Purification and substrate characterization of a human enkephalin-degrading aminopeptidase

Boomerang effect between [Met]-enkephalin derivatives and human polymorphonuclear leukocytes

[Met]-enkephalin or its precursor, pre-[Met]-enkephalin, were exposed to activated oxygen species produced by human phorbol myristate acetate (PMA)-stimulated polymorphonuclear leukocytes (PMNs) and then analyzed by high-pressure liquid chromatography (HPLC). The chromatograms recorded at the tyrosine maximum wavelength (lambda em 300 nm and lambda ex 280 nm) showed the formation of new peptides by oxidation of methionyl residue in position 5 and ortho, meta, or para hydroxylation of phenylalanyl residue in position 4. The chromatograms recorded at the dityrosine maximum wavelength (lambda em 400 nm and lambda ex 325 nm) showed the formation of new dimeric peptides which contained two [Met]-enkephalin-derivatives linked by a dityrosyl group. These new peptides were tested for chemiluminescence response to PMA-stimulated PMNs. [Met]-enkephalin, pre-[Met]-enkephalin, and the methionyl-oxidized derivatives suppressed the PMA-induced respiratory burst of PMNs. Conversely, after hydroxylation by activated oxygen species released by stimulated PMNs, these peptides enhanced the PMA-induced respiratory burst of PMNs. In the same conditions, dimeric peptides had no effect.

Oral absorption of peptides: influence of pH and inhibitors on the intestinal hydrolysis of leu-enkephalin and analogues

Leu-enkephalin (YGGFL) and several analogues were chosen as model peptides for the study of peptide absorption and hydrolysis in the rat jejunum. An HPLC assay was adapted to detect YGGFL or the analogues and metabolites. Peptide hydrolysis was studied in the rat jejunum using a single-pass perfusion method. Extensive hydrolysis of YGGFL was observed in the rat jejunum and approaches to reduce its metabolism were studied. The brush border enzymes are a major site of enkephalin hydrolysis. Lumenal peptidases were secondary to the brush border enzymes in hydrolyzing the enkephalins in this system. In the in situ perfusion system, YGGFL is hydrolyzed primarily to Tyr and GGFL by the brush border aminopeptidase and to YGG and FL by brush border endopeptidase. Lowering the jejunal pH below 5.0 significantly reduces aminopeptidase activity and, to a lesser extent, endopeptidase activity. An aminopeptidase inhibitor, amastatin, produced more pronounced inhibitory effects at higher pH and the endopeptidase inhibitors, tripeptides YGG and GGF, are effective even below pH 5.0. Coperfusion of YGGFL with a combination of aminopeptidase and endopeptidase inhibitors, e.g., amastatin and YGG, is more effective in inhibiting hydrolysis since both metabolic pathways are inhibited. Leu-D(Ala)2-enkephalin, while showing enhanced stability against aminopeptidase hydrolysis, is hydrolyzed at the Gly-Phe bond by the endopeptidase. Its hydrolysis is not affected by pH changes or amastatin but is decreased by YGG. The YGGFL wall permeability was estimated and is not a limiting factor for oral absorption.

Aminopeptidases in human retroplacental sera: purification and characterization of two enzymes

Ten different maternal serum samples were analyzed for the hydrolysis of S-Bz-Cys-pNA (substrate for CAP) and Ala-pNA. The results showed clear differences in the activities in individual sera. Similar S-Bz-Cys-pNA hydrolysis activity was detected for all sera. However, Ala-pNA hydrolysis activity differed remarkably. Serum exhibiting low Ala-pNA hydrolysis activity contained only CAP, and that exhibiting high Ala-pNA hydrolyzing activity contained CAP and AAP. The two aminopeptidases were independently purified to a homogeneous state through the same purification procedures and some of their biochemical properties were compared. The enzymes were quite different with respect to molecular mass, the substrate specificities for some aminoacyl-pNA substrates, and the effects of inhibitors. Among various natural peptides tested for hydrolysis, the enzymes hydrolyzed Met-enkephalin most rapidly, but their modes of action were different. Furthermore, only CAP degraded oxytocin and AAP exhibited a high kinin-converting activity.

Methionine enkephalin is increased in plasma in acute liver disease and is present in bile and urine

Plasma immunoreactive methionine enkephalin is increased in cirrhosis. To determine whether it was increased in acute liver disease and chronic renal failure and whether the peptide was present in bile and urine, it was measured by radioimmunoassay in appropriate samples. Plasma immunoreactive methionine enkephalin, while at its peak in 15 patients with acute liver disease (median 425 pmol/l, range 220-1460), was approximately six times greater (P less than 0.001) than in 15 patients with chronic renal failure (70 pmol/l, 50-140), 15 controls with other diseases (75 pmol/l, 50-115) and 15 healthy controls (65 pmol/l, 50-95). In eight of the patients recovering from acute liver disease, the decline of the peptide's plasma level correlated with that of the alanine aminotransferase (r = 0.813, P less than 0.01) and prothrombin time (r = 0.682, P less than 0.05) measured in the simultaneously taken blood. Immunoreactive methionine enkephalin was found to be excreted in bile and urine. The possibility that increased plasma methionine enkephalin, and possibly other opioid peptides, may contribute to some of the manifestations of acute liver failure is worthy of further investigation.

Studies on the tissue distribution of the puromycin-sensitive enkephalin-degrading aminopeptidases

An antiserum generated to the soluble form of the rat brain puromycin-sensitive enkephalin-degrading aminopeptidase was used to determine the tissue distribution of the soluble and membrane-associated forms of this enzyme. All tissues examined contained significant levels of the soluble enzyme form, with this enzyme accounting for greater than 90% of the arylamidase activity in brain, heart, and skeletal muscle. Native gel electrophoresis coupled with activity staining as well as inhibition studies were used to confirm the presence of this enzyme in various tissues. Serum was found not to contain this particular aminopeptidase. In contrast to the results obtained with the soluble enzyme form, brain was the only tissue found to contain the membrane-associated enzyme form. Although all tissues contained membrane-associated aminopeptidase activity only the brain enzyme could be maintained in solution in the absence of detergent. In addition, the brain membrane-associated enzyme could be distinguished from the membrane-associated aminopeptidase activity in other tissues on the basis of its sensitivity to inhibition by puromycin.

Studies of the in vitro human plasma degradation of methionine-enkephalin

1. Incubation of [3H]tyrosine methionine-enkephalin (6 x 10(-9) M final concentration) with human platelet-poor plasma (1:9 ratio to Trizma Base buffer, pH 7.4) results mostly (greater than 95%) in hydrolysis of the tyrosyl-glycine peptide bond. This enzymatic reaction is essentially completed within 90 min, showing a half-life, Km and Vmax of 12.8 +/- 2.5 min, 0.70 +/- 0.01 mM and 17.90 +/- 1.05 mumol/L/min, respectively. These values are comparable to those previously reported for the human plasma degradation of leucine-enkephalin. 2. As expected hydrolysis of the methionine-enkephalin tyrosyl-glycine peptide bond was blocked by the known aminopeptidase inhibitors bestatin and puromycin (IC50 1.2 +/- 0.4 and 4.3 +/- 2.4 microM, respectively) but not by either thiorphan or captopril. 3. Neither the storing (up to 60 days) nor the freezing and thawing (up to ten times during a 60 days periods) significantly changed the above kinetic parameters, showing the stability of the plasma methionine-enkephalin degrading aminopeptidase.

Target tissue distribution of the proenkephalin peptides F, E, and B

The adrenal medulla is a rich source of endogenous opioid peptides. These peptides exist predominantly in the form of larger (25-34 amino acid long) enkephalin containing peptides, whose biological roles have yet to be elucidated. We report here the tissue binding distribution of three iodinated enkephalin containing peptides, Peptides F, E, and B, in rats, rabbits, and guinea pigs following intravenous injection. Each [125I]peptide has a unique distribution profile but all three are found to bind to the pituitary in each species of animal examined. A number of lines of evidence point toward the enkephalin containing peptides, Peptides F, E, and B, having physiological roles distinct from the enkephalins. The distribution profile of these [125I]peptides reported here gives insight into their potential effector sites.

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 degradation by human erythrocytes and hemolysates studied using 1H NMR spectroscopy

High resolution (400 MHz) 1H spin-echo NMR spectroscopy was used to monitor the degradation of leucine-enkephalin, and peptide fragments of it, by human erythrocytes and hemolysates. We showed that leucine-enkephalin is rapidly degraded by the cytosolic peptidases of the human erythrocyte, and we have elucidated the most probable pathway of degradation. Computer simulations of the proposed pathway, using a model incorporating the experimentally derived steady-state kinetic parameters obtained for the individual enzyme steps, showed close agreement with the experimental results. From a methodological perspective, the work demonstrates the value of 1H spin-echo NMR spectroscopy for rapidly elucidating, both qualitatively and quantitatively, an entire peptide-degradation pathway as it operates in situ.

Mechanisms of leu-enkephalin hydrolysis in human plasma

The present work describes the kinetics of enkephalin hydrolysis by plasma enzymes and the fragmentation pattern of both the parent peptide and of the first hydrolysis by-products. The degradation kinetics were followed by positive identification of the hydrolysis fragments by chromatographic methods, by amino acid analysis and by scintillation counting of tritium-labeled enkephalin. In addition, the results presented confirm the role of the low molecular weight plasma components in the control of the hydrolysis of the peripherally-released enkephalins.

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.

'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.

Purification and characterization of an enkephalin-degrading aminopeptidase from guinea pig serum

An enkephalin-degrading aminopeptidase using Leu-enkephalin as a substrate was purified about 4100-fold from guinea pig serum. The purified preparation was apparently homogenous, showing one band on polyacrylamide gel electrophoresis. The molecular weight of the enzyme was approx. 92 000. The aminopeptidase had a pH optimum of 7.0 with Km values of 0.12 mM and 0.18 mM for Leu- and Met-enkephalin, respectively. The enzyme hydrolyzed neutral, basic and aromatic amino acid beta-naphthylamides, but did not the acidic one. The enzyme was inhibited strongly by metal-chelating agents, bestatin and amastatin and weakly by puromycin. Among several biologically active peptides, angiotensin III and substance P strongly inhibited the enzyme.

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.

Endogenous opiates: 1981

This paper is the fourth of an annual series reviewing the research concerning the endogenous opiate peptides. This installment covers only work published during 1981 and attempts to provide a comprehensive, but not exhaustive, survey of the area. Previous papers in the series have dealt with research done before 1981. Topics concerning endogenous opiates reviewed here include a delineation of their receptors, their distribution, their precursors and degradation, behavioral effects resulting from their administration, their possible involvement in physiological responses, and their interactions with other peptides and hormones. Due to the burgeoning literature in this field, the comprehensive nature of this review in the future will be limited to considerations of behavioral phenomena related to the endogenous opiates.