Amino acid hydroxamates as inhibitors of the human enkephalin-degrading aminopeptidase

Synthesis and inhibitory activities against enkephalin degrading aminopeptidase of H-Trp(Nps)-Lys-OMe analogues bearing chelating groups

With the aim of increasing the inhibitory potency of the analgesic dipeptide H-Trp(Nps)-Lys-OMe against enkephalin-degrading aminopeptidases, the following derivatives bearing chelating groups at the N-terminus have been synthesized: Ac-Trp(Nps)-Lys-OMe (3), HS(CH2)nCO-Trp(Nps)-Lys-OMe [n = 1 (4), n = 2 (5)], MeOCO(CH2)n-Trp(Nps)-Lys-OMe [n = 1 (6), n = 2 (7)] and analogues in which the N alpha-amino group has been replaced by a methoxycarbonyl group (8) and a bidentate hydroxamate function (9), respectively. The inhibitory activities of all these compounds and the S-protected derivatives EtNHCOS(CH2)nCO-Trp(Nps)-Lys-OMe [n = 1 (16), n = (17)] against the mentioned enzyme, isolated from rat striatum, are compared with those of the parent dipeptide 2 and bestatin. All the new derivatives showed, in general, inhibitory potencies of the same order of magnitude as compound 2.

Studies on the subsite specificity of the rat brain puromycin-sensitive aminopeptidase

The specificity of the puromycin-sensitive aminopeptidase from rat brain was examined. Using L-alanyl-beta-naphthylamide as substrate Vmax of the reaction was shown to be pH independent over the range of 5.5-9.0, while Km exhibited a pKa of 7.7. This latter value corresponds to the pKa of the amino group of the substrate. Using X-Ala and X-Leu to examine the specificity of the P1 site it was found that Arg and Lys exhibit the highest affinity, followed by Met, Val, Leu, Trp, and Phe, which bind congruent to 5- to 20-fold less well. Although Km varied more than 20-fold within this series, Vmax showed considerably less variation. Significantly weaker binding was observed with a P1 Gly, Ala, Ser, or Pro with no binding detectable with a P1 Glu. The presence of a P'1 Leu compared to P'1 Ala results in an approximate 10-fold decrease in Km with little change in Vmax. The effect of varying P'1 residues was examined with the series Leu-X. In this case basic and hydrophobic amino acids, with the exception of Val, all exhibit nearly the same Km. The binding of Arg-Arg and Lys-Lys showed the same Km as obtained for Arg-Leu or Lys-Leu, respectively. When Leu-Ser-Phe was compared to Leu-Ser the P'2 residue led to a 100-fold decrease in Km and slightly less than a 5-fold increase in Vmax. In contrast the addition of a P'2 Met to Leu-Trp results in only a 3-fold decrease in Km and a 3-fold increase in Vmax. The results indicate a preference for a basic or hydrophobic residue in the P1 and P'1 sites and indicate subsite-subsite interactions which primarily affect binding.

Novel inhibitors of enkephalin-degrading enzymes. II: N5'-substituted-4-thioxohydantoic acids as aminopeptidase inhibitors

Some 2-substituted-(2'-aminophenyl)-4-thioxohydantoic acids (o-amino PTC-amino acids) have antinociceptive activity when administered (icv) alone (IC50 = 0.04-0.87 microM/animal) and show a striking prolongation of the antinociceptive action of (D-Ala-2 D-Leu5)-enkephalin (DADL) in combination. The effects are thought to be mediated via opioid receptors since they are naloxone-reversible. Although inhibitors of the enkephalin degrading puromycin-insensitive, bestatin-sensitive aminopeptidase (possibly aminopeptidase M) their action is weak (IC50 = 32 microM leucine, 536 microM, glycine) and they might be considered to have a direct antinociceptive effect on opioid receptors. The titled compounds constitute novel 'lead' compounds for the development of potent aminopeptidase M inhibitors.

alpha-Aminoboronic acid derivatives: effective inhibitors of aminopeptidases

alpha-Aminoboronic acids and their derivatives have been synthesized as stable white solids. These compounds are effective inhibitors of human enkephalin degrading aminopeptidase, microsomal leucine aminopeptidase (EC 3.4.11.2), and cytosolic leucine aminopeptidase (EC 3.4.11.1) at micro- to nanomolar concentrations. The inhibition of cytosolic leucine aminopeptidase has been studied in some detail. Kinetic data correspond to the mechanism for biphasic slow-binding inhibition: E + I in equilibrium E.I in equilibrium E.I*, in which a rapid initial binding is followed by a slow transformation to a stable enzyme inhibitor complex. The initial and final binding constants are dependent on the nature of the side chain at the alpha-carbon atom but are independent of the protecting group on the boronic acid moiety and follow the trend for the hydrolysis of the corresponding amino acid amides. The first-order rate constant for the transformation of E.I to E.I* is similar for all four compounds studied. These data suggest that the slow-binding step represents the formation of tetrahedral boronate species from trigonal boronic acid.

Enkephalin-degrading activity in arthropode hemolymphe

Enkephalin and related peptides are rapidly inactivated in Astacus fluviatilis and Limulus polyphemus hemolymphe. At least three different enzymes, an aminopeptidase, a carboxypeptidase and a peptidyl-dipeptidase, acting concomitantly on the peptide substrates have been identified. The properties of these enzymes were characterized and they were compared to similar enzymes of the vertebrate blood. The opioid peptides appear to be extremely short lived in invertebrate hemolymphe, which presumably is a metabolic barrier to the action of active peptides stronger than vertebrate blood.

Inhibition of substance P degradation in rat brain preparations by peptide hydroxamic acids

A peptidase activity of rat diencephalon membranes, which acts on the C-terminal hexapeptide sequence of substance P, was characterized using the radiolabeled substrate N alpha-[( 125I]iododesaminotyrosyl)-substance P (6-11)-hexapeptide. This activity presents certain characteristics similar to those of the substance-P-degrading enzyme purified from human brain by Lee et al. [Eur. J. Biochem. 114, 315-327 (1981)]. It is inhibited by metal chelators and some thiol reagents, but is insensitive to inhibitors of serine proteases and aminopeptidases. The activity is different from angiotensin-converting enzyme and enkephalinase, since it is not affected by specific inhibitors of these enzymes. Substance P and substance P C-terminal fragments longer than the pentapeptide inhibited the degradation of the radiolabeled substrate with inhibition constants around 200 microM. Short fragments of the substance P sequence, such as Boc-Phe-Phe-OMe and Boc-Phe-Phe-Gly-OEt, were also found to inhibit the degradation of the substrate. When the metal-chelating hydroxamic acid moiety was attached to the carboxyl terminus of these short peptides, potent inhibitors of the substance-P-degrading activity were obtained, with inhibition constants in the micromolar range. The most potent of these compounds, iododesaminotyrosyl-Phe-Phe-Gly-NHOH (IBH-Phe-Phe-Gly-NHOH), is a competitive inhibitor, with a Ki value of 1.9 microM. The degradation of substance P by rat diencephalon slices was inhibited to the same extent (40-50%) by IBH-Phe-Phe-Gly-NHOH (20 microM) and by phosphoramidon (1 microM). A combination of both reagents reduced the degradation rate by 75-80%, suggesting that both enkephalinase and the substance-P-degrading activity are involved in the metabolism of substance P in this preparation. IBH-Phe-Phe-Gly-NHOH seems to be quite specific for the latter enzyme, since at a high concentration (0.1 mM) it did not affect the degradation of the radiolabeled substrate by alpha-chymotrypsin, papain, or thermolysin.

Structural requirements for specific inhibition of microsomal aminopeptidase by mercaptoamines

L-Leucinthiol, a synthetic derivative of mercaptoethylamine with a hydrophobic side chain, was recently reported to be a potent inhibitor of microsomal aminopeptidase. The structural features necessary for interaction of mercaptoamines with this enzyme have now been explored more systematically. Optimal binding requires a primary amine linked to the mercapto group via two carbon atoms. Only a substituent with L-configuration at the 1 position increased the affinity toward the enzyme. The high degree of specificity and other evidence suggest that the mode of binding of these inhibitors is similar to that of substrates. Comparison of leucinthiol with other amino compounds suggest that the mercapto group makes a much greater contribution to the binding than the hydrophobic side chain. L-Leucinthiol is fairly specific for aminopeptidase although some inhibition of thermolysin and carboxypeptidase A is observed.

Use of secondary isotope effects and varying pH to investigate the mode of binding of inhibitory amino aldehydes by leucine aminopeptidase

Ki values for leucine aldehyde, a competitive inhibitor of leucine aminopeptidase, vary with pH in a manner compatible with binding of uncharged inhibitor. The pH dependence of kcat/Km suggests likewise that the substrate leucine p-nitroanilide is productively bound as the uncharged species. Comparison of pKa values of the model compounds aminoacetone and aminoacetal indicates that the equilibrium constant for hydration of amino aldehydes is reduced by a factor of about 2 when a proton is lost from the alpha-ammonium group near pH 8. Effects of deuterium substitution at C-1 on equilibrium binding of leucine aldehyde were determined with immobilized enzyme and inhibitors doubly labeled with radioisotopes. The observed isotope effect (KD/KH) is approximately unity, suggesting that leucine aldehyde combines with the enzyme as an oxygen adduct, not as the intact aldehyde.

'In vivo' amplification of biological activity of tetragastrin by amino acid hydroxamates

Rat blood was shown to contain an aminopeptidase which rapidly hydrolyses short peptides containing an aromatic amino acid as N-terminal residue. Using tetragastrin (Trp-Met-Asp-PheNH2) as substrate, we showed that some amino acid hydroxamates inhibit rat aminopeptidase activity 'in vitro' in the following order: HTrpNHOH greater than HPheNHOH much greater than HAlaNHOH. The same hydroxamates markedly enhanced the biological activity of tetragastrin 'in vivo'. The amplification of the secretory effect, correlated with the amount of the hydroxamate used, strongly suggests that these compounds can stabilize a number of active peptides in vivo by inhibiting their proteolytic degradation.

Inhibition of a membrane-bound enkephalin-degrading aminopeptidase by bestatin analogs

A variety of bestatin analogs were examined as potent inhibitors of a membrane-bound enkephalin-degrading aminopeptidase that was purified from monkey brain. Bestatinyl amino acid derivatives showed strong inhibition of this enzyme. The most effective was bestatin-L-Arg X AcOH, with a Ki value of 0.21 X 10(-8) M with Leu-enkephalin as substrate. It exhibited competitive kinetics and was about 100-fold more potent than bestatin. This compound seems to be useful for pharmacological and other studies.

L-leucinthiol - a potent inhibitor of leucine aminopeptidase

L-leucinthiol (2-amino-4-methyl-1-pentanethiol) was designed as an inhibitor of leucine aminopeptidase by analogy with sulfhydryl inhibitors of other zinc-containing peptidases. It was synthesized from L-leucinol and shown to be a potent competitive inhibitor of the microsomal aminopeptidase from porcine kidney (Ki = 2.2 X 10(-8) M). The results suggest that the mechanism of aminopeptidase may be similar to that of other metalloproteases.

Enkephalin-degrading activity in Xenopus laevis tadpoles and adults blood

The enkephalin-degrading enzyme (alpha-aminoacyl-peptide hydrolase,EC 3.4.11.11) responsible for the rapid inactivation of enkephalins in human blood was purified and compared to the enkephalin-degrading system of Xenopus laevis tadpoles and adult blood. The specificity and the kinetic constants of the Xenopus enzyme(s) were determined after partial purification. Even if remarkable similarities between the Xenopus and the human enzyme exist, still they show differences in specificity towards peptides whose N-terminal is Phe. Amphibians are able to manufacture enkephalins and the present work shows that they are endowed with an enkephalin-degrading system comparable to the soluble one of human blood.

beta-Carboline and diazepam effect on the degradation of enkephalin by the human blood aminopeptidase

An enkephalin-degrading aminopeptidase (alpha-amino-acyl-peptide hydrolase, EC 3.4.11.11) has been purified from human plasma and has been shown to be the principle responsible for the transient half-life of enkephalin in blood. An inhibitory effect of beta-carbolines and of 3,4-dihydro-beta-carbolines on this enzyme 'in vitro' is reported. The best inhibitor is the 3-carboxylic acid (Ki congruent to 10(-4) M), while the ester, amide, and/or peptide derivatives are less potent. Since some beta-carboline derivatives have recently been shown to possess high affinity for benzodiazepine receptors in brain, the action of diazepam on the aminopeptidase activity was tested and a relevant inhibition of the human enzyme could be demonstrated.