Pro-enkephalin

Group II consists of the enkephalins which come from the 267-aniino acid piecuisoi pro-enkephalin A [88402-54-4] (Fig. 2). This proteia contains four copies of Met-enkephalin, one copy of Leu-enkephalin, and the extended peptides Met-enkephalin-Arg -Phe (the last Met-enkephalin sequence ia Fig. 2) and Met-enkephalin-Arg -Gly -Leu (the fourth Met-enkephalin sequence ia Fig. 2) (25,26). AH of these products ate formed by trypsin-like cleavage between pairs of basic residues. The extended enkephalin peptides are further cleaved by carboxypeptidase E (27) to form authentic Met-enkephalin. 

The Group III peptides come from the 256-amino acid precursor, pro-dynorphin [88402-55-5] (pro-enkephalin B). This group contains dynorphin A [80448-90-4] and B [85006-82-2] as weU as a-neoendorphin [77739-20-9] (Fig. 2), all of which can be further cleaved to form biologically active iatermediates, eg, dynorphin A g and P-neoendorphin [77739-21-0] (a-neoendorphin ) (28). The longer of these peptides are relatively basic because of the number of Lys and Arg residues. 

Pig. 3. Representation of promoter sites on the pro-enkephalin gene. The numbers represent the distance in nucleotides from the pro-enkephalin initiation codon the arrow indicates the direction of transcription. The TATA promoter box occurs immediately before the pro-enkephalin initiation site the AP-2 site, which binds immediate-early gene products, is 70 nucleotides upstream, and the CRE site, which binds a regulatory protein involved in cAMP induction of mRNA synthesis, is 107 nucleotides upstream from the initiation codon. The expanded section shows that the CRE site actually consists of two elements, ENKCRE-1 and ENKCRE-2, which separately confer cAMP sensitivity to pro-enkephalin mRNA synthesis. 

In addition to the weU-defined opioid systems in the central nervous system, the three opioid peptides and their precursor mRNA have also been identified in peripheral tissues. ( -Endorphin is most abundant in the pituitary, where it exists in corticotroph cells with ACTH in the anterior lobe and in melanotroph cells with MSH in the intermediate lobe (59). Enkephalin and pre-pro-enkephalin mRNA have been identified in the adrenal medulla (60) and this has been the source of material for many studies of pro-enkephalin synthesis and regulation. Pre-pro-enkephalin mRNA has also been identified in the anterior and posterior lobes of the pituitary (61). mRNA for all three opioid precursors has been identified in the reproductive system (62—64). POMC... 

The classic endogenous opioid peptides are derived from one of three families of precursors proopiomelanocortin (POMC), pro-dynorphin, and pro-enkephalin. Many active opioid peptides are derived from these three, but the best known are )S-endorphin, enkephalin, and dynorphin. POMC is produced by nuclei in the hypothalamus and medulla (Khachaturian et al. 1985 Watson et al. 1978 Bloom et al. 1978). Enkephalin and dynorphin neurons are distributed to all levels of the central nervous system (Hokfelt et al. 1977 Khachaturian et al. 1983 Sar et al. 1978 Khachaturian et al. 1985). 

The enkephalins, derived from pro-enkephalin, are distributed much more widely in the CNS both in local circuit neurons and projection neurons, a distribution generally paralleling to that of the opiate receptors. Leu-enkephalin predominates over Met-enkephalin and both peptides are inhibitory. Enkephalin containing neurons are sparsely distributed in upper lay-... 

Precursor peptides (1)pro-endomorphin (2)pro-opiomelanocortin, <3)pro-enkephalin, (4)pro-dynorphin, (5)pro-nociceptin ( presumed to exist). 

The availability of receptor knockout animals has also helped to illustrate cannabinoid-opioid interactions. CBi receptor knockout mice had greatly reduced morphine self-administration behavior and less severe naloxone-induced withdrawal signs than wild type animals, although the antinociceptive actions of morphine were unaffected in the knockout animals (40). The rimona-bant-precipitated withdrawal syndrome in THC-treated mice was significantly attenuated in animals with knockout of the pro-enkephalin gene (48). Knockout of the p opioid (OP3) receptor also reduced rimonabant-induced withdrawal signs in THC-treated mice, and there was an attenuated naloxone withdrawal syndrome in morphine-dependent CBi knockout mice (49,50). 

Three separate categories of endogenous substances with morphine-like activity have been identified. These families of neuropeptides are known as enkephalins, endorphins, and dynorphins. Each family is derived from a distinct precursor polypeptide (pro-enkephalin, pro-opiomelanocortin, and prodynorfin) and has a characteristic anatomical distribution. The enkephalins and dynorphins found co-exist with other neurotransmitters, such as serotonin (5-HT) and noradrenaline, but details of how the peptides modulate the activity of co-transmitters await elucidation. 

In addition to P-endorphin, P-LPH contains the amino acid sequence of another endogenous opioid, met-enkephalin. However, this peptide is not the product of P-LPH breakdown, but rather arises from a precursor molecule loiown as pro-enkephalin. Pro-enkephalin is widely distributed in neurons throughout the brain and spinal cord. Some pro-enkephalin is found in the pituitary gland, but most is localized in the catecholamine-synthesizing cells of the adrenal medulla and is co-released with epinephrine and norepinephrine. In the medulla, pro-enkephalin gives rise to met-enkephalin (Tyr-Gle-Gle-Phe-Met) and leu-enkephalin (Tyr-Gle-Gle-Phe-Leu) and to larger opioid peptides. A third family of endogenous opioid peptides is derived from prodynorphin, a prohormone stored primarily in the poste-... 

Metorphamide, adrenorphin, H-Tyr-Gly-Gly-Phe-Met-Arg-Arg-Val-NH2, an 8-peptide amide found in bovine brain and adrenal tissue. The sequence of metorphamide corresponds to bovine pro-enkephalin precursor-(206-213). Furthermore, metorphamide is similar in sequence to bovine adrenal medulla peptides. It shows high affinity to X opioid receptors [H. Matsuo et al.. Nature 1983, 305, 721 M. Sonders, E. Weber, J. Neurochem. 1987, 49, 671]. 

Neoendorphins, opioid peptides derived from the precursor protein pro-dynorphin (pro-enkephalin B). a-Neoendorphin, H-Tyr-Gly-Gly-Phe-Leu-Arg-Lys-Tyr-Pro-Lys -OH, corresponds to the partial sequence of human and porcine pro-dynorphin-(175-184), and contains the sequence of -neoendorphin ([desLys ]-a-endorphin). Both neoendorphins show potent activity in the guinea pig ileum assay [K. Kangawa et al., Biochem. Biophys. Res. Commun. 1981, 99, 871 N. Minamino et al., Biochem. Biophys. Res. Commun. 1981, 99, 864]. 

The adrenal gland has been shown to contain high levels of enkephalins and a precursor peptide pro-enkephalin [20], The structure of proenkephalin has been characterized from the adrenal gland mRNA and found to contain four repeating units of the [Met]-enkephalin sequence and one [Leu]-enkephalin sequence this ratio closely resembles that for the pentapeptides in the brain. 

Pro-enkephalin A, 426-427, 428/ Pro-enkephalin B, 426-427 Promorphinane alkaloid (-)-8,14-dihy-droflavinantine, 26-27 Propeleine, 41, 41/... 

TABLE 7.1 fragmentation of pro-enkephalin A, a precursor of human enkephalin 428... 

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