Leu-enkephalin and Met-enkephalin

June 1, 2023
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Leu-enkephalin is an endogenous opioid peptide neurotransmitter with the amino acid sequence Tyr-Gly-Gly-Phe-Leu that is found naturally in the brains of many animals, including humans.

It is one of the two forms of enkephalin; the other is met-enkephalin.

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

  • Schiller PW, Yam CF, Lis M (May 1977). “Evidence for topographical analogy between methionine-enkephalin and morphine derivatives”. Biochemistry. 16 (9): 1831–8. doi:10.1021/bi00628a011. PMID 870028.

Leu-enkephalin has agonistic actions at both the μ- and δ-opioid receptors, with significantly greater preference for the latter. It has little to no effect on the κ-opioid receptor.

  • Porreca F, Mosberg HI, Hurst R, Hruby VJ, Burks TF (August 1984). “Roles of mu, delta and kappa opioid receptors in spinal and supraspinal mediation of gastrointestinal transit effects and hot-plate analgesia in the mouse”. The Journal of Pharmacology and Experimental Therapeutics. 230 (2): 341–8. PMID 6086883.
  • Raynor K, Kong H, Chen Y, Yasuda K, Yu L, Bell GI, Reisine T (February 1994). “Pharmacological characterization of the cloned kappa-, delta-, and mu-opioid receptors”. Molecular Pharmacology. 45 (2): 330–4. PMID 8114680.

Met-enkephalin, also known as metenkefalin (INN), sometimes referred to as opioid growth factor (OGF), 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.

  • 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. doi:10.1016/0169-328x(94)90381-6. PMID 8164525.

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.

History

Met-enkephalin was discovered and characterized by John Hughes, Hans Kosterlitz, et al. in 1975 after a search for endogenous ligands of the opioid receptors.

Chemistry

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]

Biochemistry

Distribution

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. It is also present in the periphery, notably in some primary afferent fibers that innervate the pelvic viscera.

Biosynthesis

Met-enkephalin is synthesized from proenkephalin via proteolytic cleavage 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)).

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. 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), though whether they affect the opioid receptors in a similar manner as met-enkephalin is not entirely clear.

Clearance

Met- and leu-enkephalin are metabolized by a variety of different enzymes, including aminopeptidase N (APN), neutral endopeptidase (NEP), dipeptidyl peptidase 3 (DPP3), carboxypeptidase A6 (CPA6), and angiotensin-converting enzyme (ACE). These enzymes are sometimes referred to as enkephalinases.

Biological activity

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 antidepressant-like effects.

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

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

These properties are considered undesirable in pharmaceuticals as large doses would need to be administered multiple times an hour to maintain a therapeutically relevant effect, making it unlikely that met-enkephalin will ever be used as a medicine.

[D-Ala2]-Met-enkephalinamide (DALA), is a synthetic enkephalin analog which is not susceptible to degradation by brain enzymes and at low doses (5 to 10 micrograms) caused profound, long-lasting, morphine-like analgesia when microinjected into a rat’s brain.

See also

References

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  2. Christoph Stein (1999). Opioids in pain control: basic and clinical aspects. Cambridge University Press. pp. 22–23. ISBN 978-0-521-62269-1. Retrieved 25 November 2011.
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  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. doi:10.1016/j.humpath.2009.08.025. PMID 20040394.
  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. doi:10.1016/j.npep.2009.07.003. PMID 19716174. S2CID 19181608.
  8. Thanawala V, Kadam VJ, Ghosh R (October 2008). “Enkephalinase inhibitors: potential agents for the management of pain”. Current Drug Targets. 9 (10): 887–94. doi:10.2174/138945008785909356. PMID 18855623. Archived from the original on 2013-04-14.
  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. doi:10.1074/jbc.M707680200. PMID 18178555.
  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. doi:10.1016/0197-0186(82)90081-X. PMID 20487892. S2CID 23138078.
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  12. Pert, C. B.; Pert, A.; Chang, J. K.; Fong, B. T. (1976-10-15). “(D-Ala2)-Met-enkephalinamide: a potent, long-lasting synthetic pentapeptide analgesic”. Science. 194 (4262): 330–332. Bibcode:1976Sci…194..330P. doi:10.1126/science.968485. ISSN 0036-8075. PMID 968485.
  13. Colaianni L, Kung SC, Taggart DK, Picca RA, Greaves J, Penner RM, Cioffi N (July 2014). “Reduction of spectral interferences using ultraclean gold nanowire arrays in the LDI-MS analysis of a model peptide”. Analytical and Bioanalytical Chemistry. 406 (19): 4571–83. doi:10.1007/s00216-014-7876-7. PMID 24980599. S2CID 24046957.
  14. Lazarus LH, Ling N, Guillemin R (June 1976). “beta-Lipotropin as a prohormone for the morphinomimetic peptides endorphins and enkephalins”. Proceedings of the National Academy of Sciences of the United States of America. 73 (6): 2156–9. Bibcode:1976PNAS…73.2156L. doi:10.1073/pnas.73.6.2156. PMC 430469. PMID 1064883.
  15. Hughes J, Kosterlitz HW, Smith TW (February 1997). “The distribution of methionine-enkephalin and leucine-enkephalin in the brain and peripheral tissues. 1977”. British Journal of Pharmacology. 120 (4 Suppl): 428–36, discussion 426–7. doi:10.1111/j.1476-5381.1997.tb06829.x. PMC 3224324. PMID 9142421.
  16. Schiller PW, Yam CF, Lis M (May 1977). “Evidence for topographical analogy between methionine-enkephalin and morphine derivatives”. Biochemistry. 16 (9): 1831–8. doi:10.1021/bi00628a011. PMID 870028.
  17. Porreca F, Mosberg HI, Hurst R, Hruby VJ, Burks TF (August 1984). “Roles of mu, delta and kappa opioid receptors in spinal and supraspinal mediation of gastrointestinal transit effects and hot-plate analgesia in the mouse”. The Journal of Pharmacology and Experimental Therapeutics. 230 (2): 341–8. PMID 6086883.
  18. Raynor K, Kong H, Chen Y, Yasuda K, Yu L, Bell GI, Reisine T (February 1994). “Pharmacological characterization of the cloned kappa-, delta-, and mu-opioid receptors”. Molecular Pharmacology. 45 (2): 330–4. PMID 8114680.
Peptides: neuropeptides
Opioid receptor modulators

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