Pro-opioid proteins

Most of the pro-opioid proteins are usually synthesized very much within the cell nucleus, and subsequently transported meticulously to the terminals of the nerve cells from where they are being released gradually. 

Fig. 24.3. Schematic representation of a 5 enkephalinergic nerve terminal. (1) Pro-opioid proteins (proenkephalin A) are synthesized in the cell nucleus. (2) Pro-opioid proteins undergo microtubular transport to the nerve terminal. (3) Active endogenous opioids (E) are cleaved from the pro-opioid proteins by the action of processing proteases. (4) The active peptides (E) are taken up and stored in presynaptic vesicles. (5) The peptides are released when the presynaptic neuron fires. (6) The endogenous opioid peptides bind to postsynaptic receptors and activate second messenger systems. (7) For all opioid receptors, the second messenger effect is primarily mediated by a G-inhibitory (Gj/o) protein complex, which promotes the inactivation of adenylate cyclase (AC), a decrease in intracellular cyclic-adenosine-3, 5 -monophosphate (cAMP),...

Both Met-enkephalin and Leu-enkephalin have their own pro- and prepro forms.29 Bovine preproenkephalin A is a 268-residue protein containing a 20-residue signal sequence and four sequences of Met-enkephalin and one of Leu-enkephalin, each flanked by pairs of basic residues. There are also Met-enkephalin-Arg-Gly-Leu (YGGFMRGL) and Met-enkephalin-Arg-Phe sequences. Not all of these are cut out cleanly, and other peptides such as Met-enkephalin-Arg-Arg-Val-NH2 are also found in brain. Proenkephalin B contains three copies of Leu-enkephalin contained within longer peptides. One of these, P-dynorphin (Table 30-4), is also a potent opioid compound. The enkephalins are thought to act as neurotransmitters, which are rapidly degraded after their release by two or three membrane-bound... 

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... 

There is considerable evidence that opioid receptors are coupled to G-proteins and produce their effects through these proteins (see Refs. 96, 97 for reviews of opioid receptors and G-proteins). The structure of cloned opioid receptors is consistent with their belonging to this receptor superfamily (see below). G-pro-teins are heterotrimers, consisting of a, jS, and y subunits, which bind guanine nucleotides to their a-subunit and catalyze the hydrolysis of GTP to GDP. G-proteins mediate the interac-... 

Caseinomorphins. Several peptides with opioid activity have been isolated from enzymatic digests of milk proteins (see Fox and Flynn, 1992). Such peptides were first isolated from enzymatic digests of casein and characterized as a family of peptides containing 4-7 amino acids with a common N-terminal sequence, H.Tyr.Pro.Phe.Pro-, and 0-3 additional residues (Gly, Pro, He), i.e. residues 60-63/6 of -casein, and hence were called caseinomorphins (P-CM) 4 to 7, respectively. P-CM-5 is the most effective of these peptides, which are 300-4000 times less effective than morphine. P-CMs are very resistant to enzymes of the gastrointestinal tract (GIT) and appear in the contents of the small intestine following ingestion of milk. -CN f60-70 also has weak opiate activity but may be hydrolysed to smaller, more active P-CMs by peptidases in the brush border of the GIT. 

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.

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]. 

---