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Citric acid (citrate) is a tricarboxylic acid, an organic acid with three carboxylate groups. Citrate is an intermediate in the TCA cycle (also known as the Tricarboxylic Acid cycle, the Citric Acid cycle or Krebs cycle). The TCA cycle is a central metabolic pathway for all animals, plants, and bacteria. As a result, citrate is found in all living organisms, from bacteria to plants to animals. In the TCA cycle, the enzyme citrate synthase catalyzes the condensation of oxaloacetate with acetyl CoA to form citrate. Citrate then acts as the substrate for the enzyme known as aconitase and is then converted into aconitic acid. The TCA cycle ends with regeneration of oxaloacetate. This series of chemical reactions in the TCA cycle is the source of two-thirds of the food-derived energy in higher organisms. Citrate can be transported out of the mitochondria and into the cytoplasm, then broken down into acetyl-CoA for fatty acid synthesis, and into oxaloacetate. Citrate is a positive modulator of this conversion, and allosterically regulates the enzyme acetyl-CoA carboxylase, which is the regulating enzyme in the conversion of acetyl-CoA into malonyl-CoA (the commitment step in fatty acid synthesis). In short, citrate is transported into the cytoplasm, converted into acetyl CoA, which is then converted into malonyl CoA by acetyl CoA carboxylase, which is allosterically modulated by citrate. In mammals and other vertebrates, Citrate is a vital component of bone, helping to regulate the size of apatite crystals (PMID: 21127269). Citric acid is found in citrus fruits, most concentrated in lemons and limes, where it can comprise as much as 8% of the dry weight of the fruit. Citric acid is a natural preservative and is also used to add an acidic (sour) taste to foods and carbonated drinks. Because it is one of the stronger edible acids, the dominant use of citric acid is as a flavoring and preservative in food and beverages, especially soft drinks and candies. Citric acid is an excellent chelating agent, binding metals by making them soluble. It is used to remove and discourage the buildup of limescale from boilers and evaporators. It can be used to treat water, which makes it useful in improving the effectiveness of soaps and laundry detergents. The salts of citric acid (citrates) can be used as anticoagulants due to their calcium chelating ability. Intolerance to citric acid in the diet is known to exist. Little information is available as the condition appears to be rare, but like other types of food intolerance it is often described as a "pseudo-allergic" reaction.

The amino acid betaine derived from glycine." []

Citraconic acid, also known as 2-methylmaleate or methylmaleic acid, belongs to the class of organic compounds known as methyl-branched fatty acids. These are fatty acids with an acyl chain that has a methyl branch. Usually, they are saturated and contain only one or more methyl group. However, branches other than methyl may be present. Citraconic acid is a dicarboxylic acid consisting of maleic acid having a methyl substituent at the 2-position. Citraconic acid exists as a white solid. It is the cis-isomer of mesaconic acid and is one of the pyrocitric acids formed upon the heating of citric acid. Citraconic acid has been detected in the urine of both normal and fasting individuals (PMID: 6778884). Citraconic acid is also elevated in the urine of individuals with methylmalonic acidaemia who have suffered ketotic attacks (PMID: 116077). Altered serum levels of citraconic acid have been detected in patients with primary biliary cholangitis (PMID: 28400566).

Guanosine (G), also known as 2-amino-inosine, belongs to the class of organic compounds known as purine nucleosides. Purine nucleosides are compounds comprising a purine base attached to a ribosyl or deoxyribosyl sugar moiety. Guanosine consists of a guanine base attached to a ribose (ribofuranose) ring via a beta-N9-glycosidic bond. Guanosine is a white, crystalline powder with no odor and mild saline taste. It is very soluble in acetic acid, and slightly soluble in water, but insoluble in ethanol, diethyl ether, benzene, and chloroform. Guanosine exists in all living species, ranging from bacteria to plants to humans. High levels of guanosine can be found in clovers, coffee plants, and the pollen of pines. It has been detected, but not quantified in, several different foods, such as leeks, garlic, chicory roots, green bell peppers, and black-eyed peas. Guanosine plays an important role in various biochemical processes including the synthesis of nucleic acids such as RNA and intracellular signal transduction (cGMP). The antiviral drug acyclovir, often used in herpes treatment, and the anti-HIV drug abacavir, are both structurally similar to guanosine. Guanosine can be phosphorylated to become guanosine monophosphate (GMP), cyclic guanosine monophosphate (cGMP), guanosine diphosphate (GDP), and guanosine triphosphate (GTP). In humans, guanosine is involved in intracellular signalling through the adenosine receptors A1R and A2AR (PMID: 31847113). Evidence from rodent and cell models has shown a number of important neurotrophic and neuroprotective effects of guanosine. In particular, it is effective in preventing deleterious consequences of seizures, spinal cord injury, pain, mood disorders and aging-related diseases, such as ischemia, Parkinson’s and Alzheimer’s diseases (PMID: 27699087). Studies with rodent models of Parkinson’s disease have shown that guanosine decreases neuronal apoptotic cell death and increases dopaminergic neurons at substantia nigra pars compacta, accompanied by an improvement of motor symptoms in Parkinson’s disease (i.e. a reduction of bradykinesia). Guanosine promotes neurite arborization, outgrowth, proliferation and differentiation. Systemic administration of guanosine for eight weeks (8 mg/kg) has been shown to stimulate neuroprogenitors proliferation in the subventricular zone (SVZ) in a mouse model of Parkinsonism (PMID: 27699087). The effect of guanosine treatment is accompanied by an increased number of fibroblast growth factor (FGF-2)-positive cells which is an important regulator of neuroprogenitor/stem cell proliferation, survival and differentiation (PMID: 27699087). Guanosine prevents reactive oxygen species (ROS) generation and cell death in hippocampal slices subjected to the oxygen/glucose deprivation (PMID: 31847113).

Itaconic acid is a dicarboxylic acid that is methacrylic acid in which one of the methyl hydrogens is substituted by a carboxylic acid group. It has a role as a fungal metabolite and a human metabolite. It is a dicarboxylic acid and an olefinic compound. It derives from a succinic acid. It is a conjugate acid of an itaconate(2-). This dicarboxylic acid is a white solid that is soluble in water, ethanol, and acetone. Historically, itaconic acid was obtained by the distillation of citric acid, but currently it is produced by fermentation. The name itaconic acid was devised as an anagram of aconitic acid, another derivative of citric acid. Itaconic acid, also known as itaconate, belongs to the class of organic compounds known as branched fatty acids. These are fatty acids containing a branched chain. Itaconic acid is a very hydrophobic molecule, practically insoluble in water, and relatively neutral. Since the 1960s, it is produced industrially by the fermentation of carbohydrates such as glucose or molasses using fungi such as Aspergillus itaconicus or Aspergillus terreus. For A. terreus the itaconate pathway is mostly elucidated. The generally accepted route for itaconate is via glycolysis, tricarboxylic acid cycle, and a decarboxylation of cis-aconitate to itaconate via cis-aconitate-decarboxylase. The smut fungus Ustilago maydis uses an alternative route. Cis-aconitate is converted to the thermodynamically favoured trans-aconitate via aconitate-Δ-isomerase (Adi1). trans-Aconitate is further decarboxylated to itaconate by trans-aconitate-decarboxylase (Tad1). Itaconic acid is also produced in cells of macrophage lineage. It was shown that itaconate is a covalent inhibitor of the enzyme isocitrate lyase in vitro. As such, itaconate may possess antibacterial activities against bacteria expressing isocitrate lyase (such as Salmonella enterica and Mycobacterium tuberculosis). It is also sythesized in the laboratory, where dry distillation of citric acid affords itaconic anhydride, which undergoes hydrolysis to itaconic acid.

Allopurinol is only found in individuals that have used or taken this drug. It is a xanthine oxidase inhibitor that decreases uric acid production. It also acts as an antimetabolite on some simpler organisms. [PubChem]Allopurinol and its active metabolite, oxypurinol, inhibits the enzyme xanthine oxidase, blocking the conversion of the oxypurines hypoxanthine and xanthine to uric acid. Elevated concentrations of oxypurine and oxypurine inhibition of xanthine oxidase through negative feedback results in a decrease in the concentrations of uric acid in the serum and urine. Allopurinol also facilitates the incorporation of hypoxanthine and xanthine into DNA and RNA, leading to a feedback inhibition of de novo purin synthesis and a decrease in serum uric acid concentrations as a result of an increase in nucleotide concentration.

Mesaconic acid, also known as 2-methylfumarate or citronic acid, belongs to the class of organic compounds known as methyl-branched fatty acids. These are fatty acids with an acyl chain that has a methyl branch. Usually, they are saturated and contain only one or more methyl group. However, branches other than methyl may be present. Mesaconic acid is a dicarboxylic butenoic acid, with a methyl group in position 2 and the double bound between carbons 2 and 3. Mesaconic acid was first studied for its physical properties in 1874 by Jacobus van ‘t Hoff (https://web.archive.org/web/20051117102410/http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Van%27t-Hoff-1874.html). It is now known to be involved in the biosynthesis of vitamin B12 and it is also a competitor inhibitor of the reduction of fumarate.

Aconitic acid is a tricarboxylic acid that is prop-1-ene substituted by carboxy groups at positions 1, 2 and 3. It is a conjugate acid of an aconitate(3-). Aconitic acid is a natural product found in Glycine max, Zea mays, and other organisms with data available. A tricarboxylic acid with the formula (COOH)-CH2-C(COOH)=CH-COOH.

Pyroglutamic acid (5-oxoproline) is a cyclized derivative of L-glutamic acid. It is an uncommon amino acid derivative in which the free amino group of glutamic acid cyclizes to form a lactam. It is formed nonenzymatically from glutamate, glutamine, and gamma-glutamylated peptides, but it can also be produced by the action of gamma-glutamylcyclotransferase on an L-amino acid. Elevated blood levels may be associated with problems of glutamine or glutathione metabolism. This compound is found in substantial amounts in brain tissue and other tissues in bound form, especially skin. It is also present in plant tissues. It is sold, over the counter, as a "smart drug" for improving blood circulation in the brain. Pyroglutamate in the urine is a biomarker for the consumption of cheese. When present in sufficiently high levels, pyroglutamic acid can act as an acidogen and a metabotoxin. An acidogen is an acidic compound that induces acidosis, which has multiple adverse effects on many organ systems. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. Chronically high levels of pyroglutamic acid are associated with at least five inborn errors of metabolism including 5-oxoprolinuria, 5-oxoprolinase deficiency, glutathione synthetase deficiency, hawkinsinuria, and propionic acidemia. Pyroglutamic acid is an organic acid. Abnormally high levels of organic acids in the blood (organic acidemia), urine (organic aciduria), the brain, and other tissues lead to general metabolic acidosis. Acidosis typically occurs when arterial pH falls below 7.35. In infants with acidosis, the initial symptoms include poor feeding, vomiting, loss of appetite, weak muscle tone (hypotonia), and lack of energy (lethargy). These can progress to heart, liver, and kidney abnormalities, seizures, coma, and possibly death. These are also the characteristic symptoms of the untreated IEMs mentioned above. Many affected children with organic acidemias experience intellectual disability or delayed development. In adults, acidosis or acidemia is characterized by headaches, confusion, feeling tired, tremors, sleepiness, and seizures. It has been shown that pyroglutamic acid releases GABA from the cerebral cortex and displays anti-anxiety effects in a simple approach-avoidance conflict situation in the rat. In clinical pharmacology experiments, pyroglutamic acid significantly shortens the plasma half-life of ethanol during acute intoxication.

2-Hydroxyglutarate exists in 2 isomers: L-2-hydroxyglutarate acid and D-2-hydroxyglutarate. Both the D and the L stereoisomers of hydroxyglutaric acid (EC 1.1.99.2) are found in body fluids. In humans it is part of butanoate metabolic pathway and can be produced by phosphoglycerate dehydrogenase (PHGDH). More specifically, the enzyme PHGDH catalyzes the NADH-dependent reduction of ?-ketoglutarate (AKG) to D-2-hydroxyglutarate (D-2HG). 2-hydroxyglutarate is also the product of gain-of-function mutations in the cytosolic and mitochondrial isoforms of isocitrate dehydrogenase (IDH). Additionally, 2-hydroxyglutarate can be converted to ?-ketoglutaric acid through the action of 2-hydroxyglutarate dehydrogenase (HGDH). Humans have to variants of this enzyme: D-2-hydroxyglutarate dehydrogenase (D2HGDH) and L-2-hydroxyglutarate dehydrogenase (L2HGDH). A deficiency in either of these two enzymes can lead to a disease known as 2-hydroxyglutaric aciduria. L-2-hydroxyglutaric aciduria (caused by loss of L2HGDH) is chronic, with early symptoms such as hypotonia, tremors, and epilepsy declining into spongiform leukoencephalopathy, muscular choreodystonia, mental retardation, and psychomotor regression. D-2-hydroxyglutaric aciduria (caused by loss of D2HGDH or gain of function of IDH) is rare, with symptoms including cancer, macrocephaly, cardiomyopathy, mental retardation, hypotonia, and cortical blindness. 2-hydroxyglutarate was the first oncometabolite (or cancer-causing metabolite) to be formally named or identified. In cancer it is either produced by overexpression of phosphoglycerate dehydrogenase (PHGDH) or is produced in excess by gain-of-function mutations in the cytosolic and mitochondrial isoforms of isocitrate dehydrogenase (IDH). IDH is part of TCA cycle and is generated in high abundance when IDH is mutated. 2-hydroxyglutarate is sufficiently similar in structure to 2-oxogluratate (2OG) that it is able to inhibit a range of 2OG-dependent dioxygenases, including histone lysine demethylases (KDMs) and members of the ten-eleven translocation (TET) family of 5-methylcytosine (5mC) hydroxylases. This inhibitory effect leads to alterations in the hypoxia induced factor (HIF)-mediated hypoxic response and alterations in gene expression through global epigenetic remodeling. The net effect is that 2-hydroxyglutarate causes a cascading effect that leads genetic perturbations and malignant transformation. Furthermore, 2-hydroxyglutarate is found to be associated with glutaric aciduria II, which is also an inborn error of metabolism. 2-Hydroxyglutarate has also been found to be a metabolite in Aspergillus (PMID: 6057807).

trans-Aconitic acid, also known as trans-aconitate or (e)-aconitic acid, belongs to the class of organic compounds known as tricarboxylic acids and derivatives. These are carboxylic acids containing exactly three carboxyl groups. trans-Aconitic acid exists in all living species, ranging from bacteria to humans. trans-Aconitic acid is a dry, musty, and nut tasting compound. Outside of the human body, trans-aconitic acid has been detected, but not quantified in several different foods, such as garden tomato fruits, root vegetables, soy beans, and rices. trans-Aconitic acid is normally present in human urine, and it has been suggested that is present in larger amounts with Reye's syndrome and organic aciduria. trans-Aconitic acid in the urine is a biomarker for the consumption of soy products. trans-Aconitic acid is a substrate of enzyme trans-Aconitic acid 2-methyltransferase (EC2.1.1.144).

Glutaconic acid is related to the fully saturated glutaric acid and belongs to the class of compounds known as dicarboxylic acids and derivatives. These are organic compounds containing exactly two carboxylic acid groups. Glutaconic acid is soluble (in water) and a weakly acidic compound (based on its pKa). Glutaconic acid has been detected in the urine of individuals with inborn errors of metabolism. When present in sufficiently high levels, glutaconic acid can act as an acidogen, a neurotoxin, and a metabotoxin. An acidogen is an acidic compound that induces acidosis, which has multiple adverse effects on many organ systems. A neurotoxin is a compound that is toxic to neural tissues and cells. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. Chronically high levels of glutaconic acid are associated with glutaric aciduria type I (glutaric acidemia type I, glutaryl-CoA dehydrogenase deficiency, GA1, or GAT1). GA1 is an inherited disorder in which the body is unable to completely break down the amino acids lysine, hydroxylysine, and tryptophan due to a deficiency of mitochondrial glutaryl-CoA dehydrogenase (EC 1.3.99.7, GCDH). Excessive levels of their intermediate breakdown products (e.g. glutaric acid, glutaryl-CoA, 3-hydroxyglutaric acid, glutaconic acid) can accumulate and cause damage to the brain (and also other organs), but particularly the basal ganglia. GA1 is associated with a risk for intracranial and retinal hemorrhage, and non-specific white matter changes. Babies with glutaric acidemia type I are often born with unusually large heads (macrocephaly). Other symptoms include spasticity (increased muscle tone/stiffness) and dystonia (involuntary muscle contractions resulting in abnormal movement or posture), but many affected individuals are asymptomatic. Seizures and coma (encephalopathy) are rare. GA1 also causes secondary carnitine deficiency because 3-hydroxyglutaric acid, like other organic acids, is detoxified by carnitine. Glutaconic acid's neurotoxicity is thought to be partially caused by an excitotoxic mechanism in which glutaconic acid overactivates N-methyl-D-aspartate (NMDA) receptors. Accumulating trans-glutaconic (TG) acids have been proposed to be involved in the development of the striatal degeneration seen in children with glutaric acidemia type I via an excitotoxic mechanism.

cis-Aconitic acid is an intermediate in the tricarboxylic acid cycle produced by the dehydration of citric acid. The enzyme aconitase (aconitate hydratase; EC 4.2.1.3) catalyses the stereo-specific isomerization of citrate to isocitrate via cis-aconitate in the tricarboxylic acid cycle.

Uracil, also known as U, belongs to the class of organic compounds known as pyrimidones. Pyrimidones are compounds that contain a pyrimidine ring, which bears a ketone. Pyrimidine is a 6-membered ring consisting of four carbon atoms and two nitrogen centers at the 1- and 3- ring positions. Uracil is a common naturally occurring pyrimidine found in RNA. It base pairs with adenine and is replaced by thymine in DNA. Uracil is one of the four nucleobases in RNA that are represented by the letters A, G, C and U. Methylation of uracil produces thymine. The name "uracil" was coined in 1885 by the German chemist Robert Behrend, who was attempting to synthesize derivatives of uric acid. Originally discovered in 1900, uracil was isolated by hydrolysis of yeast nuclein that was found in bovine thymus and spleen, herring sperm, and wheat germ. Uracil exists in all living species, ranging from bacteria to plants to humans. Uracil's use in the body is to help carry out the synthesis of many enzymes necessary for cell function through bonding with riboses and phosphates. Uracil serves as an allosteric regulator and a coenzyme for many important biochemical reactions. Uracil (via the nucleoside uridine) can be phosphorylated by various kinases to produce UMP, UDP and UTP. UDP and UTP regulate carbamoyl phosphate synthetase II (CPSase II) activity in animals. Uracil is also involved in the biosynthesis of polysaccharides and in the transport of sugars containing aldehydes. Within humans, uracil participates in a number of enzymatic reactions. In particular, uracil and ribose 1-phosphate can be biosynthesized from uridine; which is mediated by the enzyme uridine phosphorylase 2. In addition, uracil can be converted into dihydrouracil through the action of the enzyme dihydropyrimidine dehydrogenase [NADP(+)]. Uracil is rarely found in DNA, and this may have been an evolutionary change to increase genetic stability. This is because cytosine can deaminate spontaneously to produce uracil through hydrolytic deamination. Therefore, if there were an organism that used uracil in its DNA, the deamination of cytosine (which undergoes base pairing with guanine) would lead to formation of uracil (which would base pair with adenine) during DNA synthesis. Uracil can be used for drug delivery and as a pharmaceutical. When elemental fluorine reacts with uracil, it produces 5-fluorouracil. 5-Fluorouracil is an anticancer drug (antimetabolite) that mimics uracil during the nucleic acid (i.e. RNA) synthesis and transcription process. Because 5-fluorouracil is similar in shape to, but does not undergo the same chemistry as, uracil, the drug inhibits RNA replication enzymes, thereby blocking RNA synthesis and stopping the growth of cancerous cells.

L-2-Hydroxyglutaric acid is a metabolite that accumulates in L-2-hydroxyglutaric aciduria, which is a neurometabolic disorder (OMIM: 236792), and has been reported in multiple patients who have a clinical phenotype of progressive neurodegeneration with extrapyramidal and cerebellar signs, seizures, and spongiform changes in the white matter (OMIM: 600721). In humans, 2-hydroxyglutarate is formed by a hydroxyacid-oxoacid transhydrogenase whereas in bacteria it is formed by a 2-hydroxyglutarate synthase. L-2-Hydroxyglutaric acid can be converted to alpha-ketoglutaric acid through the action of 2-hydroxyglutarate dehydrogenase (EC 1.1.99.2). In humans, there are two such enzymes (D2HGDH and L2HGDH). Both the D and L stereoisomers of hydroxyglutaric acid are found in body fluids. L-2-Hydroxyglutaric acid can also be produced via gain-of-function mutations in the cytosolic and mitochondrial isoforms of isocitrate dehydrogenase (IDH). IDH is part of the TCA cycle and this compound is generated in high abundance when IDH is mutated. Since L-2-hydroxyglutaric acid is sufficiently similar in structure to 2-oxoglutarate (2OG), it is able to inhibit a range of 2OG-dependent dioxygenases, including histone lysine demethylases (KDMs) and members of the ten-eleven translocation (TET) family of 5-methylcytosine (5mC) hydroxylases. This inhibitory effect leads to alterations in the hypoxia-inducible factor (HIF)-mediated hypoxic response and alterations in gene expression through global epigenetic remodeling. The net effect is that L-2-hydroxyglutaric acid causes a cascading effect that leads genetic perturbations and malignant transformation. Depending on the circumstances, L-2-hydroxyglutaric acid can function as an oncometabolite, a neurotoxin, an acidogen, and a metabotoxin. An oncometabolite is a compound that promotes tumour growth and survival. A neurotoxin is compound that is toxic to neurons or neural tissue. An acidogen is an acidic compound that induces acidosis, which has multiple adverse effects on many organ systems. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. As an oncometabolite, L-2-hydroxyglutaric acid is a competitive inhibitor of multiple alpha-ketoglutarate-dependent dioxygenases, including histone demethylases and the TET family of 5mC hydroxylases. As a result, high levels of 2-hydroxyglutarate lead to genome-wide histone and DNA methylation alterations, which in turn lead to mutations that ultimately cause cancer (PMID: 29038145). As a neurotoxin, L-2-hydroxyglutaric acid mediates its neurotoxicity through activation of N-methyl-D-aspartate receptors. L-2-Hydroxyglutaric acid is structurally similar to the excitatory amino acid glutamate and stimulates neurodegeneration by mechanisms similar to glutamate, NMDA, or mitochondrial toxins (PMID: 12153528). As an acidogen, L-2-hydroxyglutaric acid is classified as an alpha hydroxy acid belonging to the general class of compounds known as organic acids. Chronically high levels of L-2-hydroxyglutaric acid are characteristic of the inborn error of metabolism called L-2-hydroxyglutaric aciduria. Abnormally high levels of organic acids in the blood (organic acidemia), urine (organic aciduria), the brain, and other tissues lead to general metabolic acidosis. Acidosis typically occurs when arterial pH falls below 7.35. In infants with acidosis, the initial symptoms include poor feeding, vomiting, loss of appetite, weak muscle tone (hypotonia), and lack of energy (lethargy). These can progress to heart abnormalities, kidney abnormalities, liver damage, seizures, coma, and possibly death. These are the symptoms typical of untreated L-2-hydroxyglutaric aciduria. Many affected children with organic acidemias experience intellectual disability or delayed development. In adults, acidosis or acidemia is characterized by headaches, confusion, feeling tired, tremors, sleepiness, and seizures.

5-Aminopentanoic acid (or 5-aminovalerate) is a lysine degradation product. It can be produced both endogenously or through bacterial catabolism of lysine. 5-aminovalerate is formed via the following multi-step reaction: L-lysine leads to cadverine leads to L-piperideine leads 5-aminovalerate (PMID:405455). In other words it is a metabolite of cadaverine which is formed via the intermediate, 1-piperideine (PMID:6436440). Cadaverine is a foul-smelling diamine compound produced by protein hydrolysis during putrefaction of animal tissue. High levels of 5-aminovalerate in biofluids may indicate bacterial overgrowth or endogenous tissue necrosis. In most cases endogenous 5-aminovalerate is thought to be primarily a microbial metabolite produced by the gut or oral microflora, although it can be produced endogenously. 5-aminovalerate is a normal metabolite present in human saliva, with a tendency to elevated concentration in patients with chronic periodontitis. Bacterial contamination and decomposition of salivary proteins is primarily responsible for elevated salivary levels (PMID 3481959). Beyond being a general waste product, 5-aminovalerate is also believed to act as a methylene homologue of gamma-aminobutyric acid (GABA) and functions as a weak GABA agonist (PMID:4031870). It is also known as an antifibrinolytic amino acid analog and so it functions as a weak inhibitor of the blood clotting pathway (PMID:6703712). 5- aminovalerate is an in vivo substrate of 4-aminobutyrate:2-oxoglutarate aminotransferase (PMID:4031870). It can be found in Corynebacterium (PMID:27717386).

3-Hydroxyglutaric acid is a member of the class of compounds known as dicarboxylic acids and derivatives. These are organic compounds containing exactly two carboxylic acid groups. 3-Hydroxyglutaric acid is soluble (in water) and a weakly acidic compound (based on its pKa). When present in sufficiently high levels, 3-hydroxyglutaric acid can act as an acidogen and a metabotoxin. An acidogen is an acidic compound that induces acidosis, which has multiple adverse effects on many organ systems. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. Chronically high levels of 3-hydroxyglutaric acid are associated with glutaric aciduria type I (glutaric acidemia type I, glutaryl-CoA dehydrogenase deficiency, GA1, or GAT1). GA1 is an inherited disorder in which the body is unable to completely break down the amino acids lysine, hydroxylysine, and tryptophan due to a deficiency of mitochondrial glutaryl-CoA dehydrogenase (EC 1.3.99.7, GCDH). Excessive levels of their intermediate breakdown products (e.g. glutaric acid, glutaryl-CoA, 3-hydroxyglutaric acid, glutaconic acid) can accumulate and cause damage to the brain (and also other organs), but particularly the basal ganglia. GA1 is associated with a risk for intracranial and retinal hemorrhage, and non-specific white matter changes. Babies with glutaric acidemia type I are often born with unusually large heads (macrocephaly). Other symptoms include spasticity (increased muscle tone/stiffness) and dystonia (involuntary muscle contractions resulting in abnormal movement or posture), but many affected individuals are asymptomatic. Seizures and coma (encephalopathy) are rare. GA1 also causes secondary carnitine deficiency because 3-hydroxyglutaric acid, like other organic acids, is detoxified by carnitine.

Citramalic acid, also known as 2-Methylmalic acid, is an analog of malic acid. The structure of citramalic acid is similar to the structure of malic acid except it has an extra CH3 group on position 2. It is also classified as a 2-hydroxydicarboxylic acid. Citramalic acid exists in two isomers, L-citramalic acid and D-citramalic acid. The L-isomer is more biologically relevant isomer. Citramalic acid is found in almost all living organisms from microbes to plants to humans although citramalate is primarily produced from bacteria. L-citramalic acid was first isolated from the peel of apples in 1954 (PMID: 13160011). It has also been isolated in wine and other ripening fruit (PMID: 13807713). Citramalic acid can inhibit the production of malic acid. Citramalic acid is also an important microbial metabolite and has been found to be a byproduct of Saccharomyces yeast species, as well as Propionibacterium acnes and Aspergillus niger (PMID: 31827810) (http://drweyrich.weyrich.com/labs/oat.html) (PMID: 7628083). Citramalic acid is a component of the C5-branched dibasic acid metabolism pathway. It can be broken down by the enzyme citramalate lyase, which converts citramalate to acetate and pyruvate. Citramalate synthase is an enzyme found in bacteria that synthesizes citramalic acid from acetyl-CoA, pyruvate and water. Citramalic acid may have a useful role in medical diagnoses. It has been found in the urine of two brothers with autistic features (PMID: 7628083). Citramalic acid can also be used as a urinary marker for gut dysbiosis (PMID: 31669633). Dysbiosis is a disorder of the bacterial flora of the human digestive tract. It is usually diagnosed clinically by direct detection of an abnormal pattern of the intestinal microbiota.

But-2-enoic acid, also known as (2E)-2-butenoate or alpha-crotonic acid, belongs to the class of organic compounds known as straight chain organic acids. These are organic acids with a straight aliphatic chain. But-2-enoic acid is a very hydrophobic molecule, practically insoluble in water, and relatively neutral.

Ribonolactone, also known as D-ribono-1,4-lactone is a five-membered form of ribonolactone having D-configuration. It has a role as a metabolite. It is a ribonolactone and a butan-4-olide. It derives from a D-ribonic acid. Ribonolactone belongs to the class of organic compounds known as pentoses. These are monosaccharides in which the carbohydrate moiety contains five carbon atoms. Ribonolactone is a metabolite normally not detectable in human biofluids; however, it has been found in the urine of patients with neuroblastoma.

9-beta-D-arabinofuranosylguanine is a purine nucleoside in which guanine is attached to arabinofuranose via a beta-N(9)-glycosidic bond. It inhibits DNA synthesis and causes cell death. It has a role as an antineoplastic agent and a DNA synthesis inhibitor. It is a beta-D-arabinoside and a purine nucleoside.

Crotonoside is a purine nucleoside. Crotonoside is a natural product found in Croton tiglium with data available.

Uridine, also known as beta-uridine or 1-beta-D-ribofuranosylpyrimidine-2,4(1H,3H)-dione, is a member of the class of compounds known as pyrimidine nucleosides. Pyrimidine nucleosides are compounds comprising a pyrimidine base attached to a ribosyl or deoxyribosyl moiety. More specifically, uridine is a nucleoside consisting of uracil and D-ribose and a component of RNA. Uridine is soluble (in water) and a very weakly acidic compound (based on its pKa). Uridine can be synthesized from uracil. It is one of the five standard nucleosides which make up nucleic acids, the others being adenosine, thymidine, cytidine and guanosine. The five nucleosides are commonly abbreviated to their one-letter codes U, A, T, C and G respectively. Uridine is also a parent compound for other transformation products, including but not limited to, nikkomycin Z, 3'-(enolpyruvyl)uridine 5'-monophosphate, and 5-aminomethyl-2-thiouridine. Uridine can be found in most biofluids, including urine, breast milk, cerebrospinal fluid (CSF), and blood. Within the cell, uridine is primarily located in the mitochondria, in the nucleus and the lysosome. It can also be found in the extracellular space. As an essential nucleoside, uridine exists in all living species, ranging from bacteria to humans. In humans, uridine is involved in several metabolic disorders, some of which include dhydropyrimidinase deficiency, MNGIE (mitochondrial neurogastrointestinal encephalopathy), and beta-ureidopropionase deficiency. Moreover, uridine is found to be associated with Lesch-Nyhan syndrome, which is an inborn error of metabolism. Uridine is a nucleoside consisting of uracil and D-ribose and a component of RNA. Uridine plays a role in the glycolysis pathway of galactose. In humans there is no catabolic process to metabolize galactose. Therefore, galactose is converted to glucose and metabolized via the normal glucose metabolism pathways. More specifically, consumed galactose is converted into galactose 1-phosphate (Gal-1-P). This molecule is a substrate for the enzyme galactose-1-phosphate uridyl transferase which transfers a UDP molecule to the galactose molecule. The end result is UDP-galactose and glucose-1-phosphate. This process is continued to allow the proper glycolysis of galactose. Uridine is found in many foods (anything containing RNA) but is destroyed in the liver and gastrointestinal tract, and so no food, when consumed, has ever been reliably shown to elevate blood uridine levels. On the other hand, consumption of RNA-rich foods may lead to high levels of purines (adenine and guanosine) in blood. High levels of purines are known to increase uric acid production and may aggravate or lead to conditions such as gout.

4-amino-pentanoic acid

A homodetic cyclic peptide resulting from the formal condensation of both the amino and acid groups of L-histidine with the acid and amino groups of L-proline. It is a metabolite of thyrotropin-releasing hormone (TRH)." []

Cyclo(L-His-L-Pro) is a homodetic cyclic peptide resulting from the formal condensation of both the amino and acid groups of L-histidine with the acid and amino groups of L-proline. It is a metabolite of thyrotropin-releasing hormone (TRH). It has a role as a human blood serum metabolite, a dopamine uptake inhibitor and an anti-inflammatory agent. It is a homodetic cyclic peptide, a dipeptide, a pyrrolopyrazine and a member of imidazoles. It is functionally related to a L-histidine and a L-proline.

Maleic hydrazide is an odorless white solid. Sinks in water. (USCG, 1999) Maleic hydrazide is a pyridazinone. Maleic hydrazide (MH) was introduced into agriculture in the 1950s as a major commercial herbicide and a depressant of plant growth. It is a plant growth regulator (sprout inhibitor) and herbicide, that acts by inhibiting cell division in plants. It is used to control sprouting of potatoes and onions, suckers in tobacco, and growth of weeds, grasses and trees in/along lawns, turf, ornamental plants, non-bearing citrus, utility and highway rights-of-way, airports and industrial land. Most of the maleic hydrazide used in the U.S. is applied to tobacco (86-88%), followed by potatoes (10%), It is used to control sucker growth on tobacco plants, retardation of flowering and prolongation of dormancy period. 1,2-Dihydro-3,6-pyridazinedione. A herbicide and plant growth regulator; also used to control suckering of tobacco. Its residue in food and tobacco is highly toxic, causing CNS disturbances and liver damage.

In humans, D-2-hydroxyglutaric acid is formed by a hydroxyacid-oxoacid transhydrogenase whereas in bacteria it is formed by a 2-hydroxyglutarate synthase. D-2-Hydroxyglutaric acid is also formed via the normal activity of hydroxyacid-oxoacid transhydrogenase during conversion of 4-hydroxybutyrate to succinate semialdehyde. The compound can be converted to alpha-ketoglutaric acid through the action of a 2-hydroxyglutarate dehydrogenase (EC 1.1.99.2). In humans, there are two such enzymes (D2HGDH and L2HGDH). Both the D and the L stereoisomers of hydroxyglutaric acid are found in body fluids. D-2-Hydroxyglutaric acid is a biochemical hallmark of the inherited neurometabolic disorder D-2-hydroxyglutaric aciduria (OMIM: 600721) and the genetic disorder glutaric aciduria II. D-2-Hydroxyglutaric aciduria (caused by loss of D2HGDH or gain of function of IDH) is rare, with symptoms including cancer, macrocephaly, cardiomyopathy, mental retardation, hypotonia, and cortical blindness. An elevated urine level of D-2-hydroxyglutaric acid has been reported in patients with spondyloenchondrodysplasia (OMIM: 271550). D-2-Hydroxyglutaric acid can be converted to alpha-ketoglutaric acid through the action of 2-hydroxyglutarate dehydrogenase (D2HGDH). Additionally, the enzyme D-3-phosphoglycerate dehydrogenase (PHGDH) can catalyze the NADH-dependent reduction of alpha-ketoglutarate (AKG) to D-2-hydroxyglutarate (D-2HG). Nyhan et al. (1995) described 3 female patients, 2 of them sibs, who were found to have excess accumulation of D-2-hydroxyglutaric acid in the urine. The phenotype was quite variable, even among the sibs, but included mental retardation, macrocephaly with cerebral atrophy, hypotonia, seizures, and involuntary movements. One of the patients developed severe intermittent vomiting and was given a pyloromyotomy. The electroencephalogram demonstrated hypsarrhythmia. There was an increased concentration of protein in cerebrospinal fluid, an unusual finding in inborn errors of metabolism. D-2-Hydroxyglutaric acid can also be produced via gain-of-function mutations in the cytosolic and mitochondrial isoforms of isocitrate dehydrogenase (IDH). IDH is part of the TCA cycle and this compound is generated in high abundance when IDH is mutated. Since D-2-hydroxyglutaric acid is sufficiently similar in structure to 2-oxoglutarate (2OG), it is able to inhibit a range of 2OG-dependent dioxygenases, including histone lysine demethylases (KDMs) and members of the ten-eleven translocation (TET) family of 5-methylcytosine (5mC) hydroxylases. This inhibitory effect leads to alterations in the hypoxia-inducible factor (HIF)-mediated hypoxic response and alterations in gene expression through global epigenetic remodeling. The net effect is that D-2-hydroxyglutaric acid causes a cascading effect that leads genetic perturbations and malignant transformation. Depending on the circumstances, D-2-hydroxyglutaric acid can act as an oncometabolite, a neurotoxin, an acidogen, and a metabotoxin. An oncometabolite is a compound that promotes tumour growth and survival. A neurotoxin is compound that is toxic to neurons or nerual tissue. An acidogen is an acidic compound that induces acidosis, which has multiple adverse effects on many organ systems. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. As an oncometabolite, D-2-hydroxyglutaric acid is a competitive inhibitor of multiple alpha-ketoglutarate-dependent dioxygenases, including histone demethylases and the TET family of 5mC hydroxylases. As a result, high levels of 2-hydroxyglutarate lead to genome-wide histone and DNA methylation alterations, which in turn lead to mutations that ultimately cause cancer (PMID: 29038145). As a neurotoxin, D-2-hydroxyglutaric acid mediates its neurotoxicity through activation of N-methyl-D-aspartate receptors. D-2-Hydroxyglutaric acid is structurally similar to the excitatory amino acid glutamate and stimulates neurodegeneration by mechanisms similar to glutamate, NMDA, or mitochondrial toxins (PMID: 12153528). As an acidogen, D-2-hydroxyglutaric acid is classified as an alpha hydroxy acid belonging to the general class of compounds known as organic acids. Chronically high levels of D-2-hydroxyglutaric acid are a feature of the inborn error of metabolism called D-2-hydroxyglutaric aciduria. Abnormally high levels of organic acids in the blood (organic acidemia), urine (organic aciduria), the brain, and other tissues lead to general metabolic acidosis. Acidosis typically occurs when arterial pH falls below 7.35. In infants with acidosis, the initial symptoms include poor feeding, vomiting, loss of appetite, weak muscle tone (hypotonia), and lack of energy (lethargy). These can progress to heart abnormalities, kidney abnormalities, liver damage, seizures, coma, and possibly death. These are also the characteristic symptoms of untreated D-2-hydroxyglutaric aciduria. Many affected children with organic acidemias experience intellectual disability or delayed development. In adults, acidosis or acidemia is characterized by headaches, confusion, feeling tired, tremors, sleepiness, and seizures.