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Infobox disclaimer and references

[Met]enkephalin, sometimes referred to as opioid growth factor (OGF),[1] is a naturally occurring, endogenous opioid peptide that has opioid effects of a relatively short duration. It is one of the two forms of enkephalin, the other being [leu]enkephalin. The enkephalins are considered to be the primary endogenous ligands of the δ-opioid receptor, due to their high potency and selectivity for the site over the other endogenous opioids.[2]

History Edit

[Met]enkephalin was discovered and characterized by Hughes, Kosterlitz, et al. in 1975 after a diligent and intensive search for endogenous ligands of the opioid receptors.[3]

Chemistry Edit

[Met]enkephalin is a pentapeptide with the amino acid sequence Tyr-Gly-Gly-Phe-Met.

The tyrosine residue at position 1 is thought to be analogous to the 3-hydroxyl group on morphine.[citation needed]

Pharmacology Edit

Distribution Edit

[Met]enkephalin is found mainly in the adrenal medulla and throughout the central nervous system (CNS), including in the striatum, cerebral cortex, olfactory tubercle, hippocampus, septum, thalamus, and periaqueductal gray, as well as the dorsal horn of the spinal cord.[2] It is also present in the periphery, notably in some primary afferent fibers that innervate the pelvic viscera.[2]

Biosynthesis Edit

[Met]enkephalin is synthesized from proenkephalin A via proteolyic cleavage[4] in two metabolic steps. Proenkephalin A is first reduced by either one of two trypsin-like endopeptidase enzymes, prohormone convertase 1 (PC1) or prohormone convertase 2 (PC2); then, the resulting intermediates are further reduced by the enzyme carboxypeptidase E (CPE; previously known as enkephalin convertase (EC)).[5][6] Proenkephalin A contains four sequences of [Met]enkephalin (at the following positions: 100-104; 107-111; 136-140; 210-214), and as a result, its cleavage generates four copies of [Met]enkephalin peptides at once.[4] In addition, anabolism of proenkephalin A results in the production of one copy each of two C-terminal-extended [Met]enkephalin derivatives, the heptapeptide [Met]enkephalin-Arg-Phe (261-267), and the octapeptide [Met]enkephalin-Arg-Gly-Leu (186-193),[4] though whether they affect the opioid receptors in a similar manner as [Met]enkephalin is not entirely clear.[7]

Clearance Edit

[Met]- and [Leu]enkephalin are metabolized by a variety of different enzymes, including aminopeptidase N (APN),[8] neutral endopeptidase (NEP),[8] dipeptidyl peptidase 3 (DPP3),[8] carboxypeptidase A6 (CPA6),[9] and angiotensin-converting enzyme (ACE).[10] Collectively, these enzymes are known as the enkephalinases.

Pharmacodynamics Edit

[Met]enkephalin is a potent agonist of the δ-opioid receptor, and to a lesser extent the μ-opioid receptor, with little to no effect on the κ-opioid receptor. It is through these receptors that [Met]enkephalin produces its opioid effects, such as analgesia and mood lift.

It is also the endogenous ligand of the opioid growth factor receptor (OGFR; formerly known as the ζ-opioid receptor), which plays a role in the regulation of tissue growth and regeneration; hence why [Met]enkephalin is sometimes called OGF instead.

Pharmacokinetics Edit

[Met]enkephalin has low bioavailability, is rapidly metabolized, and has a very short half-life (minutes).[3][11]

See also Edit

References Edit

  1. Zagon IS, Isayama T, McLaughlin PJ (January 1994). Preproenkephalin mRNA expression in the developing and adult rat brain. Brain Research. Molecular Brain Research 21 (1-2): 85–98.
  2. 2.0 2.1 2.2 Christoph Stein (1999). = 4Rfr8cQayvgC&pg = PA22 Opioids in pain control: basic and clinical aspects, 22–23, Cambridge University Press. URL accessed 25 November 2011.
  3. 3.0 3.1 Thomas Carleton Moore (1993). = dJyJGevjkzQC&pg = PA179 Neurovascular immunology: vasoactive neurotransmitters and modulators in cellular immunity and memory, CRC Press. URL accessed 25 November 2011.
  4. 4.0 4.1 4.2 Fleur L. Strand (1999). = PNPU0sknfCUC&pg = PA348 Neuropeptides: regulators of physiological processes, MIT Press. URL accessed 25 November 2011.
  5. Costa E, Mocchetti I, Supattapone S, Snyder SH (July 1987). Opioid peptide biosynthesis: enzymatic selectivity and regulatory mechanisms. The FASEB Journal : Official Publication of the Federation of American Societies for Experimental Biology 1 (1): 16–21.
  6. Krajnik M, Schäfer M, Sobanski P, et al. (May 2010). Enkephalin, its precursor, processing enzymes, and receptor as part of a local opioid network throughout the respiratory system of lung cancer patients. Human Pathology 41 (5): 632–42.
  7. Vats ID, Chaudhary S, Karar J, Nath M, Pasha Q, Pasha S (October 2009). Endogenous peptide: Met-enkephalin-Arg-Phe, differently regulate expression of opioid receptors on chronic treatment. Neuropeptides 43 (5): 355–62.
  8. 8.0 8.1 8.2 Thanawala V, Kadam VJ, Ghosh R (October 2008). Enkephalinase inhibitors: potential agents for the management of pain. Current Drug Targets 9 (10): 887–94.
  9. Lyons PJ, Callaway MB, Fricker LD (March 2008). Characterization of carboxypeptidase A6, an extracellular matrix peptidase. The Journal of Biological Chemistry 283 (11): 7054–63.
  10. Benuck M, Berg MJ, Marks N (1982). Separate metabolic pathways for Leu-enkephalin and Met-enkephalin-Arg(6)-Phe(7) degradation by rat striatal synaptosomal membranes. Neurochemistry International 4 (5): 389–96.
  11. (29 August 2005) The endocrine system in sports and exercise, 203–, John Wiley & Sons. URL accessed 25 November 2011.

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