Nomenclature for enkephalin degrading peptidases

Enkephalinase regulation

After millennia of knowledge of opium, it was only recently that endogenous substances called opioids with similar properties to opium and derivatives were discovered. The first to be discovered were enkephalins. In addition to the regulation of their synthesis and expression of receptors, an important mechanism for the regulation of their functions carried out by multiple proteolytic enzymes acting at all levels of their structure is described. The action of such enzymes, known as enkephalinases, is also regulated by endogenous and exogenous factors which ultimately affect the control of the enkephalins's action. For therapeutic purposes, it is not only necessary to develop specific inhibitors but also to acquire a deep knowledge of the influence that such factors exert on their activities. This knowledge could help us to establish adapted therapeutic strategies in the treatment of pain or other processes in which enkephalinases are involved. In this chapter, some of these regulatory factors are discussed, such as regional and subcellular distribution, developmental changes, diurnal variations, hormonal influences, stress, dietary factors or interactions with other neurotransmitters.

Interaction of neuropeptidase activities in cortico-limbic regions after acute restraint stress

Brain enkephalin, vasopressin and oxytocin are anxiolytic agents involved in the stress response. Acute restraint stress influences certain neuropeptidase activities, such as some enkephalin-degrading peptidases and vasopressinase/oxytocinase, in the medial prefrontal cortex (mPFC), amygdala (AM) or hippocampus (HC), which are involved in this response. Because these regions form a unified circuit and cooperate in their response to stress, it is important to analyze the profile of the regional distribution of these activities as well as their inter-regional model of interaction in this circuit. Regarding the regional study, although most activities showed a marked predominance of the AM over the HC and mPFC, both in control and stressed animals, enkephalin-degrading activity, assayed as membrane-bound alanyl aminopeptidase activity, showed a change after stress, increasing in the HC and decreasing in the AM. The correlational study in controls indicated essentially a positive interaction between the mPFC and AM. In marked contrast, there was a highly significant change in the functional status of this circuit after stress, showing mainly a positive correlation between the mPFC and HC and between the AM and HC. The existence of correlations does not demonstrate a direct relationship between regions. However, reasons for such strong associations after restraint stress should be examined. The present study may indicate a connection between neuropeptidase activities and their corresponding neuropeptidergic substrates due to significant changes in the functional status of the cortico-limbic circuit after restraint stress.

Mutational analysis reveals only one catalytic histidine in neutral endopeptidase ("enkephalinase")

Aside from serving as zinc ligands, kinetic data has implicated one or more additional histidines as catalytic residues in neutral endopeptidase ("enkephalinase") action. One of these histidines has previously been identified as histidine 704 (Bateman et al., J. Biol. Chem., 265:8365-8368, 1990). In order to determine whether a second histidine is involved in catalysis each of these residues not previously changed have been converted to glutamine by site directed mutagenesis. The resultant recombinant enzymes possess full catalytic activity indicating that histidine 704 is the only catalytic histidine in the enzyme.

A specific enzyme assay for aminopeptidase M in rat brain

A specific enzyme assay for aminopeptidase M (APM) activity on rat brain membranes has been developed through selective use of enzyme inhibitors. Amastatin was the most potent inhibitor (amastatin > actinonin > MDL73347 > bestatin) for purified porcine kidney APM, giving 98% inhibition at a 6 microM concentration, while actinonin, yielded only 57% inhibition at this concentration. Puromycin (10 microM) was used to inhibit puromycin-sensitive aminopeptidase activity in the rat brain membrane preparation. Puromycin (10 microM) had only a slight effect on the Km of porcine kidney APM, and had negligible effect on APM velocity at the high substrate concentration (2 mM) used in the APM assay. The assay produced a linear accumulation of product for increasing amount of rat brain membranes used, and for increasing incubation time. The Km of APM on rat brain membranes for L-Leucine-p-nitroanilide (0.383 mM) was similar to the Km of purified porcine kidney APM (0.558 mM). APM-activity, involved in the metabolism of several biologically important neuropeptides in different brain regions, can be specifically measured with this enzyme assay.

Degradation of a-factor by a Saccharomyces cerevisiae alpha-mating-type-specific endopeptidase: evidence for a role in recovery of cells from G1 arrest

Mating response between opposite mating types of Saccharomyces cerevisiae is dependent upon alpha factor, a tridecapeptide, and a-factor, an isoprenylated, methyl esterified dodecapeptide whose interaction with the alpha target cell has not been characterized. We report on the first biochemical and physiological evidence of an alpha-mating-type-specific a-factor-degrading activity. Radioiodinated a-factor was used to identify the a-factor-degrading activity, which is cell associated, endoproteolytic, and not required for response to pheromone. a-factor degradation was not energy dependent, nor did it require pheromone internalization or interaction with its receptor. Phenylmethylsulfonyl fluoride and tosyl-L-arginyl-methyl ester inhibited degradation of a-factor and increased the time required by alpha cells to recover from a-factor-induced growth arrest and morphological alteration, providing evidence that a-factor degradation plays a role in the recovery of alpha cells from the pheromone response.

Degradation of a-factor by a Saccharomyces cerevisiae alpha-mating-type-specific endopeptidase: evidence for a role in recovery of cells from G1 arrest

Mating response between opposite mating types of Saccharomyces cerevisiae is dependent upon alpha factor, a tridecapeptide, and a-factor, an isoprenylated, methyl esterified dodecapeptide whose interaction with the alpha target cell has not been characterized. We report on the first biochemical and physiological evidence of an alpha-mating-type-specific a-factor-degrading activity. Radioiodinated a-factor was used to identify the a-factor-degrading activity, which is cell associated, endoproteolytic, and not required for response to pheromone. a-factor degradation was not energy dependent, nor did it require pheromone internalization or interaction with its receptor. Phenylmethylsulfonyl fluoride and tosyl-L-arginyl-methyl ester inhibited degradation of a-factor and increased the time required by alpha cells to recover from a-factor-induced growth arrest and morphological alteration, providing evidence that a-factor degradation plays a role in the recovery of alpha cells from the pheromone response.

Effect of electron withdrawing substituents on substrate hydrolysis by and inhibition of rat neutral endopeptidase 24.11 (enkephalinase) and thermolysin

A series of N-acylphenylalanylglycine dipeptides were synthesized and examined as substrates for neutral endopeptidase 24.11 (NEP) and thermolysin. Those N-acyl dipeptides containing an N-acyl group derived from an acid whose pKa is below 3.5 were considerably more reactive with both enzymes than those peptides containing an N-acyl group derived from an acid whose pKa is above 4. The data are interpreted to suggest that electron withdrawal at the scissile bond increases kappa cat for both NEP and thermolysin. The pH dependence for inhibition by the dipeptides Phe-Ala, Phe-Gly, and Leu-Ala showed binding dependent upon the basic form of an enzyme residue with a pKa of 7 for NEP and a pKa of 6 for thermolysin. In the case of thermolysin this pKa was decreased to 5.3 in the enzyme-inhibitor complex. When examined as alternate substrate inhibitors of NEP, N-acyl dipeptides showed three distinct profiles for the dependence of Ki on pH. With N-trifluoroacetyl-Phe-Gly as inhibitor, binding is dependent upon the basic form of an enzyme residue with a pKa value of 6.2. N-methoxyacetyl-Phe-Gly inhibition appears pH independent, while N-acetyl-Phe-Gly inhibition is dependent upon the acidic form of an enzyme residue with a pKa of approximately 7. All inhibitions of thermolysin by N-acyl dipeptides exhibit a dependence on the acidic form of an enzyme residue with a pKa of 5.3 to 5.8. These results suggest that with NEP, binding interactions at the active site involve one or more histidine residues while with thermolysin binding involves an active site glutamic acid residue.

Antidepressant action of imipramine and iprindole in mice is enhanced by inhibitors of enkephalin-degrading peptidases

The implication that opioid peptides are involved in the action of the antidepressants imipramine and iprindole was investigated in mice by using the forced swimming test as an experimental model of depression. Both the drugs were found to shorten the immobility time in this test. This effect of imipramine and iprindole was reversed by the opiate antagonist naloxone. Moreover, when subeffective doses of either imipramine or iprindole were given together with an intracerebroventricular injection of an inhibitor of their degradation (thiorphan or bestatin), the immobility time was again decreased. Interestingly, the reduction of the time of immobility was found to be not related to the effect of the drugs on locomotor activity. These data might be taken as further evidence for the involvement of opioid peptides in the pharmacological action of antidepressant drugs.

Effects of bestatin and phosphoramidon on the hypertensive response to physostigmine in the rat

Intracarotid (IC) injection of bestatin produced a dose-dependent biphasic change in blood pressure (BP) of the rat, consisting of an initial short-lasting fall followed by a long-lasting increase. This effect was regularly depressed or abolished by IV injection of naloxone. IC injection of Leu-enkephalin also produced a biphasic BP response, with the same characteristics as that produced by IC injection of bestatin. This effect was also easily blocked by IV injection of naloxone. IC injection of bestatin significantly potentiated the BP response to IC injection of Leu-enkephalin. This potentiated response was blocked by naloxone. IC injection of both bestatin and phosphoramidon, whether separately or in combination, significantly depressed the hypertensive response to physostigmine. This depressive action of bestatin and phosphoramidon on physostigmine hypertension can be significantly antagonized or even reversed by IV injection of naloxone. IC injection of both bestatin and phosphoramidon did not affect the BP response to either acetylcholine or catecholamines. It is concluded that bestatin and phosphoramidon, injected into the carotid artery, inhibit the activity of aminopeptidase and "enkephalinase", thus producing an accumulation of enkephalins in the central nervous system. These enkephalins activate opioidergic receptors in the brain, but concomitantly produce a depression of the cholinergic-adrenergic interaction in the central nervous system, which is known to be a prerequisite for the hypertensive response to physostigmine in the rat.

Kelatorphan potentiates the effect of [Met5]enkephalin in the substantia gelatinosa of the cat spinal cord

In anaesthetized spinal cats, kelatorphan, an inhibitor of enkephalin degradation, was administered microelectrophoretically while recording the excitation of lumbar dorsal horn neurones by noxious and innocuous peripheral stimuli. When administered near the cell bodies of laminae IV and V neurons, kelatorphan neither altered evoked responses nor potentiated the inhibition by [Met5]enkephalin of these cells. When ejected in the substantia gelatinosa, however, kelatorphan reduced the nociceptive responses of some laminae IV and V neurones, an effect blocked by electrophoretic naloxone. The selective inhibition of nociceptive responses by [Met5]enkephalin administered in the substantia gelatinosa was markedly potentiated by co-administration of kelatorphan, and this effect was also blocked by electrophoretic naloxone. Neurones inhibited by administration of kelatorphan alone in the substantia gelatinosa were excited by administration of naloxone alone at the same site. The results suggest that some dorsal horn neurones are tonically inhibited by an action of opioid peptides in the substantia gelatinosa, and indicate that enzymic degradation limits the action of both exogenous and endogenous enkephalin in this spinal region.

Immunohistochemical localization of aminopeptidase M in rat brain and periphery: relationship of enzyme localization and enkephalin metabolism

An antiserum specific for rat aminopeptidase M has been used for the immunohistochemical localization of the enzyme in rat brain and peripheral tissues. The enzyme in brain is localized exclusively on blood vessels. Within the pituitary the enzyme was associated with the vasculature in the posterior lobe, on the surface of the intermediate lobe and on the surface of some cells in the anterior lobe. In the liver, fine cell staining was observed between parenchymal cells, in the ileum the entire lumenal surface was stained, while in the kidney both proximal tubular and a central tubular staining was detected. In each tissue aminopeptidase M is localized such that it can limit diffusion across specific barriers. Aminopeptidase M activity in brain has been proposed to function in the degradation of synaptically released enkephalins. Its localization on blood vessels requires that enkephalins diffuse prior to degradation, a concept not in concert with current hypotheses. Based on these studies it is proposed that diffusion away from enkephalinergic synapses plays a key role in terminating enkephalin action.

Purification and characterization of a peptidyl dipeptidase resembling angiotensin converting enzyme from the electric organ of Torpedo marmorata

The electric organ of Torpedo marmorata contains a membrane-bound, captopril-sensitive metallopeptidase that resembles mammalian angiotensin converting enzyme (peptidyl dipeptidase A; EC 3.4.15.1). The Torpedo enzyme has now been purified to apparent homogeneity from electric organ by a procedure involving affinity chromatography using the selective inhibitor lisinopril immobilised to Sepharose via a 28-A spacer arm. The purified protein, like the mammalian enzyme, acted as a peptidyl dipeptidase in cleaving dipeptides from the C-terminus of a variety of peptide substrates, including angiotensin I, bradykinin, [Met5]enkephalin, [Leu5]enkephalin, and the model substrate hippuryl (benzoylglycyl; BzGly)-His-Leu. The hydrolysis of BzGly-His-Leu was activated by Cl-. Enzyme activity was inhibited by classical angiotensin converting enzyme inhibitors, including captopril, enalaprilat (MK422), and lisinopril (MK521). Torpedo angiotensin converting enzyme, like its mammalian counterpart, was also able to act as an endopeptidase in hydrolysing the amidated neuropeptide substance P. Hydrolysis of substance P occurred primarily at the Phe8-Gly9 bond with release of the C-terminal tripeptide, Gly-Leu-MetNH2, and this hydrolysis was blocked by selective inhibitors. The Torpedo enzyme was recognised by a polyclonal antibody to pig kidney angiotensin converting enzyme on immunoelectrophoretic (Western) blot analysis. Thus, on the basis of substrate specificity, inhibitor sensitivity, and immunological criteria, the Torpedo enzyme closely resembles mammalian angiotensin converting enzyme. However, the Torpedo enzyme appears somewhat larger (Mr = 190,000) than the pig kidney enzyme (Mr = 180,000) on sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The endogenous peptide substrate(s) for Torpedo electric organ angiotensin converting enzyme and the physiological role of the enzyme in this tissue remain to be evaluated.

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.