Enkephalins, discovery

At the time of the discovery of Met-enkephalin, its sequence was observed to be identical to that of residues 61—65 contained in the C-fragment of the pituitary hormone p-Hpotropin [12584-99-5] (p-LPH) (see Hormones), first isolated in 1964 (11). In 1976, the isolation of a larger peptide fragment, P-endorphin [60617-12-1] that also displayed opiate-like activity was reported (12). This peptide s 31-amino-acid sequence comprised residues 61—91 of P-LPH. Subsequentiy, another potent opioid peptide, dynorphin [72957-38-17, was isolated from pituitary (13). The first five amino acids (qv) of this 17-amino-acid peptide are identical to the Leu-enkephalin sequence (see Table 1). 

Since the discovery of the enkephalins in 1975 [11] a large number of endogenous opioid peptides have been detected in mammals, and at present three distinct families of opioid peptides are known (for a review, See Ref. 12). These are the enkephalins, the endorphins (a-, (J-, and y-), and the dynorphins and neoendorphins. The recently discovered endomor-phins [13] also may represent endogenous opioid peptides. Peptides with opioid activity have also been isolated from tryptic digests of milk casein... 

The recent discovery of a class of peptides, the enkephalins, which act as opiate agonists has led to a number of physical chemical studies aimed at understanding the structure-activity relationships between the enkephalins and the opiates (1 -9). 

After the early discovery of a tyrosine 0-sulfate residue in bovine fibrinopeptide B, 15 this posttranslational modification which occurs ubiquitously in proteins was also detected in a series of biologically active peptides such as the neurohormones of the gastrin/cholecysto-kinin (CCK) family of peptides, phyllokinin, Leu-enkephalin, and the thrombin inhibitor hirudin listed in Table 1. 

Tariot PN, Cohen RM, Welkowitz JA, et al Multiple-dose arecohne infusions in Alzheimer s disease. Arch Gen Psychiatry 45 901-905, 1988 Taylor AE, Saint-Cyr JA, Lang AE Frontal lobe dysfunction in Parkinson s disease the cortical focus of neostriatal outflow. Brain 109 845-883, 1986 Taylor DP, Smith DW, Hyslop DK, et al Receptor binding and atypical antidepressant drug discovery, in Receptor Binding in Drug Research. Edited by O Brien RA. New York, Marcel Dekker, 1986, pp 151-165 Tejedor-Real P, Mico JA, Maldonado R, et al Effect of mixed (RB 38A) and selective (RB 38B) inhibitors of enkephalin degrading enzymes on a model of depression in the rat. Biol Psychiatry 34 100-107, 1993... 

Endogenous opioid peptides. Extensive processing is also involved in formation of analgesic opioid peptides, which are present naturally in the brain (see also Section B). Tire formation of (1-endorphin in the hypothalamus from prepro-opiomelanocortin (Fig. 30-2) has already been mentioned. Prior to the discovery of P-endorphin, the pentapeptides Met-enkephalin and Leu-enkephalin (Table 30-4) were discovered and were found to compete with opiate drugs for receptors in the brain. Tire larger P-endorphin, which contains the Met-enkephalin sequence at its N terminus, is a far more potent opiate antagonist than are the enkephalins. Since the Met-enkephalin sequence within P-endorphin is not flanked by basic residues, it apparently is normally not released. Two other recently discovered brain peptides are endomorphin-1 (YPWF-NH2) and endomorphin-2 (YPFF-NH2). They are also potent agonists for the opioid receptors, especially the p receptor (see Section B,10).,61a,61b... 

The demonstration of the existence of strictly defined SARs. which is perhaps Ihe most important criterion of drug action at a specific receptor site, has made possible the most important pharmacologic discoveries. For example, the analgesic actions of morphine and related agents, which are indicative of specific receptors, led to die discovery of endogenous opiate peptides, i.e., the leucine and methionine enkephalins and endorphins. 

Finally, the discovery of a CNS receptor that is specific for benzodiazepines has led to some interesting speculation as to the possible existence of some type of endogenous sedative-like agent. The presence of a certain type of receptor to indicate that the body produces an appropriate agonist for that receptor makes sense. For instance, the discovery of opiate receptors initiated the search for endogenous opiate-like substances, which culminated in the discovery of the enkephalins. It has been surmised that certain endogenous steroids such as allopregnanolone (a metabolic byproduct of... 

The opium alkaloids codeine and morphine served as models for the synthesis of naloxone, an important analog used to treat and diagnose opiate addicts, and also led to the discovery of endogenous opioids (enkephalins and endorphins) (see Chapter 47). Similarly, A9-tetrahydro-cannabinol (THC), the component of Cannabis sativa responsible for the central nervous system (CNS) effect, has also been found to reduce nausea associated with cancer chemotherapy (see Chapter 18). 

Prodynorphin contains three copies of Leu-enkephalin with carboxy-termi-nus extended polypeptides of various lengths known as dynorphin A (or dynorphin 1-17), dynorphin B (dynorphin 1-13), or a- and 3-neoendorphin. These peptides derived from prodynorphin are selective to kappa receptors and can also be further broken down to Leu-enkephalin. The identification of the delta receptor (or the enkephalin receptor) was a direct consequence of the discovery of enkephalins. This chapter will review the major events that are important for the identification of delta receptors and the subsequent cloning of delta receptor genes, and eventually all other opioid receptor genes. 

PENK), prodynorphin (PDYN), and pro-opiomelanocortin (POMC) [13-15]. PENK was originally discovered in bovine adrenal cortex and pig brain, where enkephalin biosynthesis was elucidated [13]. It contains one copy of Leu-enkephalin, four copies of Met-enkephalin, and one copy each of a Met-enkephalin C-terminal-extended heptapeptide and octapeptides, all flanked by basic dipeptides, where processing generally takes place. The primary identification of delta opioid receptors was a direct consequence of the discovery of these enkephalins [16]. These peptides were first examined for their potency at opioid receptors present in the guinea pig ileum and mouse vas deferens bioassays. The result of these studies shows that enkephalins are acting at a different receptor in the mouse vas deferens than the guinea pig ileum and this receptor for enkephalins was defined as the elimination of the delta opioid receptor [17]. 

Subsequent conversion into heroin 2 was first reported in 1874 by Wright in the UK as a result of boiling morphine acetate the process was commercialised by Bayer AG in 1898. The subsequent use and abuse of these compounds is much too complex to discuss here, but one major discovery came in the early 1970s when Pert and Synder reported the identification of opioid receptors in brain tissue.17 This report was followed closely by the identification of endogenous morphine-like substances in 1975 by Kosterlitz and Hughes,18 which over the next few years led to the identification of enkephalins, endorphins and dynorphins—all of which had the common N-terminal sequence of Tyr-Gly-Gly-Phe-(Met/Leu), leading to the concept that morphine actually mimics this sequence.19... 

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