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The rubiscolins are a group of opioid peptides that are formed during digestion of the ribulose bisphosphate carboxylase/oxygenase (Rubisco) protein from spinach leaves. [1] These peptides have much in common with the better-known gluten exorphins.
There are 2 known rubiscolins with known structure:
Studies have been conducted on rubiscolin structure and biological responses following its digestion. [3] [4] The tertiary structure and biological function of spinach-derived rubiscolin has been analyzed in the laboratory. [3] When rubiscolin is digested, studies have shown that rubiscolin has the potential to bind to δ opioid receptors in the body. [3] The analysis of the amino acids responsible for this agonistic relationship of rubiscolin with δ opioid receptors can lead to replication of these proteins in the lab. [3] Rubiscolin has the capability to bind to δ opioid receptors following its digestion. [3] Upon the digestion of rubiscolin from spinach with the protease pepsin, peptides MRWRD, MRW, LRIPVA, AND IAYKPAG were found and purified. [4] These peptides were found to have binding capabilities with angiotensin I-converting enzyme (ACE), which catalyze an antihypertensive, or decreased blood pressure, response. [4] When treated to rats in the laboratory, MRW, MRWRD, and IAYKPAG resulted in antihypertensive responses in hypertensive rats 2 hours, 4 hours, and 4 hours, respectively, after ingestion of the peptides. [4] The peptide LRIPVA did not induce any antihypertensive responses from laboratory rats. [4] The tertiary structure of rubiscolin has been mapped and the δ opioid receptor and ACE binding capabilities have been researched in the lab. [3] [4]
Angiotensin is a peptide hormone that causes vasoconstriction and an increase in blood pressure. It is part of the renin–angiotensin system, which regulates blood pressure. Angiotensin also stimulates the release of aldosterone from the adrenal cortex to promote sodium retention by the kidneys.
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 enkephalin have been found, one containing leucine ("leu"), and the other containing methionine ("met"). Both are products of the proenkephalin gene.
Beta-Endorphin or β-Endorphin, is an endogenous opioid neuropeptide and peptide hormone that is produced in certain neurons within the central nervous system and peripheral nervous system. It is one of three endorphins that are produced in humans, the others of which include α-endorphin and γ-endorphin.
Gluten exorphins are a group of opioid peptides formed during digestion of the gluten protein. It has been hypothesized that people with autism and schizophrenia have abnormal leakage from the gut of these compounds, which then pass into the brain and disrupt brain function, a process collectively known as the opioid excess theory or a part of leaky gut syndrome. This is partly the basis for the gluten-free, casein-free diet. The medical evidence is mixed. Two clinical studies of autism patients who followed this diet found no benefit. Another study found a benefit. Another study suggested the diet may present a greater risk to brain development.
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 they all resemble those of opiates. Brain opioid peptide systems are known to play an important role in motivation, emotion, attachment behaviour, the response to stress and pain, and the control of food intake.
Neurotensin is a 13 amino acid neuropeptide that is implicated in the regulation of luteinizing hormone and prolactin release and has significant interaction with the dopaminergic system. Neurotensin was first isolated from extracts of bovine hypothalamus based on its ability to cause a visible vasodilation in the exposed cutaneous regions of anesthetized rats.
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. 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.
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. 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.
Big dynorphin is an endogenous opioid peptide of the dynorphin family that is composed of both dynorphin A and dynorphin B. Big dynorphin has the amino acid sequence: Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro-Lys-Leu-Lys-Trp-Asp-Asn-Gln-Lys-Arg-Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Gln-Phe-Lys-Val-Val-Thr. It has nociceptive and anxiolytic-like properties, as well as effects on memory in mice.
SNC-80 is an opioid analgesic compound that selectively activates μ–δ opioid receptor heteromers and is used in scientific research. It was discovered in 1994.
DAMGO is a synthetic opioid peptide with high μ-opioid receptor specificity. It was synthesized as a biologically stable analog of δ-opioid receptor-preferring endogenous opioids, leu- and met-enkephalin. The crystal structure of DAMGO bound to the µ opioid receptor reveals a very similar binding pose to morphinans.
Teprotide is nonapeptide which has been isolated from the snake Bothrops jararaca. It is an angiotensin converting enzyme inhibitor (ACE inhibitor), which inhibits the conversion of angiotensin I to angiotensin II and may potentiate some of the pharmacological actions of bradykinin. It has a molecular formula of C53H76N14O12 and has been looked at as an antihypertension agent.
Melanocyte-inhibiting factor (also known as Pro-Leu-Gly-NH2, Melanostatin, MSH release–inhibiting hormone or MIF-1) is an endogenous peptide fragment derived from cleavage of the hormone oxytocin, but having generally different actions in the body. MIF-1 produces multiple effects, both blocking the effects of opioid receptor activation, while at the same time acting as a positive allosteric modulator of the D2 and D4 dopamine receptor subtypes, as well as inhibiting release of other neuropeptides such as alpha-MSH, and potentiating melatonin activity.
Spinorphin is an endogenous, non-classical opioid peptide of the hemorphin family first isolated from the bovine spinal cord (hence the prefix spin-) and acts as a regulator of the enkephalinases, a class of enzymes that break down endogenous the enkephalin peptides. It does so by inhibiting the enzymes aminopeptidase N (APN), dipeptidyl peptidase III (DPP3), angiotensin-converting enzyme (ACE), and neutral endopeptidase (NEP). Spinorphin is a heptapeptide and has the amino acid sequence Leu-Val-Val-Tyr-Pro-Trp-Thr (LVVYPWT). It has been observed to possess antinociceptive, antiallodynic, and anti-inflammatory properties. The mechanism of action of spinorphin has not been fully elucidated (i.e., how it acts to inhibit the enkephalinases), but it has been found to act as an antagonist of the P2X3 receptor, and as a weak partial agonist/antagonist of the FP1 receptor.
Hemorphin-4 is an endogenous opioid peptide of the hemorphin family which possesses antinociceptive properties and is derived from the β-chain of hemoglobin in the bloodstream. It is a tetrapeptide with the amino acid sequence Tyr-Pro-Trp-Thr. Hemorphin-4 has affinities for the μ-, δ-, and κ-opioid receptors that are in the same range as the structurally related β-casomorphins, although affinity to the κ-opioid receptor is markedly higher in comparison. It acts as an agonist at these sites. Hemorphin-4 also has inhibitory effects on angiotensin-converting enzyme (ACE), and as a result, may play a role in the regulation of blood pressure. Notably, inhibition of ACE also reduces enkephalin catabolism.
Hemorphins are a class of naturally occurring, endogenous opioid peptides which are found in the bloodstream, and are derived from the β-chain of hemoglobin. They have antinociceptive effects via activation of the opioid receptors, and some may also play a role in blood pressure through inhibition of the angiotensin-converting enzyme (ACE), as well as cause an elevation of endogenous enkephalin levels. Some examples of hemorphins include hemorphin-4, spinorphin, and valorphin.
Biphalin is a dimeric enkephalin endogenous peptide (Tyr-D-Ala-Gly-Phe-NH)2 composed of two tetrapeptides derived from enkephalins, connected 'tail-to-tail' by a hydrazide bridge. The presence of two distinct pharmacophores confers on biphalin a high affinity for both μ and δ opioid receptors (with an EC50 of about 1-5 nM for both μ and δ receptors), therefore it has analgesic activity. Biphalin presents a considerable antinociceptive profile. In fact, when administered intracerebroventricularly in mice, biphalin displays a potency almost 7-fold greater than that of the ultra-potent alkaloid agonist, etorphine and 7000-fold greater than morphine; biphalin and morphine were found to be equipotent after intraperitoneal administration. The extraordinary in vivo potency shown by this compound is coupled with low side-effects, in particular, to produce no dependency in chronic use. For these reasons, several efforts have been carried out in order to obtain more information about structure-activity relationship (SAR). Results clearly indicate that, at least for μ receptor binding, the presence of two pharmacophores is not necessary; Tyr1 is indispensable for analgesic activity, while replacing Phe at the position 4 and 4' with non-aromatic, but lipophilic amino acids does not greatly change the binding properties and in general 4,4' positions are found to be important to design biphalin analogues with increased potency and modified μ/δ selectivity. The hydrazide linker is not fundamental for activity or binding, and it can be conveniently substituted by different conformationally constrained cycloaliphatic diamine linkers.
Deltorphin, also known as deltorphin A and dermenkephalin, is a naturally occurring, exogenous opioid heptapeptide and thus, exorphin, with the amino acid sequence Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2. Along with the other deltorphins (such as deltorphin I and deltorphin II) and the dermorphins, deltorphin is endogenous to frogs of the genus Phyllomedusa such as P. bicolor and P. sauvagei where it is produced in their skin, and is not known to occur naturally in any other species. Deltorphin is one of the highest affinity and most selective naturally occurring opioid peptides known, acting as a very potent and highly specific agonist of the δ-opioid receptor.
Casokefamide (INN), also known as β-casomorphin 4027 (β-CM-4027) and [D-Ala2,4,Tyr5]-β-casomorphin-5-amide, is a peripherally-specific, synthetic opioid pentapeptide with the amino acid sequence Tyr-D-Ala-Phe-D-Ala-Tyr-NH2. Derived from the β-casomorphin sequence, it was designed with the intention of improving resistance to digestive enzymes so that it could be used as an antidiarrheal medicine. Unlike other casomorphins, which are generally selective μ-opioid receptor agonists, casokefamide binds to both the μ- and δ-opioid receptors. In a clinical study, casokefamide was found to be effective via the oral route for the treatment of chronic diarrhea, and did not produce any side effects. However, further clinical development was not pursued and it was never marketed.
Modified GRF (1-29) often abbreviated as mod GRF (1-29), originally known as tetrasubstitued GRF (1-29), is a term used to identify a 29 amino acid peptide analogue of growth-hormone-releasing hormone (GHRH), a releasing hormone of growth hormone (GH). It is a modified version of the shortest fully functional fragment of GHRH, often referred to as growth hormone releasing factor (1-29), and also known by its standardized name, sermorelin.
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