Opioid peptides precursors

Cloning of the opioid receptors did not come easy, and there were many false claims along the way. Efforts began in the mid-1980s in the wake of cloning of the opioid peptide precursors. However, it was not till 1992 that the delta receptor was first cloned and provided the critical probes leading to the characterization of the entire family of opioid receptors. 

Sdentitic studies of opioid neurotransmitters during the 1970s have uncovered a complex and subtle system that exhibited impressive diversity in terms of endogenous ligands for only three major receptors. The opioid peptide precursors were subject to complex post-translational modifications resulting in the synthesis of multiple active peptides all of them sharing the common N-terminal sequence of Tyr-Gly-Gly-Phe-(Met or Leu), which has been termed the opioid motif. Based on the results of theses studies, the endogenous opioids have been implicated in circuits involved in the control of sensation, emotion, and affect and a role has been ascribed to them in addiction, not only to opiates such as morphine or heroin, but also to alcohol. ... 

Beaumont, A., Fuentes, J. A., Hughes, J., and Metters, K. M., 1980, Opioid peptide precursors in striatum, FEES Lett. 122 135-137. 

Dell, A., Etienne, A. T., Morris, H. R., Beaumont, A., Burrel, R., and Hughes, J., 1979, Neuropeptides high-resolution purification procedures and their application to the study of new opioid peptides and opioid peptide precursors, in Molecular Endocrinology (I. MacIntyre and M. Szelke, eds.), pp. 91-97, Elsevier/North-Holland Biomedical Press, Amsterdam. 

Fig. 2. Schematic drawing of the precursors for opioid peptides. Shaded areas represent the location of sequences of active peptide products which are normally released by trypsin-like enzymes acting on pairs of basic amino acid residues. Precursors are not necessarily drawn to scale.

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). 

Endogenous opioid peptides are derived from three distinct precursor molecules, which are the primary products of three separate genes (Simon and Hiller,... 

Anterior pituitary hormones include growth hormone (GH), thyrotropin (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin (PRL), and adrenocorticotropin (ACTH). Another peptide, B-lipotropin (B-LPH), is derived from the same prohormone, proopiomelanocortin, as ACTH. B-LPH is secreted from the pituitary (along with ACTH), and is a precursor of the opioid peptide B-endorphin (see Chapter 31 Opioid Analgesics Antagonists). 

Similar to mammalian prohormones, all opioid peptides in amphibian skin precursors are flanked by paired dibasic amino acids (Lys-Arg). Moreover, the precursor sequence contains an additional Gly residue at its carboxyl terminus this extra residue is required for the carboxamidation of the mature heptapeptide [20]. 

The relatively large number of opioid peptides isolated in recent years is a reflection of the complexity of the endogenous opioid system. The major opioid peptides are cleavage products of three distinct proteins, which are the primary products of three genes. These precursor proteins are proenkephalin... 

Bovine haemoglobin, the protein from erythrocytes which occurs as a minor component in meat and meat products, is also a precursor of opioid peptides (haemorphins). These opioid peptides are released by pepsin digestion in vitro and may also be produced by macrophages. Moreover, haemorphins have been found to decrease the tension of the guinea pig ileum in vitro (Nyberg et al., 1997). 

Figure 2 Proneuropeptides structural features for proteolytic processing. Neuropeptides are synthesized as proneuropeptide precursors, also known as prohormones, that require proteolytic processing to liberate the active neuropeptide. Proteolytic processing occurs at dibasic and monobasic sites, as well as at multibasic sites. The precursor proteins may contain one copy of the active neuropeptide, such as the proneuropeptides for NPY, galanin, CRF, and vasopressin. Some proneuropeptides such as proenkephalin contain multiple copies of the active neuropeptide proenkephalin contains four copies of (Met)enkephalin (ME), one copy of (Leu)enkephalin (LE), and the related opioid peptides ME-Arg-Phe (H) and ME-Arg-Cly-Leu (O).

Two of the most abundant endogenous opioid peptides, methionine-enkephalin (Met-enk Tyr-Gly-Gly-Phe-Met) and leucine-enkephalin (Leu-enk Tyr-Gly-Gly-Phe-Leu) are derived from the precursor protein, proenkephalin (PENK). PENK is a 267 amino acid precursor that contains 6 copies of Met-enk, two of which are extended forms (Tyr-Gly-Gly-Phe-Met-Arg-Phe and Tyr-Gly-Gly-Phe-Met-Arg-Gly-Leu) and one copy of Leu-enk. This ratio is essentially the same as the ratio of these peptides found in brain and adrenal chromaffin cells. The fact that there are two distinct peptides closely related in stmcture and function proved to be a major obstacle that slowed their discovery (Kosterlitz and Hughes, 1977). 

The first two opioid peptides were isolated from pig brain and shown to be active in bioassay systems by Hughes and Kosterlitz in 1975 (Hughes etal., 1975). It was some time later, though, that the precursor proteins for these small peptides were discovered. The first of these to be identified was proopiomelanocortin (POMC). It was not until 1982 that PENK was identified and sequenced in multiple species (Udenfriend and Kilpatrick, 1983). 

Another major source of endogenous opioid peptides as w ell as non-opioid peptides is proopiomelanocortin (POMC). This large peptide contains the sequences of, and is processed into, a number of smaller peptides, including ACTH, p-endoiphin, p-hpotropin, a-MSH, p-MSH and y-MSH. Which products are derived from POMC depends upon the cell in w hich the processing takes place. In the anterior lobe of the pituitary POMC is processed into ACTH and p-lipotropin. In the neurointermediate lobe the major products are p-endorphin, a-MSH and y-MSH. POMC w as the first of the peptide precursors for W hich such cell-dependent processing w as demonstrated. 

In addition to the enkephalins and endorphins,. several other opioid peptides have been extracted from pituitary, adrenal, and ncrs ous tissue, including dynorphins and neo-endorphins. The peptides LPH. ACTH, and y-MSH are derived from the. same precursor, POMC. 

The classical mammalian opioid peptides are derived from three distinct precursor proteins (Fig. 7.10) (see Refs. 75,266 for reviews). Processing of the precursor proteins occurs at pairs of basic residues. jS-Endorphin is derived from proopiomelanocortin (POMC), along with ACTH, a-MSH, and j3-lipotropin (see Ref 267 for a review). The enkephalins are derived from proenkephalin A (see Ref 268 for a review). This protein contains four copies of Met-enkephalin flanked by pairs of basic resi-... 

Based on their finding amphibian skin peptides, which were counterparts to other mammalian bioactive peptides, Erspamer and coworkers examined amphibian skin for opioid peptides (see Ref 663 for a review). This led first to the isolation and characterization of dermorphin (212, Fig. 7.41), which is a ja-se-lective peptide (see Table 7.13), from the skin of South American Phyllemedusinae hylid frogs in the early 1980s (664). Inspection of the sequence of one of the cloned cDNAs for the precursor of dermorphin suggested the existence of another heptapeptide with a similar iV-terminal sequence (665). This then led to the isolation of deltorphin (alsocalled dermenkephalin or deltorphin A, 213, Fig. 7.41), the first S-selective amphibian opioid peptide, from these frogs (666, 667). Synthesis confirmed that the amino acid in position 2 of deltorphin was o-methionine rather than L-methionine (666,668, 669). Two additional peptides [o-Ala ldeltorphin I (also referred to as deltorphin C, 214, Fig. 7.41) and [o-Ala ]deltorphin II (also referred to as deltorphin B, 25, Fig. 7.5) were subsequently discovered (106) which exhibited even greater 8-receptor affinity and exceptional selectivity... 

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