Английская Википедия:Histamine N-methyltransferase

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Шаблон:Short description

Шаблон:Use dmy dates Шаблон:Cs1 config Шаблон:Use American English Шаблон:Enzyme Шаблон:Infobox gene Histamine N-methyltransferase (HNMT) is a cytoplasmic protein encoded by the HNMT gene in humans. It belongs to the methyltransferases superfamily of enzymes and plays a crucial role in the inactivation of histamine, a biogenic amine involved in various physiological processes. Methyltransferases are present in every life form including archaeans, with 230 families of methyltransferases found across species.

Specifically, HNMT transfers a methyl (CH3) group from S-adenosyl-L-methionine (SAM-e) to histamine, forming an inactive metabolite called Nτ-methylhistamine, in a chemical reaction called Nτ-methylation. In mammals, HNMT operates alongside diamine oxidase (DAO) as the only two enzymes responsible for histamine metabolism; however, what sets HNMT apart is its unique presence within the central nervous system (CNS), where it governs intracellular histaminergic neurotransmission. By degrading and regulating levels of intracellular histamine specifically within the CNS, HNMT ensures the proper functioning of neural pathways related to arousal, sleep-wake cycles, and other essential brain functions.

Research on knockout mice lacking the Hnmt gene has revealed that the absence of this enzyme leads to increased brain histamine concentrations and behavioral changes such as heightened aggression and disrupted sleep patterns. These findings highlight the critical role played by HNMT in maintaining normal brain function through precise regulation of neuronal signaling involving histamine. Genetic variants affecting HNMT activity have also been implicated in various neurological disorders like Parkinson's disease and attention deficit disorder, still, Шаблон:As of it remains unclear whether alterations in HNMT are primary causes or secondary effects of these conditions.

Gene

Шаблон:See also Histamine N-methyltransferase is encoded by a single gene, called HNMT, which has been mapped to chromosome 2 in humans.[1]

Three transcript variants have been identified for this gene in humans, which produce different protein isoforms[2][1] due to alternative splicing, which allows a single gene to code for multiple proteins by including or excluding particular exons of a gene in the final mRNA produced from that gene.[3][4] Of those isoforms, only one has histamine-methylating activity.[2]

In the human genome, six exons from the 50-kb HNMT contribute to forming a unique mRNA species, approximately 1.6 kb in size. This mRNA is then translated into the cytosolic enzyme histamine N-methyltransferase, comprising 292 amino acids, of which 130 amino acids are a conserved sequence.[5][6] HNMT does not have promoter cis-elements, such as TATA and CAAT boxes.[7] Шаблон:As of this human gene is still insufficiently characterized.[7]

Protein

The crystallographic structure (three-dimensional structure) of human HNMT protein was first described in 2001 as a 33kDa monomeric protein consisting of two structural domains.[8][9][7]

The first domain, called the "MTase domain", contains the active site where methylation occurs. It has a classic fold found in many other methyltransferases, and consists of a seven-stranded beta-sheet surrounded by three helices on each side. This domain binds to its cofactor, S-adenosyl-L-methionine (SAM-e), which provides the methyl group for Nτ-methylation reactions.[8][9]

The second domain, called the "substrate binding domain", interacts with histamine, contributing to its binding to the enzyme molecule. This domain is connected to the MTase domain and forms a separate region. It includes an antiparallel beta sheet along with additional alpha helices and 310 helices.[8][9]

Species

Histamine N-methyltransferase belongs to methyltransferases, a superfamily of enzymes present in every life form,[6] including archaeans.[10]

These enzymes catalyze methylation, which is a chemical process that involves the addition of a methyl group to a molecule, which can affect its biological function.[6][9][11]

To facilitate methylation, methyltransferases transfer a methyl group (-CH3) from a cosubstrate (donor) to a substrate molecule (acceptor), leading to the formation of a methylated molecule.[6][9]

Most methyltransferases use S-adenosyl-L-methionine (SAM-e) as a donor, converting it into S-adenosyl-L-homocysteine (SAH).[6][9]

In various species, members of the methyltransferase superfamily of enzymes methylate a wide range of molecules, including small molecules, proteins, nucleic acids, and lipids. These enzymes are involved in numerous cellular processes such as signaling, protein repair, chromatin regulation, and gene regulation. More than 230 families of methyltransferases have been described in various species.[6][12]

This specific protein, histamine N-methyltransferase, is found in vertebrates, including mammals, birds, reptiles, amphibians, and fishes, but not in invertebrates and plants.[13][5][14]

The cDNA of Hnmt was initially cloned from a rat kidney and has since been cloned from human, mouse, and guinea pig sources.[5]

Human HNMT shares 55.37% similarity with that of zebrafish, 86.76% with that of mouse, 90.53% with that of dog, and 99.54% with that of chimpanzee.[14][15] Moreover, expressed sequence tags from cow, pig, and gorilla, as well as genome survey sequences from pufferfish, also exhibit strong similarity to human HNMT, suggesting that it is a highly conserved protein among vertebrates.[8]

To understand the role of histamine N-methyltransferase in brain function, researchers have studied Hnmt-deficient (knockout) mice.[16][13][17] Scientists discovered that disrupting the gene led to a significant rise in histamine levels in the mouse brain that highlighted the role of the gene in the brain's histamine system and suggested that HNMT genetic variations in humans could be linked to brain disorders.[13]

Tissue and subcellular distribution

On subcellular distribution, histamine N-methyltransferase protein in humans is mainly localized to the nucleoplasm (which is an organelle, i.e., a subunit of a cell) and cytosol (which is the intracellular fluid, i.e., a fluid inside cells). In addition, it is localized to the centrosome (another organelle).[18]

In humans, the protein is present in many tissues and is most abundantly expressed in the brain, thyroid gland, bronchus, duodenum, liver, gallbladder, kidney, and skin.[19][13]

Function

Файл:HNMT-methylation-of-histamine.svg
Biological inactivation of histamine via Nτ-methylation by the histamine N-methyltransferase (HNMT) enzyme using S-adenosyl-L-methionine (SAM-e) as a cosubstrate and a donor of methyl (CH3) functional group[11] (the biochemical transformation is depicted as in KEGG reaction R02155).[20] The methyl group from the cosubstrate (denoted by red oval) is transferred to histamine to Nτ position that forms Nτ-methylhistamine (NMT), which has the methyl group attached (denoted by green oval). The conversion of histamine to NMT is shown by the straight arrow. The SAM-e is thereby transformed to S-adenosyl-L-histidine (SAH), a molecule without the methyl group. The conversion of SAM-e to SAH is shown by the curved arrow.[21][13][9]

The function of the HNMT enzyme is histamine metabolism by ways of Nτ-methylation using S-adenosyl-L-methionine (SAM-e) as the methyl donor, producing Nτ-methylhistamine, which, unless excreted, can be further processed by monoamine oxidase B (MAOB) or by DAO. Methylated histamine metabolites are excreted with urine.[8][9][7]

In mammals, histamine is metabolized by two major pathways: oxidative deamination via DAO, encoded by the AOC1 gene, and Nτ-methylation via histamine N-methyltransferase.[7]

HNMT and DAO are two enzymes that play distinct roles in histamine metabolism. DAO is primarily responsible for metabolizing histamine in extracellular (outside cells) fluids, which include interstitial fluid (fluid surrounding cells) and blood plasma.[13][22] Such histamine can be exogenous (from food or intestinal flora) or endogenous (released from granules of mast cells and basophils, such as during allergic reactions).[23] DAO is predominantly expressed in the cells of the intestinal epithelium and placenta, but not in the central nervous system (CNS).[24][25][13] In contrast, HNMT is expressed in CNS and involved in the metabolism of intracellular (inside cells) histamine, which is primarily endogenous and persistently present. HNMT operates in the cytosol, which is the fluid inside cells.[13] Histamine is required to be carried into the cytosol through transporters[26] such as plasma membrane monoamine transporter (SLC29A4) or organic cation transporter 3 (SLC22A3).[13] HNMT enzyme is found in cells of diverse tissues: neurons and glia, brain, kidneys, liver, bronchi, large intestine, ovary, prostate, spinal cord, spleen, and trachea, etc.[21][27][25][13] While DAO is primarily found in the intestinal epithelium, HNMT is present in a wider range of tissues throughout the body. This difference in location also requires different transport mechanisms for histamine to reach each enzyme, reflecting the distinct roles of these enzymes in histamine metabolism.[13] Another distinction between HNMT and DAO lies in their substrate specificity. While HNMT has a strong preference for histamine, DAO can metabolize other biogenic amines such as putrescine and cadaverine, albeit with a preference for histamine. Both DAO and HNMT exhibit comparable affinities toward histamine.[28]

In the brain of mammals, histamine neurotransmitter activity is controlled by HNMT, since DAO is not present in the CNS.[1] Consequently, the deactivation of histamine via HNMT represents the sole mechanism for ending neurotransmission within the mammalian CNS.[21] This highlights the key role of HNMT for the histamine system of the brain and the brain function in general.[13]

Clinical significance

Role in health

Шаблон:See also Histamine has important roles in human physiology as both a hormone and a neurotransmitter. As a hormone, it is involved in the inflammatory response and itching. It regulates physiological functions in the gut and acts on the brain, spinal cord, and uterus. As a neurotransmitter, histamine promotes arousal and regulates the sleep-wake cycle. It also affects vasodilation, fluid production in tissues like the nose and eyes, gastric acid secretion, sexual function, and immune responses.[29][30]

HNMT plays a crucial role in maintaining the proper balance of histamine in the human body. HNMT is responsible for the breakdown and metabolism of histamine, converting it into an inactive metabolite, Nτ-methylhistamine,[29][30] which inhibits HNMT expression in a negative feedback loop.[31] By metabolizing histamine, HNMT helps prevent excessive levels of histamine from accumulating in various tissues and organs. This enzymatic activity ensures that histamine remains at appropriate levels to carry out its physiological functions without causing unwanted effects or triggering allergic reactions. In the central nervous system, HNMT plays an essential role in degrading histamine, where it acts as a neurotransmitter, since HNMT is the only enzyme in the body that can metabolize histamine in CNS, ending its neurotransmitter activity.[29][30]

HNMT also plays a role in the airway response to harmful particles,[32] which is the body's physiological reaction to immune allergens, bacteria, or viruses in the respiratory system. Histamine is stored in granules in mast cells, basophils, and in the synaptic vesicles of histaminergic neurons of the airways. When exposed to immune allergens or harmful particles, histamine is released from these storage granules and quickly diffuses into the surrounding tissues. However, the released histamine needs to be rapidly deactivated for proper regulation, which is a function of HNMT.[33][34]

Histamine intolerance

Шаблон:Main article Histamine intolerance (HIT) is a presumed set of adverse reactions to ingested histamine in food. HIT has nonspecific clinical manifestations, which can include itching, flushing, edema, gastrointestinal symptoms such as abdominal pain and diarrhea, respiratory symptoms like runny nose and nasal congestion, and other symptoms like dizziness and headache.[35] It is presumed by some scholars[36][37][38] that in the case of flawed HNMT activity, the most affected organs are the brain, liver, and mucous membrane of the bronchus, consequently, flawed HNMT activity can lead to chronic forms of HIT. For instance, the main symptoms of histamine intolerance (HIT) within the nervous system, which are associated with flawed HNMT activity, are anxiety, dizziness, fatigue, insomnia, myoclonic twitching, and unrest. Overall, the symptoms of flawed HNMT activity are typical of symptoms of HIT, including allergic rhinitis, urticaria (hives), and peptic ulcer disease.[13] However, this link between flawed HNMT activity and adverse reactions to ingested histamine in food is not shared by mainstream science, and HIT is not recognized as a separate condition that can be diagnosed.[39]

The exact mechanisms by which deficiency or impaired activity of HNMT causes HIT are not fully understood but are hypothesized to involve genetic factors: variations in genes encoding for HNMT have been presumed as potential contributors to increased susceptibility to allergic diseases and more severe courses of conditions like atopic dermatitis and allergic rhinitis: while deficiencies in DAO may be considered the primary cause of HIT, variations in both DAO and HNMT genes could play a role in its development; the interplay between these enzymes may influence effectively histamine is broken down and cleared from the body,[35][40] but the reliable studies to confirm these hypothesis are lacking.[39] Шаблон:As of despite extensive research, there are no definitive, objective measures or indicators that can conclusively validate the occurrence of adverse reactions due to the consumption of histamine in food that will allow to classify HIT as an identifiable medical condition.[39]

Activity measurements

The activity of HNMT, unlike that of DAO, cannot be measured by blood (serum) analysis.[41][42][43]

Organs that produce DAO continuously release it into the bloodstream. DAO is stored in vesicular structures associated with the plasma membrane in epithelial cells.[25] As a result, serum DAO activity can be measured for diagnosing histamine intolerance, but not HNMT. This is because HNMT is primarily found within the cells of internal organs like the brain or liver and is not released to the bloodstream. Measuring intracellular HNMT directly is challenging. Therefore, diagnosis of HNMT activity is typically done indirectly by testing for known genetic variants.[25]

Genetic variants

There is a genetic variant, registered in The Single Nucleotide Polymorphism Database (dbSNP) as rs11558538, found in 10% of the population worldwide,[44] which means that the T allele presents at position 314 of HNMT instead of a usual C allele (c.314C>T). This variant causes the protein to be synthesized with threonine (Thr) replaced with isoleucine (Ile) at position 105 (p.Thr105Ile, T105I). This variant is described as loss-of-function allele reducing HNMT activity, and is associated with diseases, typical for histamine intolerance, such as asthma, allergic rhinitis, and atopic eczema (atopic dermatitis). For individuals with this variant, the intake of HNMT inhibitors, which hamper enzyme activity, and histamine liberators, which release histamine from the granules of mast cells and basophils, could potentially influence their histamine levels.[45][46] Still, this genetic variant is associated with a reduced risk of Parkinson's disease.[47][48][9]

Experiments involving HNMT knockout mice have shown that a deficiency in HNMT indeed leads to increased brain histamine concentrations, resulting in heightened aggressive behaviors and disrupted sleep-wake cycles in these mice. In humans, genetic variants that affect HNMT activity have been implicated in various brain disorders, such as Parkinson's disease and attention deficit disorder, but it remains unclear whether these alterations in HNMT are a primary cause or secondary effect of these conditions. Additionally, reduced histamine levels in cerebrospinal fluid have been consistently reported in patients with narcolepsy and other conditions characterized by excessive daytime sleepiness. The association between HNMT polymorphisms and gastrointestinal diseases is still uncertain. While mild polymorphisms can lead to diseases such as asthma and inflammatory bowel disease, they may also reduce the risk of brain disorders like Parkinson's disease. On the other hand, severe mutations in HNMT can result in intellectual disability. Despite these findings, the role of HNMT in human health is not fully understood and continues to be an active area of research.[13][21]

Inhibitors

The following substances are known to be HNMT inhibitors: amodiaquine, chloroquine, dimaprit, etoprine, metoprine, quinacrine, SKF-91488, tacrine, and diphenhydramine.[49][50][51] HNMT inhibitors may increase histamine levels in peripheral tissues and cause of aggravate conditions associated with histamine excess, such as allergic rhinitis, urticaria, and peptic ulcer disease.[13] Шаблон:As of the effect of HNMT inhibitors on brain function is not yet fully understood. Research suggests that using new inhibitors of HNMT to increase the levels of histamine in the brain could potentially contribute to improvements in the treatment of brain disorders.[49][50][51]

Methamphetamine overdose

Шаблон:See also HNMT could be a potential target for the treatment of symptoms of methamphetamine overdose.[52] It is a central nervous system stimulant, which can be abused up to the lethal consequences: numerous deaths related to methamphetamine overdoses have been reported.[53][54] The reasoning behind this is that such overdose often leads to behavioral abnormalities, and it has been observed that elevated levels of histamine in the brain can attenuate these methamphetamine-induced behaviors. Therefore, by targeting HNMT, it might be possible to increase the levels of histamine in the brain, which could, in turn, help to mitigate the effects of a methamphetamine overdose. This effect could be achieved by using HNMT inhibitors. Studies predict that one such inhibitor can be metoprine, which crosses the blood-brain barrier and can potentially increase brain histamine levels by inhibiting HNMT, still, Шаблон:As of treatment of methamphetamine overdose by HNMT inhibitors is still an area of research.[52]

Nτ-methylhistamine

Шаблон:Main article Nτ-methylhistamine (NMH), also known as 1-methylhistamine, is a product of Nτ-methylation of histamine in a reaction catalyzed by the HNMT enzyme.[20][8][9]

NMH is considered a biologically inactive metabolite of histamine.[55][56][57] NMH is excreted in the urine and can be measured to estimate the amounts of active histamine in the body.[58] While NMH has some biological activity on its own, it is much weaker than histamine. It can bind to histamine receptors, still, NMH has a lower affinity and efficacy than histamine for these receptors, meaning that it binds less strongly and activates them less effectively. Depending on the receptor subtype and the tissue context, NMH may act as a partial agonist or an antagonist for some histamine receptors. NMH may have some modulatory effects on histamine signaling, but it is unlikely to cause significant allergic or inflammatory reactions by itself. NMH may also serve as a feedback mechanism to regulate histamine levels and prevent excessive histamine release.[59] Still, NMT, being a produce in a reaction catalyzed by HNMT, may inhibit expression of HNMT in a negative feedback loop.[31]

Urinary NMH can be measured in clinical settings when systemic mastocytosis is suspected. Systemic mastocytosis and anaphylaxis are typically associated with at least a two-fold increase in urinary NMH levels, which are also increased in patients taking monoamine oxidase inhibitors and in patients on histamine-rich diets.[58]

References

Шаблон:Reflist Шаблон:NLM content

External links

  • Шаблон:MeshName
  • PDBe-KB provides an overview of all the structure information available in the PDB for human histamine N-methyltransferase

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