Enkephalins inactivation

Inhibitors of Enkephalin-Inactivating Enzymes and Delta Opioid Responses... 

Owing to the complementary roles of NEP and APN in enkephalin inactivation, selective inhibitor of only one of the two peptidases gives weak antinoceptive effects even after ICV administration. This led us to propose the concept of mixed inhibitors, that is, compounds able to simultaneously block NEP and APN activities [reviews in 9,20]. This was possible owing to the fact that these two membrane-bound enzymes belong to the superfamily of zinc metallopeptidases. 

Some studies have suggested that there may be links between the development of dependence to cannabinoids and to opiates (42). Some of the behavioral signs of rimonabant-induced withdrawal in THC-treated rats can be mimicked by the opiate antagonist naloxone (43). Conversely, the withdrawal syndrome precipitated by naloxone in morphine-dependent mice can be partly relieved by THC (44) or endocannabinoids (45). Rats treated chronically with the cannabinoid WIN55212-2 became sensitized to the behavioral effects of heroin (46). Such interactions can also be demonstrated acutely. Synergy between cannabinoids and opiate analgesics has been described above. THC also facilitated the antinociceptive effects of RB 101, an inhibitor of enkephalin inactivation, and acute administration of THC caused... 

The identification of the morphine receptor spurred an effort in many laboratories to find an endogenous agonist for which that receptor was normally intended. Ultimately, a pair of pentapeptides that bound quite tightly to opiate receptors were isolated from mammalian brains. These peptides, called enkephalins (2, 3), show many of the activities of synthetic opiates in isolated organ systems. They do in fact show analgesic activity when injected directly into the brain. It is thought that lack of activity by other routes of administration is due to their rapid inactivation by peptide cleaving enzymes. 

Some drugs possessing amide bonds, such as prilocaine, and of course, peptides, can be hydrolyzed by peptidases and inactivated in this manner. Peptidases are also of pharmacological interest because they are responsible for the formation of highly reactive cleavage products (fibrin, p. 146) and potent mediators (angiotensin 11, p. 124 bradykinin, enkephalin, p. 210) from biologically inactive peptides. 

PEPTIDASE-DEPENDENT INACTIVATION OF ENKEPHALINS DESIGN OF DUAL INHIBITORS OF NEP/APN... 

The N-dihydroxyphosphinylphenylpropionylleucine derivative 24 from the Wellcome Foundation Ltd. 45a) acts as a morphine agonist and an inhibitor of enke-phalinase (dipeptidylcarboxypeptidase), which causes hydrolytic cleavage of the Gly3-Phe4 linkage of the enkephalin, and hence inactivation. 

Photoaffinity labels have also been investigated. These are enkephalin analogs containing a photosensitive group [e.g., 4-azidophenyl or 2-nitro-4-azidophenyl (NAP)] that form a covalent bond upon irradiation with UV light. Thus, both the peptide 11<37) and the Leu-enkephalins 12(38) bind strongly to brain membranes on photolysis, and the latter pair have been shown to cause a 20-30% inactivation of opioid receptors. Further work on affinity labels and related topics is described in the 1982 and 1983 reports of the International Narcotics Research Conference/39 ... 

Peptide-based affinity labels have been principally photoaffinity labels, including the azide derivatives of several enkephalin analogs and CTP (see Refs. 286,594 for detailed reviews). As noted earlier (Section 5.11.2), the use of photoaffinity labels, however, has been limited because opioid receptors are susceptible to inactivation by UV irradiation (643). 

SAR studies on the enkephalins have shown the importance of the tyrosine phenol ring and the tyrosine amino group. Without either, activity is lost. If tyrosine is replaced with another amino acid, then activity is also lost (the only exception being D-serine). It has also been found that the enkephalins are easily inactivated by... 

The human plasma metallo-protease carboxypeptidase N (CPN, arginine carboxypeptidase, anaphylatoxin inactivator, kininase I, EC 3.4.17.3) catalyzes the release of the basic amino acids lysine and arginine from the C-termini of peptides and proteins such as bradykinin and kallidin [95], the anaphylatoxins C3a, C4a, and C5a [96,97], fibrinopeptides 6A and 6D [98], hexapeptide enkephalins [99], protamine [100], and the creatine kinase MM-isoenzyme [101,102]. Its most likely physiological function is to protect the organism from the actions of potent peptides, which may escape from tissues or be released in the circulation. 

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