Biological inactivation of enkephalins and the role of enkephalin-dipeptidyl-carboxypeptidase ("enkephalinase") as neuropeptidase

Enkephalins and Pain Modulation: Mechanisms of Action and Therapeutic Perspectives

Enkephalins, a subclass of endogenous opioid peptides, play a pivotal role in pain modulation. Enkephalins primarily exert their effects through opioid receptors located widely throughout both the central and peripheral nervous systems. This review will explore the mechanisms by which enkephalins produce analgesia, emotional regulation, neuroprotection, and other physiological effects. Furthermore, this review will analyze the involvement of enkephalins in the modulation of different pathologies characterized by severe pain. Understanding the complex role of enkephalins in pain processing provides valuable insight into potential therapeutic strategies for managing pain disorders.

Modification and Delivery of Enkephalins for Pain Modulation

Chronic pain negatively affects patient's quality of life and poses a significant economic burden. First line pharmaceutical treatment of chronic pain, including NSAIDs or antidepressants, is often inefficient to reduce pain, or produces intolerable adverse effects. In such cases, opioids are frequently prescribed for their potent analgesia, but chronic opioid use is also frequently associated with debilitating side effects that may offset analgesic benefits. Nonetheless, opioids continue to be widely utilized due to the lack of effective alternative analgesics. Since their discovery in 1975, a class of endogenous opioids called enkephalins (ENKs) have been investigated for their ability to relieve pain with significantly reduced adverse effects compared to conventional opioids. Their low metabolic stability and inability to cross biological membranes, however, make ENKs ineffective analgesics. Over past decades, much effort has been invested to overcome these limitations and develop ENK-based pain therapies. This review summarizes and describes chemical modifications and ENK delivery technologies utilizing ENK conjugates, nanoparticles and ENK gene delivery approaches and discusses valid lessons, challenges, and future directions of this evolving field.

A Review of Preclinical Tools to Validate Anti-Diarrheal Agents

BACKGROUND

Since their inception, preclinical experimental models have played an important role in investigating and characterizing disease pathogenesis. These in vivo, ex vivo, and in vitro preclinical tests also aid in identifying targets, evaluating potential therapeutic drugs, and validating treatment protocols.

INTRODUCTION

Diarrhea is a leading cause of mortality and morbidity, particularly among children in developing countries, and it represents a huge health-care challenge on a global scale. Due to its chronic manifestations, alternative anti-diarrheal medications must be tested and developed because of the undesirable side effects of currently existing anti-diarrheal drugs.

METHODS

Several online databases, including Science Direct, PubMed, Web of Science, Google Scholar and Scopus, were used in the literature search. The datasets were searched for entries of studies up to May, 2022.

RESULTS

The exhaustive literature study provides a large number of in vivo, in vitro and ex vivo models, which have been used for evaluating the mechanism of the anti-diarrheal effect of drugs in chemically-, pathogen-, disease-induced animal models of diarrhea. The advances and challenges of each model were also addressed in this review.

CONCLUSION

This review encompasses diverse strategies for screening drugs with anti-diarrheal effects and covers a wide range of pathophysiological and molecular mechanisms linked to diarrhea, with a particular emphasis on the challenges of evaluating and predictively validating these experimental models in preclinical studies. This could also help researchers find a new medicine to treat diabetes more effectively and with fewer adverse effects.

An Effective and Safe Enkephalin Analog for Antinociception

Opioids account for 69,000 overdose deaths per annum worldwide and cause serious side effects. Safer analgesics are urgently needed. The endogenous opioid peptide Leu-Enkephalin (Leu-ENK) is ineffective when introduced peripherally due to poor stability and limited membrane permeability. We developed a focused library of Leu-ENK analogs containing small hydrophobic modifications. N-pivaloyl analog KK-103 showed the highest binding affinity to the delta opioid receptor (68% relative to Leu-ENK) and an extended plasma half-life of 37 h. In the murine hot-plate model, subcutaneous KK-103 showed 10-fold improved anticonception (142%MPE·h) compared to Leu-ENK (14%MPE·h). In the formalin model, KK-103 reduced the licking and biting time to ~50% relative to the vehicle group. KK-103 was shown to act through the opioid receptors in the central nervous system. In contrast to morphine, KK-103 was longer-lasting and did not induce breathing depression, physical dependence, and tolerance, showing potential as a safe and effective analgesic.

Characterization of a New M13 Metallopeptidase from Deep-Sea Shewanella sp. E525-6 and Mechanistic Insight into Its Catalysis

Bacterial extracellular peptidases are important for bacterial nutrition and organic nitrogen degradation in the ocean. While many peptidases of the M13 family from terrestrial animals and bacteria are studied, there has been no report on M13 peptidases from marine bacteria. Here, we characterized an M13 peptidase, PepS, from the deep-sea sedimentary strain Shewanella sp. E525-6, and investigated its substrate specificity and catalytic mechanism. The gene pepS cloned from strain E525-6 contains 2085 bp and encodes an M13 metallopeptidase. PepS was expressed in Escherichia coli and purified. Among the characterized M13 peptidases, PepS shares the highest sequence identity (47%) with Zmp1 from Mycobacterium tuberculosis, indicating that PepS is a new member of the M13 family. PepS had the highest activity at 30°C and pH 8.0. It retained 15% activity at 0°C. Its half life at 40°C was only 4 min. These properties indicate that PepS is a cold-adapted enzyme. The smallest substrate for PepS is pentapeptide, and it is probably unable to cleave peptides of more than 30 residues. PepS prefers to hydrolyze peptide bonds with P1′ hydrophobic residues. Structural and mutational analyses suggested that His531, His535 and Glu592 coordinate the catalytic zinc ion in PepS, Glu532 acts as a nucleophile, and His654 is probably involved in the transition state stabilization. Asp538 and Asp596 can stablize the orientations of His531 and His535, and Arg660 can stablize the orientation of Asp596. These results help in understanding marine bacterial peptidases and organic nitrogen degradation.

Prognostic value of neuropeptide proenkephalin A in patients with severe traumatic brain injury

High plasma proenkephalin A levels have been associated with poor clinical outcome of aneurysmal subarachnoid hemorrhage. This prospective observatory study was designed to investigate the relationship between plasma proenkephalin A levels and 1-week mortality, 6-month mortality and 6-month unfavorable outcome (defined as Glasgow Outcome Scale score of 1-3) in patients with severe traumatic brain injury. This study recruited 128 patients and 128 sex- and age-matched healthy controls. Plasma proenkephalin A levels, as measured by chemoluminescence sandwich immunoassay, were statistically significantly higher in patients than in healthy controls (239.1±93.0 pmol/L vs.81.3±22.1 pmol/L; P<0.001) and were correlated with Glasgow Coma Scale scores (r=-0.540, P<0.001). It was identified as an independent prognostic predictor of 1-week mortality [odds ratio (OR), 1.214; 95% confidence interval (CI), 1.103-1.425; P<0.001], 6-month mortality (OR, 1.162; 95% CI, 1.101-1.372; P<0.001) and 6-month unfavorable outcome (OR, 1.116; 95% CI, 1.097-1.281; P<0.001). Moreover, it had high predictive value for 1-week mortality [area under curve (AUC), 0.852; 95% CI, 0.778-0.908], 6-month mortality (AUC, 0.841; 95% CI, 0.766-0.899) and 6-month unfavorable outcome (AUC, 0.830; 95% CI, 0.754-0.891). Furthermore, its predictive value was similar to Glasgow Coma Scale score's (all P>0.05). Yet, a combined logistic-regression model did not show that it statistically significantly improved the predictive value of Glasgow Coma Scale score (all P>0.05). Thus, it was proposed that enhanced plasma proenkephalin A could be a useful, complementary tool to predict short- or long-term clinical outcome after severe traumatic brain injury.

Evaluation of neuropeptide loaded trimethyl chitosan nanoparticles for nose to brain delivery

Leucine-enkephalin (Leu-Enk) is a neurotransmitter or neuromodulator in pain transmission. Due to non-addictive opioid analgesic activity of this peptide, it might have great potential in pain management. Leu-Enk loaded N-trimethyl chitosan (TMC) nanoparticles were prepared and evaluated as a brain delivery vehicle via nasal route. TMC biopolymer was synthesized and analyzed by (1)H NMR spectroscopy. TMC nanoparticles were prepared by ionic gelation method. Mean peptide encapsulation efficiency and loading capacity were 78.28±3.8% and 14±1.3%, respectively. Mean particle size, polydispersity index and zeta potential were found to be 443±23 nm, 0.317±0.17 and +15±2 mV respectively for optimized formulations. Apparent permeability coefficient (Papp) of Leu-Enk released from nanoparticles across the porcine nasal mucosa was determined to be 7.45±0.30×10(-6) cm s(-1). Permeability of Leu-Enk released from nanoparticles was 35 fold improved from the nasal mucosa as compared to Leu-Enk solution. Fluorescent microscopy of brain sections of mice showed higher accumulation of fluorescent marker NBD-F labelled Leu-Enk, when administered nasally by TMC nanoparticles, while low brain uptake of marker solution was observed. Furthermore, enhancement in brain uptake resulted into significant improvement in the observed antinociceptive effect of Leu-Enk as evidenced by hot plate and acetic acid induced writhing assay.

Acute ethanol administration differentially alters enkephalinase and aminopeptidase N activity and mRNA levels in regions of the nigrostriatal pathway

Opioid peptides play a key role in ethanol reinforcement and may also represent important determinants in brain sensitivity to ethanol through modulation of nigrostriatal dopaminergic activity. Regulation of opioid levels by peptidase-degrading enzymes could be relevant in ethanol's actions. The aim of this work was to study the acute ethanol (2.5 g/kg) effects on the activity and mRNA expression of enkephalinase (NEP) and aminopeptidase N (APN) in the rat substantia nigra (SN) and the anterior-medial (amCP) and medial-posterior (mpCP) regions of the caudate-putamen (CP). Enzymatic activities were measured by fluorometric assays and mRNA expression by reverse transcriptase polymerase chain reaction. Acute ethanol administration differentially altered peptidase activities and mRNA expression with different kinetics. Ethanol increased and decreased NEP mRNA levels in the SN and amCP, respectively, but produced biphasic effects in the mpCP. APN mRNA levels were increased by ethanol in all brain regions. Ethanol induced a transient and long-lasting increase in NEP (mpCP) and APN (amCP) activities, respectively. Peptidase activities were not changed by ethanol in the SN. Our results indicate that striatal NEP and APN are important ethanol targets. Ethanol-induced changes in these neuropeptidases in the CP could contribute to the mechanisms involved in brain sensitivity to ethanol.

Activity and expression of enkephalinase and aminopeptidase N in regions of the mesocorticolimbic system are selectively modified by acute ethanol administration

Opioid peptides play a key role in ethanol reinforcement and alcohol drinking behavior. However, regulation of opioid levels by peptidase-degrading activities in ethanol's actions in brain is still unclear. The aim of this work was to study the acute effects of ethanol (2.5 g/kg) on enkephalinase (NEP) and aminopeptidase N (APN) activities and expression in regions of the mesocorticolimbic system, as well as on corticosterone levels in serum for up to 24 h after administration. Enzymatic activities were measured by fluorometric assays, mRNA's expression by reverse transcriptase polymerase chain reaction (RT-PCR) and corticosterone levels by radioimmunoassay. Acute ethanol administration modified peptidase activity and expression with different kinetics. Ethanol induced a transitory increase and decrease in NEP and APN activities in the frontal cortex (FC) and ventral tegmental area (VTA), whereas only increases in these activities were observed in the nucleus accumbens (NAcc). Ethanol induced an increase in NEP mRNA in the FC and decreases in APN mRNA in the FC and NAcc. In contrast, ethanol produced biphasic effects on both enzymes expression in the VTA. Corticosterone levels were not changed by ethanol. Our results suggest that NEP and APN could play a main role in ethanol reinforcement through regulation of opioid levels in mesolimbic areas.

Electroosmotic sampling. Application to determination of ectopeptidase activity in organotypic hippocampal slice cultures

We hypothesize that peptide-containing solutions pulled through tissue should reveal the presence and activity of peptidases in the tissue. Using the natural zeta-potential in the organotypic hippocampal slice culture (OHSC), physiological fluids can be pulled through the tissue with an electric field. The hydrolysis of the peptides present in the fluid drawn through the tissue can be determined using capillary HPLC with electrochemical detection of the biuret complexes of the peptides following a postcolumn reaction. We have characterized this new sampling method by measuring the flow rate, examining the use of internal standards, and examining cell death caused by sampling. The sampling flow rate ranges from 60 to 150 nL/min with a 150 microm (ID) sampling capillary with an electric field (at the tip of the capillary) from 30 to 60 V/cm. Cell death can be negligible with controlled sampling conditions. Using this sampling approach, we have electroosmotically pulled Leu-enkephalin through OHSCs to identify ectopeptidase activity in the CA3 region. These studies show that a bestatin-sensitive aminopeptidase may be critical for the hydrolysis of exogenous Leu-enkephalin, a neuropeptide present in the CA3 region of OHSCs.

The Analgesic Activity of Bestatin as a Potent APN Inhibitor

Bestatin, a small molecular weight dipeptide, is a potent inhibitor of various aminopeptidases as well as LTA4 hydrolase. Various physiological functions of Bestatin have been identified, viz.: (1) an immunomodifier for enhancing the proliferation of normal human bone marrow granulocyte-macrophage progenitor cells to form CFU-GM colonies; Bestatin exerts a direct stimulating effect on lymphocytes via its fixation on the cell surface and an indirect effect on monocytes via aminopeptidase B inhibition of tuftsin catabolism; (2) an immunorestorator and curative or preventive agent for spontaneous tumor; Bestatin alone or its combination with chemicals can prolongate the disease-free interval and survival period in adult acute or chronic leukemia, therefore, it was primarily marketed in 1987 in Japan as an anticancer drug and servers as the only marketed inhibitor of Aminopeptidase N (APN/CD13) to cure leukemia to date; (3) a pan-hematopoietic stimulator and restorator; Bestatin promotes granulocytopoiesis and thrombocytopoiesis in vitro and restores them in myelo-hypoplastic men; (4) an inhibitor of several natural opioid peptides. Based on the knowledge that APN can cleave several bioactive neuropeptides such as Met-enkaphalins, Leu-enkaphalins, beta-Endorphin, and so on, the anti-aminopeptidase action of Bestatin also allows it to protect endopeptides against their catabolism, exhibiting analgesic activity. Although many scientific studies and great accomplishments have been achieved in this field, a large amount of problems are unsolved. This article reviews the promising results obtained for future development of the analgesic activity of Bestatin that can be of vital interest in a number of severe and chronic pain syndromes.

Stage-specific modulation of neprilysin and aminopeptidase N in the limbic system during kindling progression

Aminopeptidase N (APN) and neprilysin (NEP) inactivate neuropeptides released into the brain extracellular fluid. We previously showed that the expression of pyroglutamyl peptidase II (PPII), the TRH degrading ecto-enzyme, is regulated in rat brain by amygdaline kindling, a paradigm that activates neuronal pathways in the limbic system increasing the expression of several neuropeptides including TRH and opioids. To understand the specificity of this phenomenon, we studied APN and NEP expression in brains of partially or fully kindled rats (stage II and V), sacrificed 6 h after last stimulus, compared with sham-operated animals. In situ hybridization analysis of NEP mRNA levels showed decreased expression at stage II in CA1, CA2, olfactory tubercle and medial mammillary nucleus, and increased at stage V in CA1 and CA2 cells. These changes were specific for the ipsilateral side. APN mRNA levels, semi-quantified by RT-PCR, were decreased at stage II and increased at stage V, in frontal cortex-olfactory tubercle, and hippocampus. NEP and APN enzymatic activities, determined by fluorometric assays, followed similar variations to their respective mRNA levels. The coordinated changes (in some regions) of NEP and APN expression were opposite to those previously observed for PPII mRNA and activity levels in limbic regions. These results demonstrate that expression of ectopeptidases can be regulated when peptide neurons are activated and, that regulation is enzyme-, region-, and stage-specific.

Neprilysin carboxydipeptidase specificity studies and improvement in its detection with fluorescence energy transfer peptides

We examined the substrate specificity of the carboxydipeptidase activity of neprilysin (NEP) using fluorescence resonance energy transfer (FRET) peptides containing ortho-aminobenzoyl (Abz) and 2,4-dinitrophenyl (Dnp) as a donor/acceptor pair. Two peptide series with general sequences Abz-RXFK(Dnp)-OH and Abz-XRFK(Dnp)-OH (X denotes the position of the altered amino acid) were synthesized to study P1 (cleavage at the X-F bond) and P2 (cleavage at R-F bond) specificity, respectively. In these peptides a Phe residue was fixed in P1' to fulfill the well-known NEP S1' site requirement for a hydrophobic amino acid. In addition, we explored NEP capability to hydrolyze bradykinin (RPPGFSPFR) and its fluorescent derivative Abz-RPPGFSPFRQ-EDDnp (EDDnp=2,4-dinitrophenyl ethylenediamine). The enzyme acts upon bradykinin mainly as a carboxydipeptidase, preferentially cleaving Pro-Phe over the Gly-Phe bond in a 9:1 ratio, whereas Abz-RPPGFSPFRQ-EDDnp was hydrolyzed at the same bonds but at an inverted proportion of 1:9. The results show very efficient interaction of the substrates' C-terminal free carboxyl group with site S2' of NEP, confirming the enzyme's preference to act as carboxydipeptidase at substrates with a free carboxyl-terminus. Using data gathered from our study, we developed sensitive and selective NEP substrates that permit continuous measurement of the enzyme activity, even in crude tissue extracts.

Analgesic effects of intra-nasal enkephalins

The analgesic effects of intranasal delivery of leucine enkephalin (Leu-Enk) and its synthetic analogue [D-ala(2)]-leucine enkephalinamide (YAGFL) with or without enzyme inhibitors and/or absorption enhancers were investigated using the acetic acid-induced writhing test in mice. The analgesic activity was significantly affected by the time delay after the administration of Leu-Enk; the inhibition rates for the groups administered with acetic acid 5 min and 30 min after the administration of Leu-Enk were 56.40 +/- 8.54 and 17.98 +/- 7.07%, respectively. The addition of enzyme inhibitors and absorption enhancers markedly increased the inhibition rate of Leu-Enk and YAGFL; their inhibition rates were about four times and twice those without any enzyme inhibitor or absorption enhancer, respectively. The enzyme inhibitors and absorption enhancers that produced the highest inhibition rates of Leu-Enk and YAGFL were azelaic acid (1%), thimerosal (0.5 mM, TM), ethylenediamine-tetraacetic acid (5 mM, EDTA) and L-alpha-lysophosphatidylcholine (0.5%, LPC), and TM (0.5 mM), EDTA (5 mM), LPC (0.5%) and povidone (5%), respectively. The ED50 value of both enkephalins was also determined and found to be about 13 microg kg(-1), which is 850 and 60 times more potent than literature values for ketoprofen and morphine, respectively. Based on these results it was concluded that Leu-Enk or YAGFL could exert very high analgesic activity when administered nasally with a combination of inhibitors and absorption enhancers as compared with other analgesics.

Subcellular distribution of membrane-bound aminopeptidases in the human and rat brain

We evaluated the subcellular distribution of four membrane-bound aminopeptidases in the human and rat brain cortex. The particulate enzymes under study--puromycin-sensitive aminopeptidase (PSA), aminopeptidase N (APN), pyroglutamyl-peptidase I (PG I) and aspartyl-aminopeptidase (Asp-AP)--were fluorometrically measured using beta-naphthylamide derivatives. Membrane-bound aminopeptidase activity was found in all the studied subcellular fractions (myelinic, synaptosomal, mitochondrial, microsomal and nuclear fractions), although not homogenously. Human PSA showed highest activity in the microsomal fraction. APN was significantly higher in the nuclear fraction of both species, while PG I showed highest activity in the synaptosomal and myelinic fractions of the human and rat brain. The present results suggest that in addition to inactivating neuropeptides at the synaptic cleft, these enzymes may participate in other physiological processes. Moreover, these peptidases may play specific roles depending on their activity levels at the different subcellular structures where they are localized.

In vitro modulation of cytokine expression by enkephalin-derived peptides

OBJECTIVE

We have previously reported that low doses of [Met(5)]-enkephalin (YGGFM, met-enkephalin) and two of its derivatives (YGG and YG) enhanced and accelerated delayed-type hypersensitivity responses while much higher doses of these compounds suppressed these reactions. Since the underlying mechanisms by which this and other immunomodulatory effects occur have not been established, this report explores the in vitro modulation of Th1 and Th2 cytokine expression by these peptides.

METHODS

Murine splenocytes were stimulated with suboptimal concentrations of concanavalin A (ConA) in serum-free medium in the absence or presence of met-enkephalin, YGG, YG, [des-Tyr(1)]-met-enkephalin (GGFM), [D-Ala(2)], [D-Met(5)]-enkephalin or tyrosine (Y). Cell-conditioned supernatants were assayed for interferon-gamma (IFN-gamma), interleukin (IL)-2 and IL-4. Relative IFN-gamma and IL-2 mRNA levels were assessed by reverse transcription-polymerase chain reaction. The enhancing and suppressive effects of met-enkephalin and YG on IFN-gamma production were also tested in the presence of naloxone (Nx).

RESULTS

Met-enkephalin, YGG and YG modulated the in vitro production of IFN-gamma in a biphasic manner: stimulation at low doses and inhibition at high doses. At higher concentrations, met-enkephalin and YG also suppressed the production of IL-2 (type 1) and IL-4, a type 2 cytokine. Nx reversed the enhancing effect of met-enkephalin on IFN-gamma production without affecting its suppressive action or any of the immunomodulating effects of YG. The degradation-resistant analog [D-Ala(2)], [D-Met(5)]-enkephalin enhanced IFN-gamma production but did not suppress it.

CONCLUSIONS

YG, the minimal molecular requirement for enhancement and suppression of immune responses by these metabolites, appears to mediate exclusively an across-the-board suppression via low-affinity, nonclassical, nonopioid receptors.

Sustained exogenous administration of Met5-enkephalin protects against infarction in vivo

The opioid antagonist naloxone abolishes infarct limitation by myocardial ischemic preconditioning, suggesting that one or more endogenous opioid peptides can mediate cardiac protection against ischemic damage. We tested the hypothesis that the naturally occurring opioid peptide Met5-enkephalin (ME) modulates myocardial infarct size in vivo. Experiments were conducted in barbiturate-anesthetized open-chest rabbits subjected to regional myocardial ischemia-reperfusion. ME was administered via osmotic minipump for 24 h. Infarct size was assessed with tetrazolium and is expressed as a percentage of the area at risk. Exogenous ME reduced the amount of the risk zone infarcted by approximately 60% compared with saline-treated controls. ME-induced protection was sensitive to opioid receptor blockade with naloxone [NAL 50 +/- 2% vs. ME + NAL 39 +/- 3%, P = not significant (NS)] and also to blockade of sarcolemmal and mitochondrial ATP-sensitive K+ (KATP) channels [5-hydroxydecanoate (5-HD) 33 +/- 3% vs. ME + 5-HD 43 +/- 8%, P = NS; and HMR-1098 60 +/- 3% vs. ME + HMR-1098 54 +/- 7%, P = NS]. We conclude that ME limits ischemic injury in vivo by an opioid receptor-mediated mechanism that involves both sarcolemmal and mitochondrial KATP channels.

Degradation of endomorphin-2 at the supraspinal level in mice is initiated by dipeptidyl peptidase IV: an in vitro and in vivo study

Endomorphin-2 (Tyr-Pro-Phe-PheNH(2)) was discovered as an endogenous ligand for the mu-opioid receptor. The physiological function of endomorphin-2 as a neurotransmitter or neuromodulator may cease through the rapid enzymatic process in the synapse of brain, as for other neuropeptides. The present study was conducted to examine the metabolism of endomorphin-2 by synaptic membranes prepared from mouse brain. Major metabolites were free tyrosine, free phenylalanine, Tyr-Pro and PheNH(2). Both the degradation of endomorphin-2 and the accumulation of major metabolites were inhibited by specific inhibitors of dipeptidyl peptidase IV, such as diprotin A and B. On the other hand, the accumulation of Phe-PheNH(2) and Pro-Phe-PheNH(2) was increased in the presence of bestatin, an aminopeptidase inhibitor, whereas that of free phenylalanine and PheNH(2) was decreased. Furthermore, purified dipeptidyl peptidase IV hydrolyzed endomorphin-2 at the cleavage site, Pro(2)-Phe(3) bond. Thus, degradation of endomorphin-2 by brain synaptic membranes seems to take place mainly through the cleavage of Pro(2)-Phe(3) bond by dipeptidyl peptidase IV, followed by release of free phenylalanine and PheNH(2) from the liberated fragment, Phe-PheNH(2) by aminopeptidase. We have also examined that the effect of diprotin A on the antinociception induced by intracerebroventricularly administered endomorphin-2 in the mouse paw withdrawal test. Diprotin A simultaneously injected with endomorphin-2 enhanced endomorphin-2-induced antinociception. These results indicate that dipeptidyl peptidase IV may be an important peptidase responsible for terminating endomorphin-2-induced antinociception at the supraspinal level in mice.

Acetorphan, an Enkephalinase Inhibitor, Modulates Dopaminergic Transmission in Rat Olfactory Tubercle, but not in the Nucleus Accumbens and Striatum

The present study focused on the effects of acetorphan, a parenterally active enkephalinase inhibitor, on dopaminergic transmission in rat olfactory tubercle, nucleus accumbens and striatum. Acetorphan was administered i.v. (10 mg/kg) 15 min before measurement of the in vivo specific binding of [3H]N-propylnorapomorphine ([3H]NPA) or measurement of the levels of dopamine (DA) and its metabolites 3-methoxytyramine-homovanillic acid (3MT-HVA) in the three areas. Acetorphan decreased the in vivo specific binding of [3H]NPA in the olfactory tubercle, this effect being antagonized by naloxone 1.5 mg/kg s.c. DA release in this brain structure was also significantly increased by acetorphan 10 mg/kg, as indicated by the 3MT:DA and HVA:DA ratios. Neither the specific binding of [3H]NPA nor DA metabolism and release were modified by the inhibitor in the striatum and the nucleus accumbens. The stimulant effect of acetorphan was significantly decreased in rats in which a bilateral lesion of dopaminergic endings in the olfactory tubercle had been produced by 6-hydroxydopamine (6-OHDA). These results suggest that dopaminergic transmissions in the olfactory tubercle are particularly sensitive to the modulation exerted by endogenous enkephalins, this modulation being at least partly involved in the increased locomotion induced by the enkephalinase inhibitor.

Characterization of human nasal primary culture systems to investigate peptide metabolism

The objectives of this study were to validate and compare the suitability of different primary cell culture systems as models to investigate peptide enzymatic stability following nasal administration. The degradation kinetics of a model peptide, leucine enkephalin (Tyr-Gly-Gly-Phe-Leu, Leu-Enk), was determined in four nasal cell culture systems: immersion, air-liquid interface, sequential monolayer-suspension, floating collagen. The influence of enzyme inhibitors (bestatin, puromycin) and Leu-Enk metabolite analogs (Tyr-Gly, Phe-Leu, Tyr-Gly-Gly, Gly-Phe-Leu) on the Leu-Enk degradation profile was also investigated. The disappearance of Leu-Enk in all the cell culture systems followed first order kinetics. The specific activity in the cell culture systems followed the rank: sequential monolayer-suspension (32.60 microM min(-1) mg(-1)) >air-liquid interface (15.19 microM min(-1) mg(-1)) >immersion (11.49 microM min(-1) mg(-1)) >floating collagen (4.57 microM min(-1) mg(-1)). At equimolar concentration, bestatin had a higher inhibitory effect than puromycin. The rate of hydrolysis of Leu-Enk was reduced significantly by co-incubation with Leu-Enk metabolite analogs. This study showed that immersion, sequential monolayer-suspension and air-liquid interface culture systems may be potentially suitable for further studies on peptide enzymatic stability following nasal administration.

Nasal absorption enhancement strategies for therapeutic peptides: an in vitro study using cultured human nasal epithelium

This study examined the potential usefulness of cultured human nasal epithelium as a model to investigate nasal absorption enhancement strategies for therapeutic peptides. The transport of leucine enkephalin (Leu-Enk) in the presence of bestatin and puromycin, respectively and various combinations of these protease inhibitors with absorption enhancers capable of inhibiting proteases or protecting peptides against protease degradation (glycocholate, dimethyl-beta-cyclodextrin (DM beta CD)) was studied. Epithelial membrane perturbation, protein leakage, bestatin/puromycin absorption and rebound aminopeptidase activity were used as toxicological end-points. The combination of puromycin with glycocholate or DM beta CD resulted in a higher absorption enhancement of Leu-Enk (9-14%) than when the absorption enhancers were combined with bestatin (1-3%) or when the inhibitors were used alone (2-4%). The higher absorption enhancement resulting from the combination of protease inhibitors with absorption enhancers caused a significant reduction of epithelial resistance and increased sodium fluorescein transport. Although only puromycin permeated the human nasal epithelium, both protease inhibitors induced a significant rebound aminopeptidase activity (25-61%), which can be associated with protein leakage (21-46%). This study highlighted (i) the potential usefulness of cultured human nasal epithelium as a model to study nasal absorption enhancement of therapeutic peptides; (ii) further studies using in vivo nasal models are required to ascertain whether the membrane perturbation and cytotoxicity observed with various combinations of the protease inhibitors and absorption enhancers really raise safety concerns.

Regional and subcellular distribution of soluble aminopeptidase in the human and the rat brain: a comparative study

In an attempt to elucidate the cellular function of the soluble aminopeptidases, we have analysed their activity in several subcellular fractions (synaptosomal, mitochondrial, microsomal, nuclear and cytosolic fraction) and in different areas (amygdala, hypothalamus, hippocampus, striatum, frontal cortex, occipital cortex and parietal cortex) of the human and the rat brain. The enzymes assayed in this study were five cytosolic aminopeptidases identified inmammalian brain tissues: alanyl-aminopeptidase, arginyl-aminopeptidase, leucyl-aminopeptidase, pyroglutamyl-peptidase I and aspartyl-aminopeptidase. The regional comparative study revealed significantly higher activities of alanyl-aminopeptidase activity in the human brain, with arginyl-aminopeptidase activities being higher in the rat brain. In the subcellular study, while the alanyl- and arginyl-aminopeptidase activities were quite homogeneous in all the subcellular fractions, the leucyl-aminopeptidase, pyroglutamyl-peptidase I and aspartyl-aminopeptidase activities were significantly higher in the synaptosomal fraction. The differential distribution of these enzymes could suggest that these activities have different functions in the distinct subcellular structures of the human and the rat brain.

Transplacental transfer of the opioid growth factor, [Met(5)]-enkephalin, in rats

Placental transfer of the pentapeptide [Met5]-enkephalin, known to function as a growth regulating factor and neuromodulatory agent, was studied in pregnant Sprague-Dawley rats. Using separation by reversed phase high-performance liquid chromatography, and analysis by derivative spectroscopy, [Met5]-enkephalin was detected in 20-day-old fetal tissue including brain, heart, lung, and kidney. Fetal tissues from pregnant rats given an injection of 40 mg/kg [Met5]-enkephalin on gestation day 20 had markedly elevated levels of peptide within 1 h, indicating the transplacental transfer of this opioid. [Met5]-enkephalin levels were increased from control samples at 1, 2, 4, and 14 h post-injection of peptide, but not at 24 h. Evaluation of breakdown products of [Met5]-enkephalin, along with the related peptide [Leu5]-enkephalin, revealed that elution times differed substantially from [Met5]-enkephalin. These data indicate that [Met5]-enkephalin is present in fetal organs, crosses the placenta, does not appear to be restrictive in organ specificity, and is sustained in fetal tissues at detectable levels for at least 14 h. Given that [Met5]-enkephalin tonically inhibits DNA synthesis in the fetus, these results raise the question of whether an elevated level of this peptide (either maternally or from the fetus) may be detrimental to cellular ontogeny in the fetus, and perhaps have long-term implications for postnatal development.

Human neutral brush border endopeptidase EC 3.4.24.11 in urine, its isolation, characterisation and activity in renal diseases

Human neutral brush border endopeptidase (NEP) was purified from the urine of patients suffering from acute toxic tubulointerstitial nephropathy. An enzyme preparation with specific activity of 102 Ug(-1) protein was obtained. The urinary activities of neutral endopeptidase and alanine aminopeptidase were measured in patients with renal disease and in 30 control patients, resulting in a reference range from 0.1 to 0.7 Ug(-1) creatinine and 1.4-14.1 Ug(-1) creatinine, respectively. Urine enzyme activities were highest in patients with acute tubulotoxic renal diseases. Neutral endopeptidase and alanine aminopeptidase activities were found to be 6.5- and 10-fold higher than the upper value of the reference range, respectively. Smaller increases in the rate of excretion of these enzymes (2.5- and 3.5-fold), respectively, were observed in patients suffering from acute tubular insufficiency and even lower increases, 2- and 1.5-fold, respectively, were observed in patients with chronic renal diseases. In diabetics and kidney transplant patients the enzyme excretion rates were within the reference range. Assay of both transmembrane metalloproteinases in urine may prove valuable in serving as markers for renal toxicity. Together with beta-NAG these enzymes could be employed as differentiation markers between acute and chronic tubular insufficiency.

Allergen inhalation challenge induces decrease of serum neutral endopeptidase (NEP) in asthmatics

There is accumulating evidence that tachykinins are implicated in inflammation, including asthma. Therefore, we hypothesized that the neutral endopeptidase (NEP), under challenge conditions, could be affected. Serum from 21 asthmatics and six healthy volunteers was sampled before, 30, and 120 min after allergen challenge. NEP-IR was determined using an ELISA and was found in all subjects. Compared to prechallenge, no difference was seen between asthmatics and controls; however, under challenge conditions, NEP-IR in asthmatics was significantly lower (30 min, P = 0.058; 120 min, P = 0.0017, respectively). This finding supports indirectly the hypothesis that tachykinins are released during allergen exposure, and suggests a regulatory role of NEP.

Met5-enkephalin protects isolated adult rabbit cardiomyocytes via delta-opioid receptors

In rats and rabbits, endogenous opioid peptides participate in ischemic preconditioning. However, it is not known which endogenous opioid(s) can trigger cardioprotection. We examined preconditioning-induced and opioid-induced limitation of cell death in isolated, calcium-tolerant, adult rabbit cardiomyocytes. Cells were subjected to simulated ischemia by pelleting and normothermic hypoxic incubation. Preconditioning was elicited with 15 min of simulated ischemia followed by 15 min of resuspension and reoxygenation. All cells underwent 180 min of simulated ischemia. Cell death was assessed by trypan blue permeability. Morphine protected cells, as did preconditioning; naloxone blocked the preconditioning-induced protection. Exogenous Met5-enkephalin (ME) induced protection, but exogenous beta-endorphin did not. ME-induced protection was blocked by the delta-selective antagonist naltrindole. Additionally, two other proenkephalin products, Leu5-enkephalin and Met5-enkephalin-Arg-Phe, provided protection equipotent to ME. These data suggest that one or more proenkephalin products interact with delta-opioid receptors to endogenously trigger opioid-mediated protection.

Immunolocalization of tripeptidyl peptidase II, a cholecystokinin-inactivating enzyme, in rat brain

Tripeptidyl peptidase II (EC 3.4.14.10) is a serine peptidase apparently involved in the inactivation of cholecystokinin octapeptide [Rose C. et al. (1996) Nature 380, 403-409]. We have compared its distribution with that of cholecystokinin in rat brain, using a polyclonal antibody raised against a highly purified preparation for immunohistochemistry at the photon and electron microscope levels. Tripeptidyl peptidase II-like immunoreactivity was mostly detected in neurons, and also in ependymal cells and choroid plexuses, localizations consistent with a possible participation of the peptidase in the inactivation of cholecystokinin circulating in the cerebrospinal fluid. Immunoreactivity was mostly detected in cell bodies, large processes and, to a lesser extent, axons of various neuronal populations. Their localization, relative to that of cholecystokinin terminals, appears to define three distinct situations. The first corresponds to neurons with high immunoreactivity in areas containing cholecystokinin terminals, as in the cerebral cortex or hippocampal formation, where pyramidal cell bodies and processes surrounded by cholecystokinin axons were immunoreactive. A similar situation was encountered in many other areas, namely along the pathways through which cholecystokinin controls satiety, i.e. in sensory vagal neurons, the nucleus tractus solitarius and hypothalamic nuclei. The second situation corresponds to cholecystokinin neuronal populations containing tripeptidyl peptidase II-like immunoreactivity, as in neurons of the supraoptic or paraventricular nuclei, axons in the median eminence or nigral neurons. In both situations, localization of tripeptidyl peptidase II-like immunoreactivity is consistent with a role in cholecystokinin inactivation. The third situation corresponds to areas with mismatches, such as the cerebellum, a region devoid of cholecystokinin, but in which Purkinje cells displayed high tripeptidyl peptidase II-like immunoreactivity, possibly related to a role in the inactivation of neuropeptides other than cholecystokinin. Also, some areas with cholecystokinin terminals, e.g., the molecular layer of the cerebral cortex, were devoid of tripeptidyl peptidase II-like immunoreactivity, suggesting that processes other than cleavage by tripeptidyl peptidase II may be involved in cholecystokinin inactivation. Tripeptidyl peptidase II-like immunoreactivity was also detected at the ultrastructural level in the cerebral cortex and hypothalamus using either immunoperoxidase or silver-enhanced immunogold detection. It was mainly associated with the cytoplasm of neuronal somata and dendrites, often in the vicinity of reticulum cisternae, Golgi apparatus or vesicles, and with the inner side of the dendritic plasma membrane. Hence, whereas a fraction of tripeptidyl peptidase II-like immunoreactivity localization at the cellular level is consistent with its alleged function in cholecystokinin octapeptide inactivation, its association with the outside plasma membrane of neurons remains to be confirmed.

Hydrolysis of peripheral leucine enkephalin in allergic asthma

Plasma hydrolysis of leucine enkephalin was studied in a group of patients affected by seasonal allergic asthma in acute and quiescent stage; data were compared with those obtained from a control group of healthy volunteers. Results obtained indicate a statistically significant reduction of leu-enkephalin hydrolysis in allergic subjects. In the quiescent stage, substrate degradation is reduced, and the pattern of the hydrolysis by-products is modified with respect to normal controls. In the acute stage, hydrolysis is further reduced, and the pattern of the hydrolysis by-products is further modified with respect to the quiescent stage. The variations of leu-enkephalin hydrolysis appear to be controlled by decreased activity of proteolytic enzymes and by increased activity of the low-molecular-weight plasma inhibitors active on these enzymes. The sum of these processes is conducive to a distribution of enkephalin-hydrolyzing enzymes, as well as a hydrolysis pattern, that appears to be specific for the allergic subjects and distinct from that seen in the controls.

Delta opiate receptors account for the castration-induced unmasking of gonadotropin-releasing hormone binding sites in the rat pituitary

Under control incubation conditions, gonadotropin-releasing hormone (GnRH) binds only a fraction of its receptors in rat-cultivated pituitary cells. Unmasking of the remaining receptors, which have been termed 'cryptic', requires drug- or peptide-induced protein kinase activation. Spontaneous masking however is not observed on pituitary cells sampled from castrated male rats, suggesting the presence of an intrinsic unmasking factor. Many endogenous factors could theoretically account for the effect. Here we attempted to identify the factor involved by taking advantage of their differential dependency upon second messengers and transduction cascades. Spontaneous unmasking of GnRH binding was found reversed by pertussis toxin (PTX), an inhibitor of alphai and alphao subunits of heterotrimeric G proteins, and by U73122, a phospholipase C (PLC) inhibitor. In contrast, desensitization of protein kinase C (PKC) or inhibition of tyrosine kinase by herbimycin were ineffective. Among endogenous pituitary factors able to unmask GnRH receptors in pituitary cells from normal male rats, as EGF, NPY or opiate peptides, only the latter were found to correspond to this transduction profile. In an attempt to characterize the pharmacology of opiate effects, naloxone (10 microM), a poorly selective opiate antagonist, restored masking of GnRH binding in cells from castrates. Only the delta antagonist naltrindole (1 microM) was able to mimick the action of naloxone. Conversely, when tested on cells from intact animals, morphine (10 microM), as well as dslet (1 microM) and met-ENK (10 nM), preferential delta agonists, but not dago and beta-endorphin or U50488 H and dynorphin, respectively micro and kappa agonists, were able to suppress masking. Among opioid peptides endogenous to the pituitary, only met-ENK was able to unmask cryptic receptors, an effect antagonized by naltrindole. We conclude that an opiate delta receptor subtype is endogenously activated in the pituitary of castrated male rats to prevent masking of GnRH binding.

Purification and characterization of a neutral endopeptidase-like enzyme from human urine

OBJECTIVE

The aims of this study were to purify and characterize a neutral endopeptidase-like enzyme (NEP-like) in human urine and propose a rapid, sensitive and specific assay for this enzyme using the fluorogenic substrate Abz-FDQ-EDDnp, where Abz = O-aminobenzoic acid and EDDnp = N-(2,4-dinitrophenyl)ethylenediamine.

METHODS

Soluble urinary NEP was purified from human urine using a DEAE-cellulose Cellex D column and gel filtration on an AcA-44 column. NEP-like activity was assayed by its ability to hydrolyse bradykinin (BK) and the fluorogenic substrates Abz-BKQ-EDDnp and Abz-FDQ-EDDnp. The Km was determined using Abz-FDQ-EDDnp as a substrate. The hydrolysis products of BK and Abz-FDQ-EDDnp were analysed by high-performance liquid chromatography (HPLC). The mol. wt was estimated by polyacrylamide gel electrophoresis and the enzyme analysed by Western blot using the antibody obtained from purified recombinant NEP expressed in Pichia pastoris yeast.

RESULTS

The NEP-like was purified from human urine until homogeneity and presented a mol. wt of 94000. The substrate Abz-FDQ-EDDnp was selectively hydrolysed at the F-D bond by NEP-like and by recombinant NEP. For this substrate, the NEP-like activity was maximal at pH 7.0, although a small peak of activity was observed at pH 8.0, and the determined Km was 14 microM. The enzymatic activity was inhibited by thiorphan and phosphoramidon. In Western blot analysis, NEP-like reacted strongly with a polyclonal antibody for NEP.

CONCLUSION

A NEP-like enzyme was purified from human urine. Based on the mol. wt of the isolated NEP-like enzyme, it was concluded that this enzyme was produced in the kidney. In the kidney, this enzyme may cleave the kinins filtered through the glomerulus and also the kinins produced in the distal nephron. An internally quenched fluorogenic peptide, Abz-FDQ-EDDnp, was selectively hydrolysed by NEP-like and by recombinant NEP.

Leucine enkephalin degradation in allergopathic versus normal human plasma

The enzyme hydrolysis of labelled leu-enkephalin in the presence of plasma enzymes was studied by kinetic and chromatographic techniques in a group of allergopathic patients in the acute and quiescent stage; data obtained have been compared with those obtained with normal controls. Results shown indicate that in the quiescent stage substrate degradation is reduced, and that the pattern of the hydrolysis by-products is modified with respect to the controls. In the acute as compared to the quiescent stage, enkephalin hydrolysis is further reduced, and the pattern of hydrolysis by-products is further modified. ANOVA analysis of these data indicates that the dependency of hydrolysis reduction upon the grouping of subjects (i.e., controls, quiescent and acute stage) is statistically very significant. Reduced substrate hydrolysis, and modified hydrolysis pattern, appears to be associated with decreased activity of the enzymes involved and more significantly with increased activity of the low molecular weight plasma inhibitors. The combination of these two factors appears to define a hydrolysis pattern characteristic of the allergopathic subjects, similar in the quiescent and acute phase, and different from that observed in the controls.

Neutral endopeptidase-24.11 (NEP)-like activity in molluscan hemocytes

Using a spectrofluorimetric procedure, we found that the plasma membrane from hemocytes of two freshwater snails, Planorbarius corneus and Viviparus ater, shows neutral endopeptidase-24.11 (NEP)-like activity. Moreover, the addition of ACTH(1-24) to the hemolymph provokes an increase in NEP-like activity. This increased NEP-like activity is blocked by phosphoramidon, a potent inhibitor of mammalian NEP. These findings suggest that this peptidase has been well conserved in the course of evolution and plays an important role in immune-neuroendocrine mechanisms.

Inhibition of dynorphin-converting enzymes prolongs the antinociceptive effect of intrathecally administered dynorphin in the mouse formalin test

The effects of peptidase inhibitors on the antinociceptive induced by intrathecally (i.t.) administered by dynorphin A and dynorphin B in the mouse formalin test were examined. When administered i.t. 5 min before the injection of 0.5% formalin solution into the dorsal surface of a hindpaw, dynorphin A (0.5-2 nmol) and dynorphin B (2-8 nmol) produced a dose-dependent and significant reduction of the paw-licking response. Dynorphin A (2 nmol) and dynorphin B (8 nmol)-induced antinociception disappeared completely within 90 min and 60 min, respectively. p-Hydroxymercuribenzoate, a cysteine proteinase inhibitor, and phosphoramidon, and endopeptidase 24.11 inhibitor simultaneously administered with dynorphin A or dynorphin B. Significantly prolonged antinociception induced by both dynorphins. However, captopril, and angiotensin-converting enzyme inhibitor, bestatin (a general aminopeptidase inhibitor) and a serine proteinase inhibitor phenylmethanesulfonyl fluoride, were active. Dynorphin converting enzyme(s) transform dynorphin-related peptides to [Leu5]enkephalin and [Leu5]enkephalin-Arg6. Neither [Leu5]enkephalin nor [Leu5]enkephalin-Arg6, even at high dose (10 nmol), produced any antinociceptive effect. However, [Leu5[enkephalin-Arg6, but not [Leu5]enkephalin, produced a significant antinociceptive effect when co-administered with phosphoramidon. Therefore, the prolongation of the antinociception induced by both dynorphins in the presence of phosphoramidon, may be due to inhibition of [Leu5]enkephalin-Arg6 degradation. The present results indicate that dynorphin-converting enzyme(s) may be important enzyme(s) responsible for terminating dynorphin-A- and dynorphin-B-induced antinociception at the spinal cord level in mice.

A C-terminal cyclic 8-13 neurotensin fragment analog appears less exposed to neprilysin when it crosses the blood-brain barrier than the cerebrospinal fluid-brain barrier in mice

A C-terminal cyclic 8-13 neurotensin fragment analog. JMV 1193, a direct agonist of central neurotensin receptors, is able to cross both the cerebrospinal fluid-brain barrier and the blood-brain barrier. When administered intracerebroventricularly (i.c.v.), its hypothermic effect was potentiated by the enkephalinase inhibition induced either by thiorphan (simultaneous intracerebroventricular administration of 10 micrograms) or by the thiorphan prodrug. acetorphan (intravenous (i.v.) administration of 10 mg/kg). Such a potentiation was not observed when both JMV 1193 and acetorphan were administered intravenously. Therefore it appears that the sensitivity of JMV 1193 to enkephalinase depends on its route of administration. It is exposed to this peptidase after i.c.v. injection (when crossing the cerebrospinal fluid-brain barrier), while it is not after i.v. administration (when crossing the blood-brain barrier).

Metabolism of enkephalins in head membranes of the leech Theromyzon tessulatum by peptidases: isolation of an enkephalin-degrading aminopeptidase

Metabolism of leucine and methionine enkephalins by enzyme preparations from head parts of the leech Theromyzon tessulatum was investigated. Leech homogenate degraded enkephalins by cleavage of the Tyr1-Gly2 and Gly3-Phe4 bonds. The Tyr1-Gly2-Gly3 was detected as a major metabolite when amastatin (aminopeptidase inhibitor) was present to prevent Tyr1-Gly2 breakdown. Around 50% of enkephalin-degrading activity was isolated in a 20000 x g membrane fraction and was shown to be almost entirely due to an aminopeptidase activity. This enzyme, a homodimer of approx. 70 kDa, has been purified to homogeneity by a combined approach including gel permeation and anion exchange chromatographies followed by reversed-phase HPLC. This enkephalin-degrading aminopeptidase is a typical integral membrane 'zincin' metalloprotein with an apparent k(m) of 30 microM, a specific activity of 12 nmol GGFM min-1 mg protein-1 and a catalytic efficiency (kcat/k(m)) of 46 x 10(6) mol-1 min-1. This enzyme is specifically inhibited by amastatin (IC50 = 0.5 microM), but not by bestatin and actinonin. In leech membranes, the other degrading activities performed at the same time were due to a neuropeptide-endopeptidase (NEP)-like enzyme attack, inhibited by phosphoramidon (IC50 = 0.1 microM) and in the case of the Met-enkephalin by a combined action of an angiotensin-converting-like enzyme, inhibited by captopril (IC50 = 0.2 microM) and the NEP-like enzyme. These two enzymes were previously isolated from head membranes of T. tessulatum and possess towards Met-enkephalin a catalytic efficiency (kcat/k(m)) of, respectively, 12 x 10(6) mol-1 min-1 and 78 x 10(6) mol-1 min-1. These findings constitute the first report in leeches on the nature and the sites of attack of the membrane peptidases involved in the metabolism of enkephalins and also the first biochemical evidence for a novel member of the aminopeptidase family.

Role of enzymes and inhibitors in leu-enkephalin metabolism in rabbit and human plasma

The hydrolysis of leucine enkephalin by the proteolytic enzymes present in human and rabbit plasma has been studied by kinetic and chromatographic techniques. Data obtained indicate the existence of noticeable intraspecific differences in the kinetics of leu-enkephalin degradation, and of formation of its hydrolysis by-products. The separation of the enzymes active on the substrate and of the inhibitors active on these enzymes evidences the existence of a species specific distribution of both groups of substances. Yet, the dissimilar kinetics of the substrate hydrolysis and of formation of its hydrolysis by-products appear to arise more from diversities in the competition between the enzymes present in plasma and in the role of inhibitors than from the differences in the enkephalin-degrading enzymes. It is suggested that differences observed may be related to the existence of species specific populations of the information-carrying plasma peptides.