Met-enkephalin, also known as metenkefalin (
INN
Inns are generally establishments or buildings where travelers can seek lodging, and usually, food and drink. Inns are typically located in the country or along a highway; before the advent of motorized transportation they also provided accommo ...
), sometimes referred to as opioid growth factor (OGF),
is a
naturally occurring
A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical sy ...
,
endogenous opioid peptide
Opioid peptides are peptides that bind to opioid receptors in the brain; opiates and opioids mimic the effect of these peptides. Such peptides may be produced by the body itself, for example endorphins. The effects of these peptides vary, but t ...
that has
opioid
Opioids are substances that act on opioid receptors to produce morphine-like effects. Medically they are primarily used for pain relief, including anesthesia. Other medical uses include suppression of diarrhea, replacement therapy for opioid use ...
effects of a relatively short duration. It is one of the two forms of
enkephalin
An enkephalin is a pentapeptide involved in regulating nociception in the body. The enkephalins are termed endogenous ligands, as they are internally derived and bind to the body's opioid receptors. Discovered in 1975, two forms of enkephali ...
, the other being
leu-enkephalin
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-e ...
. The enkephalins are considered to be the primary endogenous ligands of the
δ-opioid receptor
The δ-opioid receptor, also known as delta opioid receptor or simply delta receptor, abbreviated DOR or DOP, is an inhibitory 7-transmembrane G-protein coupled receptor coupled to the G protein Gi/G0 and has enkephalins as its endogenous lig ...
, due to their high
potency
Potency may refer to:
* Potency (pharmacology), a measure of the activity of a drug in a biological system
* Virility
* Cell potency, a measure of the differentiation potential of stem cells
* In homeopathic dilutions, potency is a measure of how ...
and
selectivity
Selectivity may refer to:
Psychology and behaviour
* Choice, making a selection among options
* Discrimination, the ability to recognize differences
* Socioemotional selectivity theory, in social psychology
Engineering
* Selectivity (radio), a ...
for the site over the other endogenous opioids.
History
Met-enkephalin was discovered and characterized by
John Hughes,
Hans Kosterlitz
Hans Walter Kosterlitz FRS (27 April 1903 – 26 October 1996) was a German-born British biochemist.
Biography
Hans Walter Kosterlitz was born on 27 April 1903 in Berlin. He was the elder son of Bernhard Kosterlitz, a physician, and Selma ...
, ''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
Protein primary structure is the linear sequence of amino acids in a peptide or protein. By convention, the primary structure of a protein is reported starting from the amino-terminal (N) end to the carboxyl-terminal (C) end. Protein biosynthe ...
tyr-gly-gly-phe-met. The
tyrosine
-Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the G ...
residue
Residue may refer to:
Chemistry and biology
* An amino acid, within a peptide chain
* Crop residue, materials left after agricultural processes
* Pesticide residue, refers to the pesticides that may remain on or in food after they are applied ...
at position 1 is thought to be
analogous
Analogy (from Greek ''analogia'', "proportion", from ''ana-'' "upon, according to" lso "against", "anew"+ ''logos'' "ratio" lso "word, speech, reckoning" is a cognitive process of transferring information or meaning from a particular subject ...
to the 3-
hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydro ...
group
A group is a number of persons or things that are located, gathered, or classed together.
Groups of people
* Cultural group, a group whose members share the same cultural identity
* Ethnic group, a group whose members share the same ethnic ide ...
on
morphine
Morphine is a strong opiate that is found naturally in opium, a dark brown resin in poppies (''Papaver somniferum''). It is mainly used as a pain medication, and is also commonly used recreationally, or to make other illicit opioids. T ...
.
Biochemistry
Distribution
Met- enkephalin is found mainly in the
adrenal medulla and throughout the
central nervous system
The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...
(CNS), including in the
striatum,
cerebral cortex
The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consistin ...
,
olfactory tubercle
The olfactory tubercle (OT), also known as the tuberculum olfactorium, is a multi-sensory processing center that is contained within the olfactory cortex and ventral striatum and plays a role in reward cognition. The OT has also been shown to ...
,
hippocampus
The hippocampus (via Latin from Greek , ' seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, ...
,
septum,
thalamus
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
, and
periaqueductal gray
The periaqueductal gray (PAG, also known as the central gray) is a brain region that plays a critical role in autonomic function, motivated behavior and behavioural responses to threatening stimuli. PAG is also the primary control center for d ...
, as well as the
dorsal horn of the
spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
.
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
Introduction
Proenkephalin (PENK), formerly known as proenkephalin A (since proenkephalin B was renamed prodynorphin), is an endogenous opioid polypeptide hormone which, via proteolyic cleavage, produces the enkephalin peptides etnkephal ...
via
proteolytic
Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids. Uncatalysed, the hydrolysis of peptide bonds is extremely slow, taking hundreds of years. Proteolysis is typically catalysed by cellular enzymes called protease ...
cleavage in two
metabolic steps. Proenkephalin A is first reduced by either one of two
trypsin
Trypsin is an enzyme in the first section of the small intestine that starts the digestion of protein molecules by cutting these long chains of amino acids into smaller pieces. It is a serine protease from the PA clan superfamily, found in the d ...
-like
endopeptidase
Endopeptidase or endoproteinase are proteolytic peptidases that break peptide bonds of nonterminal amino acids (i.e. within the molecule), in contrast to exopeptidases, which break peptide bonds from end-pieces of terminal amino acids. For this ...
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s,
prohormone convertase 1 (PC1) or
prohormone convertase 2 (PC2); then, the resulting
intermediates are further reduced by the enzyme
carboxypeptidase E
Carboxypeptidase E (CPE), also known as carboxypeptidase H (CPH) and enkephalin convertase, is an enzyme that in humans is encoded by the ''CPE'' gene. This enzyme catalyzes the release of C-terminal arginine or lysine residues from polypeptides.
...
(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
The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...
-extended met-enkephalin
derivatives, the
heptapeptide
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
A ...
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
Membrane alanyl aminopeptidase () also known as alanyl aminopeptidase (AAP) or aminopeptidase N (AP-N) is an enzyme that in humans is encoded by the ANPEP gene.
Function
Aminopeptidase N is located in the small-intestinal and renal microvilla ...
(APN),
neutral endopeptidase
Neprilysin (), also known as membrane metallo-endopeptidase (MME), neutral endopeptidase (NEP), cluster of differentiation 10 (CD10), and common acute lymphoblastic leukemia antigen (CALLA) is an enzyme that in humans is encoded by the ''MME'' ge ...
(NEP),
dipeptidyl peptidase 3
Dipeptidyl-peptidase 3 is an enzyme that in humans is encoded by the ''DPP3'' gene.
This gene encodes a protein that is a member of the S9B family in clan SC of the serine proteases. This cytoplasmic protein binds a single zinc ion with its zinc- ...
(DPP3),
carboxypeptidase A6
Carboxypeptidase A6 (CPA6) is a metallocarboxypeptidase enzyme that in humans is encoded by the ''CPA6'' gene. It is highly expressed in the adult mouse olfactory bulb and is broadly expressed in the embryonic brain and other tissues.
The prote ...
(CPA6),
and
angiotensin-converting enzyme (ACE).
These enzymes are sometimes referred to as
enkephalinase Enkephalinases are enzymes that degrade endogenous enkephalin opioid peptides. They include:
* Aminopeptidase N (APN)
* Neutral endopeptidase (NEP)
* Dipeptidyl peptidase 3 (DPP3)
* Carboxypeptidase A6 (CPA6)
* Leucyl/cystinyl aminopeptidase ( ...
s.
Biological activity
Met-enkephalin is a
potent agonist of the
δ-opioid receptor
The δ-opioid receptor, also known as delta opioid receptor or simply delta receptor, abbreviated DOR or DOP, is an inhibitory 7-transmembrane G-protein coupled receptor coupled to the G protein Gi/G0 and has enkephalins as its endogenous lig ...
, and to a lesser extent the
μ-opioid receptor
The μ-opioid receptors (MOR) are a class of opioid receptors with a high affinity for enkephalins and beta-endorphin, but a low affinity for dynorphins. They are also referred to as μ(''mu'')-opioid peptide (MOP) receptors. The prototypical Π...
, with little to no effect on the
κ-opioid receptor
The κ-opioid receptor or kappa opioid receptor, abbreviated KOR or KOP, is a G protein-coupled receptor that in humans is encoded by the ''OPRK1'' gene. The KOR is coupled to the G protein Gi/G0 and is one of four related receptors that bind op ...
. It is through these receptors that met-enkephalin produces its opioid effects, such as
analgesia
Pain management is an aspect of medicine and health care involving relief of pain (pain relief, analgesia, pain control) in various dimensions, from acute and simple to chronic and challenging. Most physicians and other health professional ...
and
antidepressant-like effects.
It is also the endogenous
ligand
In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elec ...
of the
opioid growth factor receptor
Opioid growth factor receptor, also known as OGFr or the ζ-opioid receptor, is a protein which in humans is encoded by the ''OGFR'' gene. The protein encoded by this gene is a Receptor (biochemistry), receptor for opioid growth factor (OGF), al ...
(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
In pharmacology, bioavailability is a subcategory of absorption and is the fraction (%) of an administered drug that reaches the systemic circulation.
By definition, when a medication is administered intravenously, its bioavailability is 100%. Ho ...
, is rapidly
metabolized
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
, and has a very short
half-life
Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable at ...
(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.
-Ala2Met-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
*
Leu-enkephalin
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-e ...
References
{{DEFAULTSORT:Met-enkephalin
Opioid peptides
Delta-opioid receptor agonists
Pentapeptides