Difference between revisions of "Peptide" - New World Encyclopedia
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| − | '''Peptides''' (from the Greek πεπτος, "digestible") are | + | '''Peptides''' (from the Greek πεπτος, "digestible") are a family of [[organic compound]]s that consist of short chains of [[amino acid]]s linked together via [[peptide bond]]s. They function primarily as [[hormone]]s, signaling molecules, or [[antibiotic]]s in living organisms. |
| − | + | The number of amino-acid molecules present in a peptide is indicated by a prefix. For example, a ''dipeptide'' has two amino acids; a ''tripeptide'', three. An ''oligopeptide'' contains a few molecules; a ''polypeptide'', many. The distinction between polypeptides and [[protein]]s is largely academic and imprecise. One convention places an informal dividing line at approximately 50 amino acids in length. However, this definition is somewhat arbitrary — some peptides such as [[amyloid beta|alzheimer's beta peptide]] can be considered proteins, and some proteins (such as [[insulin]]) are close to the upper limit for peptides. There is a considerable movement within the scientific community to ascribe the more specific definition that "a peptide is an amino acid molecule without secondary structure; on gaining defined structure, it is a protein." Thus, the same molecule could be classified as either a peptide or a protein depending on its environment (though there are peptides that cannot be proteins). | |
| − | + | Functions of peptides in body and in research | |
| − | + | ==The components of peptides== | |
| + | [[Image:Peptide_bonding.gif|thumb|Summary of the formation of a peptide bond. Click on image to see the reaction.]] | ||
| + | Like proteins, peptides are built from combinations of 20 different '''amino acids''', which are organic molecules composed of an amino group (-NH2), a carboxylic acid group (-COOH), and a unique R group, or [[side chain]]. Two amino acids (specifically, ''alpha-amino acids'') are linked together by a peptide bond, which forms when the amino group of one amino acid reacts with the carboxyl group of a second amino acid. An amino acid residue is what is left of an amino acid once it has coupled with another amino acid to form a peptide bond. | ||
| − | + | Peptides are then created by the polymerization of amino acids, a process in which amino acids are joined together in chains. Shorter strings of amino acids may be referred to as polypeptides, peptides, or, less commonly, oligopeptides. | |
| − | |||
| − | + | Note the use of peptides in building proteins. '''Peptide fragments''' are components of proteins that research use to identify or quantify the source protein. Often these fragments are the products of enzymatic degradation performed in the laboratory on a controlled sample, but they can also be forensic or paleontological samples that have been degraded by natural effects. | |
| − | + | ==Peptide synthesis== | |
| + | Like protein molecules, '''ribosomal peptides''' are synthesized from amino acids according to an [[mRNA]] template, which is itself synthesized from a [[DNA]] template inside the [[cell (biology)|cell]]'s [[Cell nucleus|nucleus]]. Peptide precursors are then processed in several stages, typically in the [[endoplasmic reticulum]], resulting in "propeptides". The propeptides are then packaged into membrane-bound secretory vesicles, which can be released into the bloodstream in response to specific stimuli. | ||
| − | + | '''Nonribosomal peptides''', found primarily in [[unicellular organism]]s, [[plant]]s, and [[fungi]], are synthesized using a [[Modularity (biology)|modular]] enzyme complex (which functions much like a conveyor belt in a factory). All of these complexes are laid out in a similar fashion, and they can contain many different modules to perform a diverse set of chemical manipulations on the developing peptide. In general, nonribosomal peptides often have highly complex cyclic structures, although linear nonribosomal peptides are also common. | |
| − | + | ==Some key peptide groups and their biological function== | |
| + | Ribosomal peptides function as [[hormone]]s and signaling molecules, typically in higher organisms. Some lower organisms produce peptides as antibiotics, such as [[microcin J25]]. | ||
| − | == | + | === Peptide hormones === |
| − | + | '''Peptide hormones''' are a class of peptides that function in living animals as ‘’hormones’’ - i.e., as chemical messengers from one cell (or group of cells) to another. | |
| − | + | Several important peptide hormones are secreted from the [[pituitary gland]], an [[endocrine gland]] about the size of a pea that sits in a small, bony cavity at the base of the [[brain]]. The [[anterior pituitary]] secretes [[luteinizing hormone]] and [[follicle stimulating hormone]], which act on the [[gonads]]; [[prolactin]], which acts on the [[mammary gland]], [[adrenocorticotrophic hormone]] (ACTH), which acts on the [[adrenal cortex]] to regulate the secretion of [[glucocorticoids]]; and [[growth hormone]], which acts on [[bone]], [[muscle]] and the [[liver]]. The [[posterior pituitary]] gland secretes [[vasopressin]], and [[oxytocin]]. Peptide hormones are produced by many different organs and tissues, however, including the [[heart]] ([[atrial-natriuretic peptide]] (ANP) or atrial natriuretic factor (ANF)) and [[pancreas]] ([[insulin]] and [[somatostatin]]), the gastrointestinal tract ([[cholecystokinin]], [[gastrin]]), and [[fat]] stores ([[leptin]]). | |
| − | + | '''Arginine vasopressin''' ('''AVP'''), also known as '''argipressin''' or '''antidiuretic hormone''' ('''ADH'''), is a human [[hormone]] that is mainly released when the body is low on [[water]]; it causes the [[kidney]]s to conserve water by concentrating the [[urine]] and reducing urine volume. It also has various functions in the [[brain]] and [[blood vessel]]s. | |
| − | Inhibitory peptides are also used in clinical research to examine the effects of peptides on the inhibition of cancer proteins and other diseases. | + | Vasopressin released within the brain has many actions: |
| + | * It has been implicated in [[memory]] formation, including delayed reflexes, image, short- and long-term memory, though the mechanism remains unknown, and these findings are controversial. However, the synthetic [[vasopressin analogue]] [[desmopressin]] has come to interest as a likely [[nootropic]]. | ||
| + | |||
| + | * Vasopressin is released into the brain in a [[circadian rhythm]] by neurons of the [[Suprachiasmatic nucleus|suprachiasmatic nucleus of the hypothalamus]]. | ||
| + | |||
| + | * Vasopressin released from centrally-projecting hypothalamic neurons is involved in aggression, blood pressure regulation and temperature regulation. | ||
| + | |||
| + | In recent years there has been particular interest in the role of vasopressin in social behavior. It is thought that vasopressin, released into the brain during sexual activity, initiates and sustains patterns of activity that support the pair-bond between the sexual partners; in particular, vasopressin seems to induce the male to become aggressive towards other males. | ||
| + | |||
| + | Evidence for this comes from experimental studies, in several species, which indicate that the precise distribution of vasopressin and vasopressin receptors in the brain is associated with species-typical patterns of social behavior. In particular, there are consistent differences between monogamous species and promiscuous species in the distribution of vasopressin receptors, and sometimes in the distribution of vasopressin-containing axons, even when closely-related species are compared. Moreover, studies involving either injecting vasopressin agonists into the brain, or blocking the actions of vasopressin, support the hypothesis that vasopressin is involved in aggression towards other males. There is also evidence that differences in the vasopressin receptor gene between individual members of a species might be predictive of differences in social behavior. | ||
| + | |||
| + | '''Oxytocin''' (Greek: "quick birth") is a [[mammal]]ian [[hormone]] that also acts as a [[neurotransmitter]] in the [[brain]]. In women, it is released mainly after distension of the [[cervix]] and [[vagina]] during labor, and after stimulation of the [[nipple]]s, facilitating [[childbirth|birth]] and [[breastfeeding]], respectively. Oxytocin is released during [[orgasm]] in both sexes. In the brain, oxytocin is involved in social recognition and bonding, and might be involved in the formation of trust between people. | ||
| + | |||
| + | Many [[neurotransmitters]] are secreted and released in a similar fashion to peptide hormones, and some '[[neuropeptides]]' may be used as neurotransmitters in the [[nervous system]] in addition to acting as hormones when released into the blood. | ||
| + | |||
| + | === Neuropeptides === | ||
| + | A '''neuropeptide''' is any of the variety of peptides found in [[neural tissue]]; e.g. [[endorphin]]s and [[enkephalin]]s. Approximately 100 different peptides are currently known to be released by different populations of neurons in the mammalian brain. | ||
| + | |||
| + | Neurons use many different chemical signals to communicate information, including neurotransmitters, peptides, [[cannabinoids]], and even some gases, like [[nitric oxide]]. Peptide signals play a role in information processing that is different to that of conventional neurotransmitters, and many appear to be particularly associated with specific behaviors. For example, [[oxytocin]] and [[vasopressin]] have striking and specific effects on social behaviors, including maternal behavior and pair bonding. | ||
| + | |||
| + | [[Neurotransmitters]] generally affect the excitability of other neurons, by depolarizing them or by hyperpolarizing them. Peptides have much more diverse effects; among other things, they can affect gene expression, local blood flow, [[synaptogenesis]], and [[glial]] cell morphology. Peptides tend to have prolonged actions, and some have striking effects on behavior. | ||
| + | |||
| + | Neurons very often produce both a conventional neurotransmitter (such as [[glutamate]], [[GABA]] or [[dopamine]]) and one or more neuropeptides. Peptides are generally packaged in large dense-core vesicles, while the co-existing neurotransmitters are contained in small synaptic vesicles. The large dense-core vesicles are often found in all parts of a neuron, including the [[soma]], [[dendrites]], axonal swellings and nerve endings, whereas the small synaptic vesicles are mainly found in clusters at presynaptic locations. | ||
| + | |||
| + | === Opioid peptides === | ||
| + | An '''opioid''' is a chemical substance (natural or synthetic) that has a morphine-like action in the body. Its main use is for [[analgesia|pain relief]]. These agents work by binding to [[opioid receptor]]s, which are found principally in the [[central nervous system]] and the [[gastrointestinal tract]]. Opioid peptides that are produced in the body include [[endorphin]]s, [dynorphin]]s, and [[enkephalin]]s. | ||
| + | |||
| + | When certain food proteins such as [[gluten]], [[casein]], [[egg (food)|egg]] protein, and [[spinach]] protein are broken down, [[opioid peptides]] are formed. These peptides mimic the effects of morphine, and those individuals that are unable to break them down will experience mental illness (vague). A '''gluten-free casein-free diet''' (or '''GFCF diet''') is a restrictive diet which entirely eliminates intake of the naturally-occuring proteins [[gluten]] (found naturally in wheat, barley, and rye), and [[casein]] (found in [[milk]]). The GFCF diet is recommended by advocacy groups, such as the [[Autism Research Institute]], to reduce the prevalence of certain behaviors associated with pervasive developmental disorders, especially [[autism]]. While multiple success stories have been published and circulated, results from clinical trials and double-blind studies have produced little to confirm the diet's efficacy. | ||
| + | |||
| + | In the 1960s, Dohan speculated that the low incidence of [[schizophrenia]] in certain South Pacific Island societies was a result of a diet low in wheat- and milk- based foods.<ref> Dohan, F.C. (1966) Cereals and Schizophrenia, data and hypothesis ''Acta Physiologica Scandinavica'', ''42'', 125-132.</ref> Dohan proposed genetic defect as a probable [[etiology]] for schizophrenia, wherein individuals were incapable of completely metabolizing gluten and casein. The consequence of incomplete metabolism is excess [[peptide]] levels in the gastrointestinal tract, which Dothan hypothesizes are responsible for schizophrenic behaviors. | ||
| + | |||
| + | The possible relationship between gluten, casein, and autism was first articulated by Kalle Reichelt, M.D. in 1991.<ref> Reichelt KL, Knivsberg A-M, Lind G, Nødland M. Probable etiology and possible treatment of childhood autism. Brain Dysfunct 1991; 4: 308-19 </ref> Based on studies showing correlation between autism and increased urinary peptide levels, Reichelt hypothesized that some of these peptides may have an [[opiate]] effect. Further work determined [[opioid peptide]]s such as [[casomorphine]]s<ref> Sun, Z. and Cade, J.R. (1999) A peptide found in schizophrenia and autism causes behavioral changes in rats. ''Autism'', ''3''(1), 85-95. </ref> (from [[casein]]) and [[gluten exorphine]]s and [[gliadorphin]] (from [[gluten]]) as probable suspects, due to their chemical similarity to opiates. | ||
| + | Reichelt hypothesizes that long term exposure to these [[opiate]] [[peptides]] may have effects on brain maturation and contribute to social awkwardness and isolation. On this basis, Reichelt and others have proposed a gluten-free casein-free diet to minimize the buildup of opiate peptides and promote typical development of brain function. | ||
| + | |||
| + | Reichelt's hypothesis is not generally regarded as a definitive [[etiology]] for autism, nor is the GFCF diet advocated as a "cure". While considerable attention has been given to metabolic and/or gastrointestinal factors related to autism, <ref> White, John F. (2003) "Intestinal Pathology in Autism" ''Experimental Biology and Medicine'', ''228'', pp. 639-649.</ref>, Reichelt's hypothesis is regarded as either incorrect, incomplete, or requiring further investigation.<ref name="elder">Elder, J., et al. 2006. "The Gluten-Free, Casein-Free Diet in Autism: Results of a Preliminary Double Blind Clinical Trial." Journal of Autism and Developmental Disorders 36:413-420. </ref> | ||
| + | |||
| + | == Peptides are an important research tool == | ||
| + | Peptides have received prominence in [[molecular biology]] in recent times for several reasons: | ||
| + | |||
| + | #Peptides allow researchers to generate antibodies in animals without the need to purify the protein of interest. The researcher can simply make antigenic peptides of sections of the protein. | ||
| + | #Peptides have become instrumental in [[mass spectrometry]], allowing the identification of proteins of interest based on peptide masses and sequences. | ||
| + | #Peptides have recently been used in the study of protein structure and function. For example, synthetic peptides can be used as probes to determine where protein-peptide interactions occur. | ||
| + | #Inhibitory peptides are also used in clinical research to examine the effects of peptides on the inhibition of cancer proteins and other diseases. | ||
==Peptide families== | ==Peptide families== | ||
| − | + | Below is a more detailed list of the major families of ribosomal peptides. | |
| − | |||
| − | |||
| + | Vasopressin and oxytocin | ||
* [[Vasopressin]] | * [[Vasopressin]] | ||
* [[Oxytocin]] | * [[Oxytocin]] | ||
| − | + | The [[Tachykinin peptides]] | |
* [[Substance P]] | * [[Substance P]] | ||
* [[Kassinin]] | * [[Kassinin]] | ||
| Line 44: | Line 88: | ||
* [[Neurokinin B]] | * [[Neurokinin B]] | ||
| − | + | [[Vasoactive intestinal peptides]] | |
* [[Vasoactive intestinal peptide|VIP]] ''Vasoactive intestinal peptide'' | * [[Vasoactive intestinal peptide|VIP]] ''Vasoactive intestinal peptide'' | ||
* [[PACAP]] ''[[Pituitary adenylate cyclase activating peptide]]'' | * [[PACAP]] ''[[Pituitary adenylate cyclase activating peptide]]'' | ||
| Line 53: | Line 97: | ||
* [[Secretin]] | * [[Secretin]] | ||
| − | + | [[Pancreatic polypeptide-related peptides]] | |
* [[NPY]] | * [[NPY]] | ||
* [[PYY]] ''[[Peptide YY]]'' | * [[PYY]] ''[[Peptide YY]]'' | ||
| Line 59: | Line 103: | ||
* [[HPP]] ''[[Human pancreatic polypeptide]]'' | * [[HPP]] ''[[Human pancreatic polypeptide]]'' | ||
| − | + | [[Opioid peptides]] | |
* [[Proopiomelanocortin]] ([[POMC]]) Peptides | * [[Proopiomelanocortin]] ([[POMC]]) Peptides | ||
* The [[Enkephalin pentapeptides]] | * The [[Enkephalin pentapeptides]] | ||
* The [[Prodynorphin peptides]] | * The [[Prodynorphin peptides]] | ||
| − | + | Calcitonin peptides | |
* [[Calcitonin]] | * [[Calcitonin]] | ||
* [[Amylin]] | * [[Amylin]] | ||
*[[AGG01]] | *[[AGG01]] | ||
| − | == | + | ==References== |
| − | * | + | *Stryer, L. 1995. ''Biochemistry'', 4th edition. New York, NY: W.H. Freeman. |
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==External links== | ==External links== | ||
*[http://www.peptideresource.com Peptide Resource Page] | *[http://www.peptideresource.com Peptide Resource Page] | ||
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{{credit|98760524}} | {{credit|98760524}} | ||
[[Category:Life sciences]] | [[Category:Life sciences]] | ||
Revision as of 16:41, 15 March 2007
Peptides (from the Greek πεπτος, "digestible") are a family of organic compounds that consist of short chains of amino acids linked together via peptide bonds. They function primarily as hormones, signaling molecules, or antibiotics in living organisms.
The number of amino-acid molecules present in a peptide is indicated by a prefix. For example, a dipeptide has two amino acids; a tripeptide, three. An oligopeptide contains a few molecules; a polypeptide, many. The distinction between polypeptides and proteins is largely academic and imprecise. One convention places an informal dividing line at approximately 50 amino acids in length. However, this definition is somewhat arbitrary — some peptides such as alzheimer's beta peptide can be considered proteins, and some proteins (such as insulin) are close to the upper limit for peptides. There is a considerable movement within the scientific community to ascribe the more specific definition that "a peptide is an amino acid molecule without secondary structure; on gaining defined structure, it is a protein." Thus, the same molecule could be classified as either a peptide or a protein depending on its environment (though there are peptides that cannot be proteins).
Functions of peptides in body and in research
The components of peptides
Like proteins, peptides are built from combinations of 20 different amino acids, which are organic molecules composed of an amino group (-NH2), a carboxylic acid group (-COOH), and a unique R group, or side chain. Two amino acids (specifically, alpha-amino acids) are linked together by a peptide bond, which forms when the amino group of one amino acid reacts with the carboxyl group of a second amino acid. An amino acid residue is what is left of an amino acid once it has coupled with another amino acid to form a peptide bond.
Peptides are then created by the polymerization of amino acids, a process in which amino acids are joined together in chains. Shorter strings of amino acids may be referred to as polypeptides, peptides, or, less commonly, oligopeptides.
Note the use of peptides in building proteins. Peptide fragments are components of proteins that research use to identify or quantify the source protein. Often these fragments are the products of enzymatic degradation performed in the laboratory on a controlled sample, but they can also be forensic or paleontological samples that have been degraded by natural effects.
Peptide synthesis
Like protein molecules, ribosomal peptides are synthesized from amino acids according to an mRNA template, which is itself synthesized from a DNA template inside the cell's nucleus. Peptide precursors are then processed in several stages, typically in the endoplasmic reticulum, resulting in "propeptides". The propeptides are then packaged into membrane-bound secretory vesicles, which can be released into the bloodstream in response to specific stimuli.
Nonribosomal peptides, found primarily in unicellular organisms, plants, and fungi, are synthesized using a modular enzyme complex (which functions much like a conveyor belt in a factory). All of these complexes are laid out in a similar fashion, and they can contain many different modules to perform a diverse set of chemical manipulations on the developing peptide. In general, nonribosomal peptides often have highly complex cyclic structures, although linear nonribosomal peptides are also common.
Some key peptide groups and their biological function
Ribosomal peptides function as hormones and signaling molecules, typically in higher organisms. Some lower organisms produce peptides as antibiotics, such as microcin J25.
Peptide hormones
Peptide hormones are a class of peptides that function in living animals as ‘’hormones’’ - i.e., as chemical messengers from one cell (or group of cells) to another.
Several important peptide hormones are secreted from the pituitary gland, an endocrine gland about the size of a pea that sits in a small, bony cavity at the base of the brain. The anterior pituitary secretes luteinizing hormone and follicle stimulating hormone, which act on the gonads; prolactin, which acts on the mammary gland, adrenocorticotrophic hormone (ACTH), which acts on the adrenal cortex to regulate the secretion of glucocorticoids; and growth hormone, which acts on bone, muscle and the liver. The posterior pituitary gland secretes vasopressin, and oxytocin. Peptide hormones are produced by many different organs and tissues, however, including the heart (atrial-natriuretic peptide (ANP) or atrial natriuretic factor (ANF)) and pancreas (insulin and somatostatin), the gastrointestinal tract (cholecystokinin, gastrin), and fat stores (leptin).
Arginine vasopressin (AVP), also known as argipressin or antidiuretic hormone (ADH), is a human hormone that is mainly released when the body is low on water; it causes the kidneys to conserve water by concentrating the urine and reducing urine volume. It also has various functions in the brain and blood vessels.
Vasopressin released within the brain has many actions:
- It has been implicated in memory formation, including delayed reflexes, image, short- and long-term memory, though the mechanism remains unknown, and these findings are controversial. However, the synthetic vasopressin analogue desmopressin has come to interest as a likely nootropic.
- Vasopressin is released into the brain in a circadian rhythm by neurons of the suprachiasmatic nucleus of the hypothalamus.
- Vasopressin released from centrally-projecting hypothalamic neurons is involved in aggression, blood pressure regulation and temperature regulation.
In recent years there has been particular interest in the role of vasopressin in social behavior. It is thought that vasopressin, released into the brain during sexual activity, initiates and sustains patterns of activity that support the pair-bond between the sexual partners; in particular, vasopressin seems to induce the male to become aggressive towards other males.
Evidence for this comes from experimental studies, in several species, which indicate that the precise distribution of vasopressin and vasopressin receptors in the brain is associated with species-typical patterns of social behavior. In particular, there are consistent differences between monogamous species and promiscuous species in the distribution of vasopressin receptors, and sometimes in the distribution of vasopressin-containing axons, even when closely-related species are compared. Moreover, studies involving either injecting vasopressin agonists into the brain, or blocking the actions of vasopressin, support the hypothesis that vasopressin is involved in aggression towards other males. There is also evidence that differences in the vasopressin receptor gene between individual members of a species might be predictive of differences in social behavior.
Oxytocin (Greek: "quick birth") is a mammalian hormone that also acts as a neurotransmitter in the brain. In women, it is released mainly after distension of the cervix and vagina during labor, and after stimulation of the nipples, facilitating birth and breastfeeding, respectively. Oxytocin is released during orgasm in both sexes. In the brain, oxytocin is involved in social recognition and bonding, and might be involved in the formation of trust between people.
Many neurotransmitters are secreted and released in a similar fashion to peptide hormones, and some 'neuropeptides' may be used as neurotransmitters in the nervous system in addition to acting as hormones when released into the blood.
Neuropeptides
A neuropeptide is any of the variety of peptides found in neural tissue; e.g. endorphins and enkephalins. Approximately 100 different peptides are currently known to be released by different populations of neurons in the mammalian brain.
Neurons use many different chemical signals to communicate information, including neurotransmitters, peptides, cannabinoids, and even some gases, like nitric oxide. Peptide signals play a role in information processing that is different to that of conventional neurotransmitters, and many appear to be particularly associated with specific behaviors. For example, oxytocin and vasopressin have striking and specific effects on social behaviors, including maternal behavior and pair bonding.
Neurotransmitters generally affect the excitability of other neurons, by depolarizing them or by hyperpolarizing them. Peptides have much more diverse effects; among other things, they can affect gene expression, local blood flow, synaptogenesis, and glial cell morphology. Peptides tend to have prolonged actions, and some have striking effects on behavior.
Neurons very often produce both a conventional neurotransmitter (such as glutamate, GABA or dopamine) and one or more neuropeptides. Peptides are generally packaged in large dense-core vesicles, while the co-existing neurotransmitters are contained in small synaptic vesicles. The large dense-core vesicles are often found in all parts of a neuron, including the soma, dendrites, axonal swellings and nerve endings, whereas the small synaptic vesicles are mainly found in clusters at presynaptic locations.
Opioid peptides
An opioid is a chemical substance (natural or synthetic) that has a morphine-like action in the body. Its main use is for pain relief. These agents work by binding to opioid receptors, which are found principally in the central nervous system and the gastrointestinal tract. Opioid peptides that are produced in the body include endorphins, [dynorphin]]s, and enkephalins.
When certain food proteins such as gluten, casein, egg protein, and spinach protein are broken down, opioid peptides are formed. These peptides mimic the effects of morphine, and those individuals that are unable to break them down will experience mental illness (vague). A gluten-free casein-free diet (or GFCF diet) is a restrictive diet which entirely eliminates intake of the naturally-occuring proteins gluten (found naturally in wheat, barley, and rye), and casein (found in milk). The GFCF diet is recommended by advocacy groups, such as the Autism Research Institute, to reduce the prevalence of certain behaviors associated with pervasive developmental disorders, especially autism. While multiple success stories have been published and circulated, results from clinical trials and double-blind studies have produced little to confirm the diet's efficacy.
In the 1960s, Dohan speculated that the low incidence of schizophrenia in certain South Pacific Island societies was a result of a diet low in wheat- and milk- based foods.[1] Dohan proposed genetic defect as a probable etiology for schizophrenia, wherein individuals were incapable of completely metabolizing gluten and casein. The consequence of incomplete metabolism is excess peptide levels in the gastrointestinal tract, which Dothan hypothesizes are responsible for schizophrenic behaviors.
The possible relationship between gluten, casein, and autism was first articulated by Kalle Reichelt, M.D. in 1991.[2] Based on studies showing correlation between autism and increased urinary peptide levels, Reichelt hypothesized that some of these peptides may have an opiate effect. Further work determined opioid peptides such as casomorphines[3] (from casein) and gluten exorphines and gliadorphin (from gluten) as probable suspects, due to their chemical similarity to opiates. Reichelt hypothesizes that long term exposure to these opiate peptides may have effects on brain maturation and contribute to social awkwardness and isolation. On this basis, Reichelt and others have proposed a gluten-free casein-free diet to minimize the buildup of opiate peptides and promote typical development of brain function.
Reichelt's hypothesis is not generally regarded as a definitive etiology for autism, nor is the GFCF diet advocated as a "cure". While considerable attention has been given to metabolic and/or gastrointestinal factors related to autism, [4], Reichelt's hypothesis is regarded as either incorrect, incomplete, or requiring further investigation.[5]
Peptides are an important research tool
Peptides have received prominence in molecular biology in recent times for several reasons:
- Peptides allow researchers to generate antibodies in animals without the need to purify the protein of interest. The researcher can simply make antigenic peptides of sections of the protein.
- Peptides have become instrumental in mass spectrometry, allowing the identification of proteins of interest based on peptide masses and sequences.
- Peptides have recently been used in the study of protein structure and function. For example, synthetic peptides can be used as probes to determine where protein-peptide interactions occur.
- Inhibitory peptides are also used in clinical research to examine the effects of peptides on the inhibition of cancer proteins and other diseases.
Peptide families
Below is a more detailed list of the major families of ribosomal peptides.
Vasopressin and oxytocin
- Vasopressin
- Oxytocin
The Tachykinin peptides
- Substance P
- Kassinin
- Neurokinin A
- Eledoisin
- Neurokinin B
Vasoactive intestinal peptides
- VIP Vasoactive intestinal peptide
- PACAP Pituitary adenylate cyclase activating peptide
- PHI 27
- PHM 27
- GHRH 1-24 Growth hormone releasing hormone 1-24
- Glucagon
- Secretin
Pancreatic polypeptide-related peptides
- NPY
- PYY Peptide YY
- APP Avian pancreatic polypeptide
- HPP Human pancreatic polypeptide
Opioid peptides
- Proopiomelanocortin (POMC) Peptides
- The Enkephalin pentapeptides
- The Prodynorphin peptides
Calcitonin peptides
- Calcitonin
- Amylin
- AGG01
ReferencesISBN links support NWE through referral fees
- Stryer, L. 1995. Biochemistry, 4th edition. New York, NY: W.H. Freeman.
External links
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- ↑ Dohan, F.C. (1966) Cereals and Schizophrenia, data and hypothesis Acta Physiologica Scandinavica, 42, 125-132.
- ↑ Reichelt KL, Knivsberg A-M, Lind G, Nødland M. Probable etiology and possible treatment of childhood autism. Brain Dysfunct 1991; 4: 308-19
- ↑ Sun, Z. and Cade, J.R. (1999) A peptide found in schizophrenia and autism causes behavioral changes in rats. Autism, 3(1), 85-95.
- ↑ White, John F. (2003) "Intestinal Pathology in Autism" Experimental Biology and Medicine, 228, pp. 639-649.
- ↑ Elder, J., et al. 2006. "The Gluten-Free, Casein-Free Diet in Autism: Results of a Preliminary Double Blind Clinical Trial." Journal of Autism and Developmental Disorders 36:413-420.
