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The opioid excess theory postulates that autism is the result of a metabolic disorder in which opioid peptides produced through metabolism of gluten and casein pass through an abnormally permeable intestinal membrane and then proceed to exert an effect on neurotransmission through binding with opioid receptors. [1]
Opioid-like peptides may also be absorbed from partially digested food (casomorphins, exorphins, and rubiscolins). Opioid peptides from food typically have lengths between 4–8 amino acids. Endogenous opioids are generally much longer.1 Opioid peptides are released by post-translational proteolytic cleavage of precursor proteins.
The class of endorphins consists of three endogenous opioid peptides: α-endorphin, β-endorphin, and γ-endorphin. [23] The endorphins are all synthesized from the precursor protein, proopiomelanocortin, and all contain a Met-enkephalin motif at their N-terminus: Tyr-Gly-Gly-Phe-Met.
The enkephalins are termed endogenous ligands, as they are internally derived (and therefore endogenous) and bind as ligands 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. [2]
An animated view of the human κ-opioid receptor in complex with the antagonist JDTic. Opioid receptors are a group of inhibitory G protein-coupled receptors with opioids as ligands. [1] [2] [3] The endogenous opioids are dynorphins, enkephalins, endorphins, endomorphins and nociceptin. The opioid receptors are ~40% identical to somatostatin ...
Endogenous opioid peptides, produced naturally in the body, such as endorphins, enkephalins, dynorphins, and endomorphins. Endogenous opioids, non-peptide: Morphine, and some other opioids, which are produced in small amounts in the body, are included in this category.
β-Endorphin (beta-endorphin) is an endogenous opioid neuropeptide and peptide hormone that is produced in certain neurons within the central nervous system and peripheral nervous system. [1] It is one of three endorphins that are produced in humans, the others of which include α-endorphin and γ-endorphin .
Endomorphin-2 (EM-2) is an endogenous opioid peptide and one of the two endomorphins. [1] It has the amino acid sequence Tyr-Pro-Phe-Phe-NH 2.It is a high affinity, highly selective agonist of the μ-opioid receptor, and along with endomorphin-1 (EM-1), has been proposed to be the actual endogenous ligand of this receptor (that is, rather than the endorphins).