage of GRP10 and SP would inactivate the peptides. It is concluded that a membrane-bound peptidase in the stomach wall catabolizes and inactivates GRP10 and SP and that, in its specificity and sensitivity to phosphoramidon, this peptidase resembles endopeptidase-24.11.     

Metabolism of neuropeptides. Hydrolysis of the angiotensins, bradykinin, substance P and oxytocin by pig kidney microvillar membranes.

Microvillar membranes derived from the brush border of the renal proximal tubule are very rich in peptidases. Pig kidney microvilli contain endopeptidase-24.11 associated with a battery of exopeptidases. The manner by which some neuropeptides are degraded by the combined attack of the peptidases of this membrane has been investigated. The contribution of individual peptidases was assessed by including inhibitors (phosphoramidon, captopril, amastatin and di-isopropyl fluorophosphate) with the membrane fraction when incubated with the peptides. Substance P, bradykinin and angiotensins I, II and III and insulin B-chain were rapidly hydrolysed by kidney microvilli. Oxytocin was hydrolysed much more slowly, but no products were detected from [Arg8]vasopressin or insulin under the conditions used for other peptides. The peptide bonds hydrolysed were identified and the contributions of the different peptidases were quantified. For each of the susceptible peptides, the main contribution came from endopeptidase-24.11 (inhibited by phosphoramidon). Peptidyl dipeptidase A (angiotensin-I-converting enzyme) was of less importance, even in respect of angiotensin I and bradykinin. When [2,3-Pro3,4-3H]bradykinin was also investigated at a lower concentration (20 nM), the conclusions in regard to the contributions of the two peptidases were unchanged. The possibility that endopeptidase-24.11 might attack within the six-residue disulphide-bridged rings of oxytocin and vasopressin was examined by dansyl(5-dimethylaminonaphthalene-1-sulphonyl)ation and by reduction and carboxymethylation of the products after incubation. Additional peptides were only observed after prolonged incubation, consistent with hydrolysis at the Tyr2-Ile3 and Tyr2-Phe3 bonds respectively. These results show that a range of neuropeptides are efficiently degraded by microvillar membranes and that endopeptidase-24.11 plays a key role in this process.     

Ectoenzymes of the kidney microvillar membrane. Affinity purification, characterization and localization of the phospholipase C-solubilized form of renal dipeptidase.

Renal dipeptidase (EC 3.4.13.11) was solubilized from pig kidney microvillar membranes with bacterial phosphatidylinositol-specific phospholipase C and then purified by affinity chromatography on cilastatin-Sepharose. The enzyme was apparently homogeneous on SDS/polyacrylamide-gel electrophoresis with an Mr of 47,000. Immunohistochemical analysis of the distribution of the dipeptidase showed it to be concentrated in the brush-border region of the proximal tubules in close association with endopeptidase-24.11) (EC 3.4.24.11). The purified dipeptidase was shown to contain 1 mol of inositol/mol and to possess the cross-reacting determinant characteristic of the glycosyl-phosphatidylinositol membrane-anchoring domain. The glycoprotein nature of renal dipeptidase was confirmed by chemical and enzymic deglycosylation. These results establish renal dipeptidase as a glycosyl-phosphatidylinositol-anchored ectoenzyme of the microvillar membrane.     

Membrane Localization of Endopeptidase-24.11 and Peptidyl Dipeptidase A (Angiotensin Converting Enzyme) in the Pig Brain: A Study Using Subcellular Fractionation and Electron Microscopic Immunocytochemistry

Brains from piglets were dissected and a block of tissue including the substantia nigra, globus pallidus, and entopeduncular nucleus was homogenized and then fractionated on discontinuous Percoll gradients. Ligand-binding assays using (-)-[3H]nicotine and [3H]quinuclidinyl benzilate served to delineate fractions containing nicotinic and muscarinic acetylcholine receptors. In this system endopeptidase-24.11 exhibited a biphasic distribution, consistent with its presence on both pre- and postsynaptic membranes. Peptidyl dipeptidase A (angiotensin converting enzyme; ACE) was associated with membrane fractions containing muscarinic receptors. An immunoblot of these fractions with an affinity-purified polyclonal antibody to ACE revealed only the neuronal form of ACE (Mr 170,000), the endothelial form (Mr 180,000) being undetectable. Electron microscopic immunoperoxidase staining of the substantia nigra, with an affinity-purified antibody to endopeptidase-24.11 at the preembedding stage, showed this antigen to be confined to the plasma membranes of boutons, axons, and some dendrites. Both pre- and postsynaptic membranes were stained, and occasionally other regions of the dendritic membrane were positive. No staining of synaptic vesicles within the boutons was observed. Thus, two independent approaches indicate that endopeptidase-24.11 is present on both pre- and postsynaptic membranes in the pig substantia nigra. The subcellular fractionation suggests that neuronal ACE is confined to dendritic membranes.     

Substance P and neurokinin A metabolism by cultured human skeletal muscle myocytes and fibroblasts

A recent study determined that cultured human skeletal muscle adult myoblasts, myotubes, and fibroblasts degraded angiotensins and kinins via neutral endopeptidase-24.11 (NEP-24.11; EC 3.4.24.11) and aminopeptidase N (APN; EC 3.4.11.2). Due to the possible importance of other peptides to skeletal muscle blood flow and function, the present study looked specifically at the metabolism of the neurokinins substance P (SP) and neurokinin A (NKA) by skeletal muscle peptidases. The results show that SP is degraded not only by NEP-24.11, but also sequentially by dipeptidyl(amino)peptidase IV (DAP IV; EC 3.4.14.5)/APN. NKA is unaffected by DAP IV but is metabolized by NEP-24.11 and APN. NEP-24.11 was inhibited by phosphoramidon (IC₅₀ = 80 nM), thiorphan and ZINCOV, DAP IV by diprotin A (IC₅₀ = 8 μM), and APN by amastatin (IC₅₀ = 50 nM) and bestatin (IC₅₀ = 100 μM). Skeletal muscle myocyte and fibroblast metabolism of SP and NKA may regulate local skeletal muscle vascular and extravascular functions including SP- and NKA-mediated nerve-induced vasodilation. Inhibition of both NEP-24.11 and DAP IV/APN may increase skeletal muscle blood flow and decrease peripheral vascular resistance via potentiation of local neurokinin levels.     

Membrane peptidases in the pig choroid plexus and on other cell surfaces in contact with the cerebrospinal fluid.

A comprehensive survey of 11 peptidases, all of which are markers for renal microvillar membranes, has been made in membrane fractions prepared from pig choroid plexus. Two fractionation schemes were explored, both depending on a MgCl2-precipitation step, the preferred one having advantages in speed and yield of the activities. The specific activities of the peptidases in the choroid-plexus membranes were, with the exception of carboxypeptidase M, lower than in renal microvillar membranes: those of aminopeptidase N, peptidyl dipeptidase A ('angiotensin-converting enzyme') and gamma-glutamyltransferase were 3-5-fold lower, those of aminopeptidase A and endopeptidase-24.11 were 12-15 fold lower, and those of dipeptidyl peptidase IV and aminopeptidase W were 50-70-fold lower. Carboxypeptidase M had a similar activity in both membranes. Alkaline phosphatase and (Na+ + K+)-activated ATPase were more active in the choroid-plexus membranes. No activity for microsomal dipeptidase, aminopeptidase P and carboxypeptidase P could be detected. Six of the peptidases and (Na+ + K+)-activated ATPase were also studied by immunoperoxidase histochemistry at light- and electron-microscopic levels. Endopeptidase-24.11 and (Na+ + K+)-activated ATPase were uniquely located on the brush border, and the other two peptidases appeared to be much more abundant on the endothelial lining of microvessels. Dipeptidyl peptidase IV and aminopeptidase W were also detected in microvasculature. Pial membranes associated with the brain and spinal cord also stained positively for endopeptidase-24.11, aminopeptidase N and peptidyl dipeptidase A. The immunohistochemical studies indicated the subcellular fractionation did not discriminate between membranes derived from epithelial cells (i.e. microvilli) and those from endothelial cells. The possible significance of these studies in relation to neuropeptide metabolism and the control of cerebrospinal fluid production is discussed.     

Hydrolysis of alpha-human atrial natriuretic peptide in vitro by human kidney membranes and purified endopeptidase-24.11. Evidence for a novel cleavage site.

alpha-Human atrial natriuretic peptide (hANP) is secreted by the heart and acts on the kidney to promote a strong diuresis and natriuresis. In vivo it has been shown to be catabolized partly by the kidney. Crude microvillar membranes of human kidney degrade 125I-ANP at several internal bonds generating metabolites among which the C-terminal fragments were identified. Formation of the C-terminal tripeptide was blocked by phosphoramidon, indicating the involvement of endopeptidase-24.11 in this cleavage. Subsequent cleavages by aminopeptidase(s) yielded the C-terminal dipeptide and free tyrosine. Using purified endopeptidase 24.11, we identified seven sites of hydrolysis in unlabelled alpha-hANP: the bonds Arg-4-Ser-5, Cys-7-Phe-8, Arg-11-Met-12, Arg-14-Ile-15, Gly-16-Ala-17, Gly-20-Leu-21 and Ser-25-Phe-26. However, the bonds Gly-16-Ala-17 and Arg-4-Ser-5 did not fulfil the known specificity requirements of the enzyme. Cleavage at the Gly-16-Ala-17 bond was previously observed by Stephenson & Kenny [(1987) Biochem. J. 243, 183-187], but this is the first report of an Arg-Ser bond cleavage by this enzyme. Initial attack of alpha-hANP by endopeptidase-24.11 took place at a bond within the disulphide-linked loop and produced a peptide having the same amino acid composition as intact ANP. The bond cleaved in this metabolite was determined as the Cys-7-Phe-8 bond. Determination of all the bonds cleaved in alpha-hANP by endopeptidase-24.11 should prove useful for the design of more stable analogues, which could have therapeutic uses in hypertension.     

Calmodulin-dependent enzymic phosphorylation of dolichol in Tetrahymena pyriformis.

Endopeptidase-24.11 (EC 3.4.24.11) was purified from pig intestinal microvilli by immunoadsorbent chromatography, using antibodies raised to kidney endopeptidase-24.11. In many respects, the kidney and intestinal enzymes were indistinguishable, but some structural differences were demonstrated. In particular, the detergent form of the intestinal enzyme had an apparent subunit Mr of 95000, which, on treatment with trypsin, fell to a value of 89000, identical with that of the kidney form. The intestinal enzyme contained 3-4% more carbohydrate and many more fucose residues than that from kidney. Although these results show that post-translational processing was different in the two cell types, the possibility that the primary translation products also differed cannot be excluded.     

Proteolytic Inactivation of Substance P and Neurokinin A in the Longitudinal Muscle Layer of Guinea Pig Small Intestine

Membrane vesicles, showing a 21 +/- 2-fold enrichment in the activity of 5'-nucleotidase and a 11 +/- 4-fold enrichment in the activity of angiotensin-converting enzyme relative to homogenate, were prepared from the myenteric plexus-containing longitudinal muscle layer of guinea pig ileum. Incubation of the vesicles with substance P and neurokinin A led to degradation of the peptides, and metabolites were isolated by reverse-phase HPLC and identified by amino acid composition. Cleavages of substance P between Glu6-Phe7, Phe7-Phe8, and Gly9-Leu10 and of neurokinin A between Gly8-Leu9 were observed and could be inhibited in a dose-dependent manner by phosphoramidon, an inhibitor of neutral endopeptidase 24.11. Formation of these metabolites was not completely inhibited by this agent, indicating that a phosphoramidon-insensitive form of endopeptidase 24.11 was present in the gut. Substance P was resistant to degradation by aminopeptidases, but neurokinin A was a substrate for bestatin-sensitive aminopeptidase(s), so that the neurokinin A (3-10) fragment represented the predominant metabolite in the chromatograms. The rate of formation of all the metabolites was not inhibited by enalapril and not enhanced by an increased Cl- concentration, indicating that angiotensin-converting enzyme was unimportant in the degradation process. Degradation of neurokinin A by the vesicles (Km 30 microM; Vmax 7.2 +/- 0.8 nmol min-1 mg of protein-1) was more rapid than degradation of substance P (Km 25 microM; Vmax 4.4 +/- 0.4 nmol min-1 mg of protein-1).     

Endopeptidase-24.11 Is the Integral Membrane Peptidase Initiating Degradation of Somatostatin in the Hippocampus

Abstract: The membrane metalloenzyme endopeptidase-24.11 has been localized by immunocytochemistry in the porcine hippocampus in the stratum oriens and stratum radiatum. Endopeptidase-24.11 was found to be ∼10-fold more abundant in a striatal than a hippocampal membrane preparation. Both somatostatin-28 and somatostatin-14 were metabolized by endopeptidase-24.11, but the kinetics of hydrolysis markedly favoured the smaller form of the neuropeptide. After phase separation with Triton X-114 of striatal and hippocampal membrane preparations, and by using selective inhibitors, the major (>80%) somatostatin-metabolizing activity was found to partition into the detergent-rich phase and was attributable predominantly to endopeptidase-24.11. The residual activity observed in the presence of the selective endopeptidase-24.11 inhibitor phosphoramidon was blocked by Pro-Ile or N -[1-( RS )-carboxy-3-phenylpropyl]-Ala-Ala-Phe- p -aminobenzoate, inhibitors of endopeptidase-24.16 and endopeptidase-24.15, respectively. However, Pro-Ile, at comparable concentrations, was shown to inhibit endopeptidase-24.11, challenging the validity of its use as a selective inhibitor of endopeptidase-24.16. The immunocytochemical and Triton X-114 phase-separation data implicate endopeptidase-24.11, rather than endopeptidase-24.16 or endopeptidase-24.15, as the major physiological somatostatin-degrading neuropeptidase in the striatum and hippocampus.