Opioid precursor proteins

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... 

Endorphins are peptides produced by the intermediate pituitary that react with the brain s opioid receptors and presumably act as endogenous analgesics. The two best known endorphins from bovine brain are the pentapeptides, met-enkephalin and leu-enkephalin, whose structures are Tyr-Gly-Gly-Phe-Met and Tyr-Gly-Gly-Phe-Leu, respectively. There is evidence that the enkephalins, MSH, and ACTH are produced from the same precursor protein, pro-opiomelanocortin, molecular weight (MW) 31,000. This protein is present in both the anterior and intermediate pituitary glands. In the anterior pituitary area, this protein loses the element of ACTH (39 amino acids). In the intermediate pituitary, this protein... 

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). 

The endogenous ligands for the opioid receptors are peptides known as the endorphins (endogenous morphine) or opio-peptins. These include the pentapep tides methionine-enkephalin and leucine-enkephalin and a hep tapep tide and octapeptide version of methionine-enkephalin, all derived from preproenkephalin p-endorphin derived from proopiomelanocortin a-and p-dynorphin derived from prodynorphin endomorphin-1 and -2, whose precursor has not been definitively identified and orphanin FQ or nociceptin, derived from OFQ/N precursor protein. These peptides are discussed in more detail in Chapter 34. 

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-... 

Three distinct families of classical opioid peptides have been identified the enkephalins, endorphins, and dynorphins. Each family derives from a distinct precursor protein, prepro-opiome-lanocortin (POMC), preproenkephalin, and preprodynorphin, respectively, which are encoded by distinct genes. Each precursor is subject to complex cleavages and posttranslational modifications that result in the synthesis of multiple active peptides. The opioid peptides share a common amino-terminal sequence of Tyr-Gly-Gly-Phe-(Met or Leu), the opioid motif. This motif is followed by C-terminal extensions yielding peptides ranging from 5 to 31 residues (Table 21-1). 

Akil et al, (1984) observed that the endogenous opioid peptides are invariably synthesized as essential component associated with the structures of specific large precursor proteins. Evidently, each of the major types of opioid peptides does have an altogether different and specific precursor protein. 

The endogenous opioid peptides are synthesized as part of the structures of large precursor proteins (10). There is a different precursor protein for each of the major types of opioid peptides (Fig. 24.1). Proopiomelanocortin is the precursor for P-endorphin. Proenkephalin A is the precursor for Met- and Leu-enkephalin. Proenkephalin B (prodynorphin) is the precursor for dynorphin and... 

Dynorphins (Dyn), opioid peptides released from the precursor protein prepro-dynorphin, (prepro-enkephalin B). Dynorphin A (Dyn A), YGGFLRRIRP KLKWDNQ, dynorphin B (Dyn B), YG GFLRRDFK WT, a-neoendorphin (— neoendorphins), prodynorphin-derived... 

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]. 

GUal precursors in the ventricular and subventricular zones can also express opioid receptors (Zhu et al. 1998 Reznikov et al. 1999 Stiene-Martin et al. 2001 Kim et al. 2006 Tripathi et al. 2008). Immature glia and adult progenitors express opioid receptors, which affect ceUular maturation (Stiene-Martin and Hauser 1990, 1991 Stiene-Martin et al. 1991 Persson et al. 2003,2006) and cell fate decisions (Kim et al. 2006). Importantly, gUal precursors (Khurdayan et al. 2004 Lawrence et al. 2004 Buch et al. 2007), and espedaUy immature oUgodendroglia (Khurdayan et al. 2004 Hauser et al. 2008), also appear to be vulnerable to opioids and HIV-1 proteins. 

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). 

Opium, derived from poppies, relieves pain and induces euphoria by binding to "opiate receptors" in the brain. These opioid drugs mimic the actions of three peptide families in the brain known as the endorphins, the enkephalins, and the dynorphins. These peptides, along with several nonopioid peptides (MSH, ACTH, lipotropin) are cleaved from the protein precursors pro-opiomelanocortin (POMC), proenkephalin, and prodynorphin (Fig. 3.5). 

Figure 33 Cleavage of pro-opiomelanocortin (POMC) into endorphin and non-opioid peptides. POMC serves as a precursor (pro-) for the opiate peptide, -endorphin (-opio-) for a-, J3- and y-melanocyte-stimulating hormone (MSH, -melano-) for adrenocortkotrophic hormone (ACTH, -cort-) and for a- and /3- lipotropin (-in). Notice that the peptides are not located end-to-end, but that some of the smaller peptide sequences are embedded in larger sequences (e.g., a-lipotropin, P-MSH and P-endorphin are contained within P-lipotropin). Two other protein precursors contain enkephalins and dynorphins. Proenkephalin contains seven met-enkephalin sequences (Tyr-Gly-Gly-Phe-Met). Prodynorphin contains three leu-enkephalin sequences (Tyr-Gly-Gly-Phe-Leu) and the sequences for dynorphin A, Dynorphin B, a-neodynorphin and P-neoendorphin.

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