Inhibition of porcine kidney "enkephalinase" by substituted-N-carboxymethyl dipeptides

Inhibition of thermolysin and neutral endopeptidase 24.11 by a novel glutaramide derivative: X-ray structure determination of the thermolysin-inhibitor complex

Determination of the X-ray structure of thermolysin-inhibitor complexes has proven useful in aiding our understanding of the mode of binding of inhibitors of related, physiologically important, mammalian zinc peptidases including neutral endopeptidase EC 3.4.24.11 and angiotensin-converting enzyme. Here we describe the mode of binding to crystalline thermolysin of N-[1-(2(R,S)-carboxy-4-phenylbutyl)-cyclopentylcarbonyl]-(S) -tryptophan (CCT). CCT is an analogue of both candoxatrilat, a potent inhibitor of neutral endopeptidase 24.11, and of the 5-indanyl ester prodrug candoxatril, which is under clinical evaluation as a potential therapy for congestive heart failure. CCT differs from the previously studied N-carboxyalkyl dipeptide CLT [N-(S)-(1-carboxy-3-phenylpropyl)-(S)-leucyl-(S)-tryptophan] in several important respects. It has a highly constrained gem-cyclopentyl P1' substituent and lacks the characteristic imino nitrogen substituent of CLT. The structure determination shows that, notwithstanding the conformational influence of the gem-cyclopentyl substituent, CCT binds within the active site of thermolysin in a similar manner to CLT. Although the characteristic hydrogen bond between the imino nitrogen of CLT and thermolysin is absent in CCT, the affinities of the two inhibitors for the enzyme are virtually identical. These results illustrate the importance of considering not only those hydrogen bonds that are formed in an enzyme-ligand complex but also the other hydrogen bonds that may be lost due to desolvation of the enzyme and ligand on formation of the complex. In addition, the overall conformational demands placed upon a ligand in order to achieve receptor interaction may be critically important.

Inhibitors of enkephalin-degrading enzymes as potential therapeutic agents

A limited number of enzymes such as membrane metalloendopeptidase (enkephalinase) and angiotensin converting enzyme appear to be involved in deactivation and modulation of circulatory regulatory peptides. Peptides such as the enkephalins are also involved in a large number of physiological processes. This multiplicity of physiological roles has made it difficult to establish the therapeutic role of enkephalin-degrading enzyme inhibitors. Other factors such as difficulty in quantification and thus measurement of processes involved in pain and mental illness have also hindered the process of establishing any therapeutic role of enkephalin-degrading enzyme inhibitors in these conditions. However, they have proved to be useful pharmacological 'tools'. The most likely therapeutic role at present appears to be in the treatment of cardiovascular disorders. As a 'profile' of pharmacological actions of enkephalin-degrading enzymes emerges, it is becoming apparent that bioavailability rather than a high degree of specificity or inhibitory potency may be the most important factor. This may be used to an advantage in future developments by the use of less specific or combined inhibitors in the form of prodrugs, designed to be active at specific sites such as the central nervous system.

Potent inhibition of endopeptidase 24.16 and endopeptidase 24.15 by the phosphonamide peptide N-(phenylethylphosphonyl)-Gly-L-Pro-L-aminohexanoic acid

A phosphonamide peptide, N-(phenylethylphosphonyl)-Gly-L-Pro-L-aminohexanoic acid, previously shown to block Clostridium histolyticum collagenases, was examined as a putative inhibitor of endopeptidase 24.16 and endopeptidase 24.15. Hydrolysis of two endopeptidase 24.16 substrates, i.e. 3-carboxy-7-methoxycoumarin (Mcc)-Pro-Leu-Gly-Pro-D-Lys-dinitrophenyl (Dnp) and neurotensin, were completely and dose-dependently inhibited by the phosphonamide inhibitor with KI values of 0.3 and 0.9 nM respectively. In addition, the phosphonamide peptide inhibited the hydrolysis of benzoyl (Bz)-Gly-Ala-Ala-Phe-(pAB) p-aminobenzoate and neurotensin by endopeptidase 24.15 with about a 10-fold lower potency (KI values of 5 and 7.5 nM respectively). The selectivity of this inhibitor towards several exo- and endo-peptidases belonging to the zinc-containing metallopeptidase family established that a 1 microM concentration of this inhibitor was unable to affect leucine aminopeptidase, carboxypeptidase A, angiotensin-converting enzyme and endopeptidase 24.11. The present paper therefore reports on the first hydrophilic highly potent endopeptidase 24.16 inhibitor and describes the most potent inhibitory agent directed towards endopeptidase 24.15 developed to date. These tools should allow one to assess the contribution of endopeptidase 24.16 and endopeptidase 24.15 to the physiological inactivation of neurotensin as well as other neuropeptides.

Specific inhibition of endopeptidase 24.16 by dipeptides

The inhibitory effect of various dipeptides on the neurotensin-degrading metallopeptidase, endopeptidase 24.16, was examined. These dipeptides mimick the Pro10-Tyr11 bond of neurotensin that is hydrolyzed by endopeptidase 24.16. Among a series of Pro-Xaa dipeptides, the most potent inhibitory effect was elicited by Pro-Ile (Ki approximately 90 microM) with Pro-Ile greater than Pro-Met greater than Pro-Phe. All the Xaa-Tyr dipeptides were unable to inhibit endopeptidase 24.16. The effect of Pro-Ile on several purified peptidases was assessed by means of fluorigenic assays and HPLC analysis. A 5 mM concentration of Pro-Ile does not inhibit endopeptidase 24.11, endopeptidase 24.15, angiotensin-converting enzyme, proline endopeptidase, trypsin, leucine aminopeptidase, pyroglutamyl aminopeptidase I and carboxypeptidase B. The only enzyme that was affected by Pro-Ile was carboxypeptidase A, although it was with a 50-fold lower potency (Ki approximately 5 mM) than for endopeptidase 24.16. By means of fluorimetric substrates with a series of hydrolysing activities, we demonstrate that Pro-Ile can be used as a specific inhibitor of endopeptidase 24.16, even in a complex mixture of peptidase activities such as found in whole rat brain homogenate.

UK-69,578, a novel inhibitor of EC 3.4.24.11 which increases endogenous ANF levels and is natriuretic and diuretic

A search for potent inhibitors of EC 3.4.24.11, an enzyme which is found most abundantly in the kidney and which degrades atrial natriuretic factor, has led to the identification of UK-69,578. Structure-activity studies starting from substituted N-carboxymethyl dipeptide inhibitors resulted in the introduction of a cyclo-alkane P1' residue and in the replacement of the aza-link between P1 and P1' residues by a methylene group, with a net ten-fold potency gain. UK-69,578 increases endogenous ANF levels and produces natriuretic and diuretic responses intravenously in mice.

Enkephalin-degrading enzyme inhibitors. Crucial role of the C-terminal residue on the inhibitory potencies of retro-hydroxamate dipeptides

The retro-inversion of the amide bond in kelatorphan and analogs, the first series of complete inhibitors of enkephalin metabolism, led to compounds highly efficient only against the neutral endopeptidase 24-11 (NEP). In order to increase the recognition of the aminopeptidase N (APN) and dipeptidylaminopeptidase (DAP), without loss of affinity for NEP, the malonyl group of these retro-inhibitors was replaced by diversely substituted succinyl moieties. All the molecules synthesized are highly efficient NEP inhibitors with Ki's in the 0.2-1 nM range, indicating that NEP possesses a relatively large and not very selective S'2 subsite. In contrast, inhibition of DAP activity is crucially dependent on the size and the position of the substituent in the succinyl moiety. Inhibitory potencies in the nanomolar range are obtained with compounds containing a benzyl group in the alpha-position related to the retro amide bond. Finally, a relatively modest inhibition of APN was observed with Ki's in the 0.5-1 microM range for compounds with benzyl or cyclohexyl group in P'2 position. However, these data demonstrate that efficient and complete inhibition of enkephalin degradation can be obtained with hydroxamate dipeptides containing a retro amide bond. The analgesic potency of the most active inhibitors was measured using the hot plate test in mice. Significant antinociceptive responses were obtained but these effects were rather weaker than those expected from the in vitro inhibitory potencies of these compounds on the three enkephalin-degrading enzymes.

Neuropeptide-specific peptidases: does brain contain a specific TRH-degrading enzyme?

The particulate fraction of brain homogenates contains an enzyme that cleaves the pyroglutamyl-histidyl bond of thyrotropin-releasing hormone (TRH) but is clearly distinct from the more widely distributed pyroglutamyl peptidase (EC 3.4.19.3). This particulate enzyme is highly localized to brain where it is found on synaptosomal membranes. It exhibits an unusual degree of substrate specificity. For example, it does not cleave the pyroglutamyl-histidyl bond of luteinizing hormone-releasing hormone (LHRH) or the pyroglutamyl histidyl bond of the chromogenic substrate pyroglutamyl-histidyl-2-naphthylamide. Evidence is reviewed supporting the possibility that this enzyme, first detected in serum and originally referred to as "thyroliberinase", may be the first neuropeptide-specific peptidase to be characterized.

Substrate and inhibitor studies of thermolysin-like neutral metalloendopeptidase from kidney membrane fractions. Comparison with bacterial thermolysin

The inhibitory constants of a series of synthetic N-carboxymethyl peptide inhibitors and the kinetic parameters (Km, kcat, and kcat/Km) of a series of model synthetic substrates were determined for the membrane-bound kidney metalloendopeptidase isolated from rabbit kidney and compared with those of bacterial thermolysin. The two enzymes show striking similarities with respect to structural requirements for substrate binding to the hydrophobic pocket at the S1' subsite of the active site. Both enzymes showed the highest reaction rates with substrates having leucine residues in this position while phenylalanine residues gave the lowest Km. The two enzymes were also inhibited by the same N-carboxymethyl peptide inhibitors. Although the mammalian enzyme was more susceptible to inhibition than its bacterial counterpart, structural variations in the inhibitor molecules affected the inhibitory constants for both enzymes in a similar manner. The two enzymes differed significantly, however, with respect to the effect of structural changes in the P1 and P2' positions of the substrate on the kinetic parameters of the reaction. The mammalian enzyme showed the highest reaction rates and specificity constants with substrates having the sequence -Phe-Gly-Phe- or -Phe-Ala-Phe- in positions P2, P1, and P1', respectively, while the sequence -Ala-Phe-Phe- was the most favored by the bacterial enzyme. The sequence -Gly-Gly-Phe- as found in enkephalins was not favored by either of the enzymes. Of the substrates having an aminobenzoate group in the P2' position, the mammalian enzyme favored those with the carboxyl group in the meta position while the bacterial enzyme favored those with the carboxyl group in the para position.(ABSTRACT TRUNCATED AT 250 WORDS)

Carboxyalkyl peptide inhibitors of kininase II: chiral synthesis

Heretofore, carboxyalkyl peptide inhibitors of kininase II (e.g. N-[1-carboxy-3-phenylpropyl]-Ala-Pro, "enalaprilic acid") have been synthesized by means that yield racemic product. Typically, the secondary amine bond is formed by reacting an amino acid or dipeptide with a 2-keto carboxylic acid ester or imide. The group providing the 2-keto function must be used in excess, and the desired S,S,S isomer must be obtained by resolution procedures. We have developed a procedure whereby enalaprilic acid, RAC-X-64 and related compounds are synthesized stereospecifically and in relatively high yields.

Recent developments in the design of angiotensin-converting enzyme inhibitors

Orally-active angiotensin-converting enzyme inhibitors are rapidly establishing themselves in the therapy of hypertension and congestive heart failure. Concerted efforts in a number of laboratories have now led to the discovery or synthesis of an unparalleled variety of potent inhibitors. The manner in which several of these inhibitors bind to ACE is beginning to be understood. It is hoped that some of the insights to be derived from the SAR and structural studies done with ACE inhibitors will be applicable to other enzyme targets as well. The success of ACE inhibitors as pharmacological tools and in the clinic will also quite certainly encourage future efforts to develop new enzyme inhibitor approaches to drug therapy.

Comparison of dipeptidyl carboxypeptidase and endopeptidase activities in the three enkephalin-hydrolysing metallopeptidases: "angiotensin-converting enzyme", thermolysin and "enkephalinase"

Angiotensin-converting enzyme (ACE), thermolysin and "enkephalinase", three metallopeptidases cleaving the Gly3-Phe4 amide bond of enkephalins, were compared regarding substrate specificity and effects of butanedione, an arginyl-directed reagent. The hydrolysis of enkephalins and analogues was more affected by the nature of P1 and P2 residues in the case of thermolysin than in those of ACE or "enkephalinase"; amidation of the C-terminal carboxylate decreased drastically the hydrolysis by ACE but only marginally by thermolysin and the effect was intermediate for "enkephalinase". With adequate model substrates, the ratio of dipeptidylcarboxypeptidase to tripeptidylcaroxypeptidase (endopeptidase) activities were of 25 for ACE, 3 for "enkephalinase" and only 0.3 for thermolysin. Finally a butanedione treatment increased thermolysin activity, but abolished ACE activity; it reduced "enkephalinase" activity by 80% when measured with a free C-terminal carboxylate enkephalin analogue but only slightly with the corresponding amidated derivative. A critical role of an Arg residue in ACE and, to a lesser extent, in "enkephalinase" (but not in thermolysin) is suggested to be responsible for the preferential dipeptidylcarboxypeptidase activity of these two enzymes.

Ketomethyldipeptides I. A new class of angiotensin converting enzyme inhibitors

The design rationale for a new series of angiotensin-converting enzyme (ACE) inhibitors which incorporate a ketone substituent into a peptide backbone is described. Molecular regions which were expected to mimic the binding of an N-acyl tripeptide substrate at secondary binding sites S1 and S1' were systematically varied in order to study the specificity of inhibitor binding and optimize inhibition against ACE. The most effective ketomethyldipeptides inhibit ACE in the 10(-9) M range.

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.

Human polymorphonuclear leukocyte collagenase and gelatinase. Comparison of certain enzymatic properties

Collagenase and gelatinase of human PMN leukocytes were separated by serial chromatography. The enzymes were shown to be similar in latency, activatability, chromatographic and electrophoretic behavior and the response to inhibitors. They recognize the same peptide linkage for cleavage, only each with a distinct difference in the effect caused by the secondary binding sites of the substrate molecules.

Benzolactams. A new class of converting enzyme inhibitors

A series of potent inhibitors of angiotensin-converting enzyme (dipeptidyl carboxypeptidase, E.C. 3.4.15.1) derived from benzofused 1-carboxyalkyl-3-(1-carboxy-3-phenyl-propylamino) lactams (III) is described. In the most effective inhibitors (I50 2-4 X 10(-9)M) the lactam is 7 or 8 membered and the N-1 side chain is carboxymethyl or carboxyethyl. Conformational and steric factors pertinent to binding to the enzyme are discussed.