88373-73-3

Hazard Information

Questions And Answer

• Discovery

  Endogenous morphines first isolated from the brain, enkephalin pentapeptides are associated with nociception by analgesic functions. There are two types of ENK peptides: methionine ENK (Met-ENK, [Met⁵]-ENK) and leucine ENK (Leu-ENK, [Leu⁵]-ENK). The isolation of these peptides from the porcine brain was first reported in 1975.;

• Structure

  The amino acid sequences of Met-ENK (Tyr-Gly-Gly-Phe-Met) and Leu-ENK (Tyr-Gly-Gly-Phe-Leu) are common in mammals. These are generated from the common precursor proenkephalin (PENK or, alternatively, proenkephalin-A [PENK-A]) by proteolytic cleavage.  In addition to Met-ENK and Leu-ENK, several distinct PENK-derived peptides with C-terminal or N-terminal extensions are present in the bovine brain. In mammals, including humans, PENK contains six copies of Met-ENK and one copy of Leu-ENK, whereas in amphibians, lungfish, and sharks, PENK contains seven Met-ENK sequences. In contrast, zebrafish PENK contains five copies of Met-ENK and one copy of Leu-ENK.
  ;

• Gene, mRNA, and precursor

  The human PENK gene, PENK, location 8q23-q24, consists of three exons and has transcription elements such as ENKCRE-1 and ENKCRE-2. The latter acts as an enhancer. A glucocorticoid response element is also present. Human prePENK is composed of 267 aa residues. A signal peptide consisting of 24 aa residues is followed by a cysteine-containing N-terminal sequence and the region containing the repeated ENK sequences.;

• Receptors

  Met-ENK and Leu-ENK are agonists for the δ-opioid receptor (DOR, also known as δ receptor, DOR-1, OP1, etc.) or μ-opioid receptor (MOR; also known as μ receptor, MOR-1, OP3, etc.), both of which are the subtypes of opioid receptors that belong to the GPCR family. Human DOR is located on chromosome 1 (1p36.1-p34.3). MetENK also interacts with a nonclassical opioid receptor called the opioid growth factor receptor.;

• Clinical implications

  Although MOR agonists are the most commonly used drugs for the treatment of pain, mu agonists show variable efficacy in the treatment of chronic pain, partly because of the development of tolerance. Delta-opioid agonists have been shown to have beneficial effects in chronic pain and emotional disorders, and may potentially be used for treatment in these symptoms.Acute alcohol intoxication stimulates the release of the endogenous opioid peptides β-endorphin, enkephalin, and dynorphin, and nonselective antagonists for opioid receptors reduce alcohol consumption in humans as well as alcohol consumption and self-administration in rats. Selective antagonists of MOR and DOR have been shown to reduce alcohol self-administration. Thus, MOR and DOR are viable targets for reducing the positive reinforcing effects of alcohol in nondependent cohorts.;

• Agonists and Antagonists

  DSLET, diprenorphine, DADLE, (-)-bremazocine, DPDPE, nalmefene, hydromorphone, and morphine are agonists. Naltriben, naltrindole, naltrexone, quadazocine, alvimopan, and naloxone are antagonists.;

• Biological functions

  The effects of morphine represent the functions of endogenous opioid peptides via DOR, KOR, and MOR. In the central nervous system, morphine causes analgesia, euphoria, sedation, miosis (constriction of the pupils), truncal rigidity, nausea, and vomiting, and decreases the rate of respiration and the cough reflex. In the gastrointestinal system, morphine causes constipation, constriction of the biliary smooth muscle, and esophageal reflux, and reduces gastric motility, digestion in the small intestine, and peristaltic waves in the colon. In other smooth muscles, morphine causes urinary retention, depresses renal function, and decreases uterine tone. In the skin, morphine causes itching, sweating, and flushing of the face, neck, and thorax. In the cardiovascular system, morphine decreases blood pressure and the heart rate if the cardiovascular system is stressed. In the immune system, morphine decreases the cytotoxic activity of natural killer cells and the formation of rosettes by human lymphocytes. Morphine also induces behavioral restlessness. Pharmacological studies using both delta agonists and delta antagonists in rodents show that the anxiolytic activity of the opioid tone is mediated by DOR. Although Met-ENK was originally identified as a neuromodulator that interacts with DOR, this peptide was subsequently revealed to be a tonically active regulator of cell proliferation as well.;

• Synthesis and release

  In the rat, glucocorticoid is required for the maintenance of basal Penk mRNA expression in the forebrain, and insulin injection induces an increase in mRNA levels in the adrenal medulla. In humans, as in rats, Met-ENK that is probably derived from the adrenal medulla is present in the circulation. Acupuncture relief of chronic pain syndrome correlates with increased plasma Met-ENK concentration, whereas the level of plasma Met-ENK in chronic alcoholics is reduced.;

Supplier