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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.

Isocitric acid, also known as isocitrate belongs to the class of organic compounds known as tricarboxylic acids and derivatives. These are carboxylic acids containing exactly three carboxyl groups. Isocitric acid is a TCA (tricarboxylic acid) cycle intermediate. It is a structural isomer of citric acid and is formed from citrate with the help of the enzyme aconitase. More specifically, Isocitric acid is synthesized from citric acid via the intermediate cis-aconitic acid by the enzyme aconitase (aconitate hydratase). Isocitrate is acted upon by isocitrate dehydrogenase (IDH) to form alpha-ketoglutarate. This is a two-step process, which involves oxidation of isocitrate to oxalosuccinate (a ketone), followed by the decarboxylation of the carboxyl group beta to the ketone, forming alpha-ketoglutarate. In humans, IDH exists in three isoforms: IDH3 catalyzes the third step of the citric acid cycle while converting NAD+ to NADH in the mitochondria. The isoforms IDH1 and IDH2 catalyze the same reaction outside the context of the citric acid cycle and use NADP+ as a cofactor instead of NAD+. They localize to the cytosol as well as the mitochondrion and peroxisome. Isocitric acid exists in all living species, ranging from bacteria to plants to humans. Isocitric acid is a minor organic acid found in most fruit juices, especially in blackberries, youngberries, and boyberries, and in vegetables, especially in carrots. The determination of D-isocitric acid has become of importance in the analysis of fruit juices for the detection of illegal additives (adulteration). Since the quantities of citric and isocitric acids are correlated in fruit juices, a high ratio of citric to isocitric acid can indicate the addition of citric acid as an alduterant. In authentic orange juice, for example, the ratio of citric acid to D-isocitric acid is usually less than 130. Isocitric acid is mostly used in the food industry (food additive) as a food acidulant.

Hydroquinone, also known as benzene-1,4-diol, is an aromatic organic compound which is a type of phenol, having the chemical formula C6H4(OH)2. Its chemical structure has two hydroxyl groups bonded to a benzene ring in a para position. Hydroquinone is commonly used as a biomarker for benzene exposure. The presence of hydroquinone in normal individuals stems mainly from direct dietary ingestion, catabolism of tyrosine and other substrates by gut bacteria, ingestion of arbutin-containing foods, cigarette smoking, and the use of some over-the-counter medicines. Hydroquinone is a white granular solid at room temperature and pressure. The hydroxyl groups of hydroquinone are quite weakly acidic. Hydroquinone can lose an H+ from one of the hydroxyls to form a monophenolate ion or lose an H+ from both to form a diphenolate ion. Hydroquinone has a variety of uses principally associated with its action as a reducing agent which is soluble in water. It is a major component of most photographic developers where, with the compound Metol, it reduces silver halides to elemental silver.

D-glucaro-1,4-lactone is a delta-lactone that is D-glucono-1,4-lactone in which the hydroxy group at position 6 has been oxidised to the corresponding carboxylic acid. It is a delta-lactone and an aldarolactone. It is functionally related to a D-glucono-1,4-lactone. It is a conjugate acid of a D-glucaro-1,4-lactone(1-).

Dimethyl maleate belongs to the class of organic compounds known as fatty acid esters. These are carboxylic ester derivatives of a fatty acid. Dimethyl maleate is the methyl ester of maleic acid and the cis-isomer of dimethyl fumarate. Dimethyl maleate is a commonly used thiol-alkylating agent and enjoys widespread use in many organic synthesis (e.g. as a dienophile for diene syntheses). In the production of plastics, pigments, pharmaceuticals, and agricultural products, dimethyl maleate is used as an additive and an intermediate. In the production of paints, adhesives, and copolymers, dimethyl maleate is used as an intermediate.

1,3-Benzenediol, also known as resorcin or m-hydroquinone, belongs to the class of organic compounds known as resorcinols. Resorcinols are compounds containing a resorcinol moiety, which is a benzene ring bearing two hydroxyl groups at positions 1 and 3. 1,3-Benzenediol exists in all living organisms, ranging from bacteria to humans. 1,3-Benzenediol is a creamy, hawthorn, and musty tasting compound. 1,3-Benzenediol has been detected, but not quantified, in several different foods, such as alcoholic beverages, cereals and cereal products, coffee and coffee products, eggplants, and java plums. This could make 1,3-benzenediol a potential biomarker for the consumption of these foods. 1,3-Benzenediol is a potentially toxic compound. In addition, exogenous ochronosis is associated with prolonged exposure to resorcinol . Data regarding the specific mechanisms of action of resorcinol does not appear to be readily accessible in the literature. Nevertheless, the role played by iodide ions in the irreversible inactivation of the enzymes is not yet fully elucidated . Resorcinol works by helping to remove hard, scaly, or roughened skin. In particular, it appears that resorcinol indicated for treating acne, dermatitis, or eczema in various skin care topical applications and peels revolves around the compound's ability to precipitate cutaneous proteins from the treated skin . In LPO and TPO, the resulting π-cation radical of the porphyrin can isomerize to a radical cation with the radical in an aromatic side chain of the enzyme . In vitro and in vivo studies have demonstrated that resorcinol can inhibit peroxidases in the thyroid and subsequently block the synthesis of thyroid hormones and cause goiter .

A dicarboxylic acid that is cis,cis-muconic acid in which the hydrogens at position 3 is substituted by a methyl group." []

Imidazole-4-acetaldehyde is a naturally occurring aldehyde metabolite of histamine formed by the action of histaminase (E.C. 1.4.3.6), and can be synthesized by oxidation of histidine. Aldehyde dehydrogenase (EC 1.2.1.3) is the only enzyme in the human liver capable of catalyzing dehydrogenation of aldehydes arising via monoamine, diamine, and plasma amine oxidases. NAD-linked dehydrogenation of short chain aliphatic aldehydes has been found in virtually every organ of the mammalian body. Imidazole-4-acetaldehyde is a good substrate for all aldehyde dehydrogenase isozymes. Experimentally, the prebiotic formation of histidine has been accomplished by the reaction of erythrose with formamidine followed by a Strecker synthesis. Imidazole-4-acetaldehyde could have been converted to histidine on the primitive earth by a Strecker synthesis, and several prebiotic reactions could convert imidazole-4-glycol and imidazole-4-ethanol to imidazole-4-acetaldehyde. (PMID: 2071588, 2957640, 11536478).

Furoic acid is a metabolite that appears in the urine of workers occupationally exposed to furfural and is a marker of exposure to this compound. Furfural is a heterocyclic aldehyde that is commonly used as a solvent in industry. It is readily absorbed into the body via the lungs and has significant skin absorption. Furfural is an irritant of the eyes, mucous membranes, and skin and is a central nervous system depressant. Furfural as a confirmed animal carcinogen with unknown relevance to humans (It has been suggested that is a substance that produces hepatic cirrhosis). Once in the body, furfural is metabolized rapidly via oxidation to the metabolite furoic acid, which is then conjugated with glycine and excreted in the urine in both free and conjugated forms. (PMID: 3751566, 4630229, 12587683). 2-Furoic acid is a biomarker for the consumption of beer

3-Furoic acid is an organic acid regularly occurring in urine of healthy individuals. (PMID 2338430). 3-Furoic acid is also a compound found in honey and honeydew samples (PMID 11403496), and is a structural analog of nicotinic acid (niacin, a vitamin of the B complex). (PMID 12563315).

A muconic semialdehyde compound having a hydroxy substituent at the 2-position and a methyl substituent at the 5-position." []

A delta-lactone that is D-galactono-1,4-lactone in which the hydroxy group at position 6 has been oxidised to the corresponding carboxylic acid." []

2-Vinylthiophene is a maillard produc

A dicarboxylic acid monoester comprising succinic acid having an oxo group at the 2-position and the methyl ester at the 4-position." []

Benzenethiol is a flavouring agent Thiophenol is a chemical compound with the formula C6H5SH, and sometimes abbreviated as PhSH. The foul-smelling liquid is the principal aromatic thiol. The chemical structures of thiophenols are analogous to phenols except the oxygen atom in the hydroxyl group (-OH) bonded to the aromatic ring is replaced by a sulfur atom. The prefix thio- implies a sulfur-containing compound and when used before a root word name for a compound which would normally contain an oxygen atom, thio- commonly means that the oxygen atom is replaced by a sulfur atom. Thiophenols also describes a class of compounds formally derived from thiophenol itself. All have a sulfhydryl group (-SH) covalently bonded to an aromatic ring. The organosulfur ligand in medicine merthiolate is a thiophenol.

Citraconic anhydride is a natural product found in Coffea arabica and Schisandra chinensis with data available. Methylmaleic anhydrides.

Orotic acid is classified as a pyrimidinemonocarboxylic acid. That is it is a uracil bearing a carboxy substituent at position C-6. It is also classified as a pyrimidinedione and a carboxylic acid. Orotic acid is a minor dietary constituent. Indeed, until it was realized that it could be synthesized by humans, orotic acid was known as vitamin B-13. The richest dietary sources of orotic acid are cow's milk and other dairy products as well as root vegetables such as carrots and beets. Dietary intake probably contributes to a basal rate of orotic acid excretion in urine because fasting decreases excretion by ~50%. However, it is now apparent that most urinary orotic acid is synthesized in the body, where it arises as an intermediate in the pathway for the synthesis of pyrimidine nucleotides. Orotic acid is converted to UMP by UMP synthase, a multifunctional protein with both orotate phosphoribosyltransferase and orotidylate decarboxylase activity. The most frequently observed inborn error of pyrimidine nucleotide synthesis is a mutation of the multifunctional protein UMP synthase (UMP synthase deficiency or orotic aciduria). This disorder prevents the conversion of orotic acid to UMP, and thus to other pyrimidines. As a result, plasma orotic acid accumulates to high concentrations, and increased quantities appear in the urine. Indeed, urinary orotic acid is so markedly increased in individuals harboring a mutation in UMP synthase that orotic acid crystals can form in the urine. The urinary concentration of orotic acid in individuals suffering from orotic aciduria can be of the order of millimoles of orotic acid per millimole creatinine. By comparison, the urinary level in unaffected individuals is ~ 1 ¬umol/mmol creatinine (PMID: 17513443). Orotic aciduria is characterized by megaloblastic anemia and orotic acid crystalluria that is frequently associated with some degree of physical and mental retardation. These features respond to appropriate pyrimidine replacement therapy and most cases appear to have a good prognosis. When present in sufficiently high levels, orotic 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 orotic acid are associated with at least seven inborn errors of metabolism, including argininemia, LPI syndrome (lysinuric protein intolerance), hyperornithinemia-hyperammonemia-homocitrullinuria (HHH), OTC deficiency, citrullinemia type I, purine nucleoside phosphorylase deficiency, and orotic aciduria. Orotic acid is broadly classified as 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 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.

N-acetylimidazole is a N-acylimidazole. N-Acetylimidazole is a natural product found in Nicotiana tabacum with data available.

3-Hydroxy-4-aminopyridine is a metabolite of dalfampridine. 4-Aminopyridine (fampridine, USAN dalfampridine) is an organic compound with the chemical formula C5H4N–NH2. The molecule is one of the three isomeric amines of pyridine. It is used primarily as a research tool, in characterizing subtypes of potassium channel, and has also been used to manage some of the symptoms of multiple sclerosis, and is indicated for symptomatic improvement of walking in adults with several variations of the disease. (Wikipedia)

Methoxypyrazine is found in animal foods. Methoxypyrazine is a flavouring agent. Methoxypyrazine is present in cooked beef and cocoa Methoxypyrazines are a class of chemical compounds that produce odors. The odors may be desirable, as in the case of certain wines, or undesirable, as in the case of the Asian Lady Beetle which produces isopropyl methoxy pyrazine (IPMP). They have also been identified as additives in cigarette manufacture. Detection thresholds are very low, typically near 2 parts per trillion (1 ng/L).

2,3-Methyleneglutaric acid belongs to the family of Branched Fatty Acids. These are fatty acids containing a branched chain.

A monochlorobenzoic acid carrying a chloro substituent at position 4." []

5-Methyl-2-furancarboxaldehyde, also known as 5-methyl-2-furfural or 2-formyl-5-methylfuran, belongs to the class of organic compounds known as aryl-aldehydes. Aryl-aldehydes are compounds containing an aldehyde group directly attached to an aromatic ring. 5-Methyl-2-furancarboxaldehyde is an almond, burnt sugar, and caramel tasting compound. 5-methyl-2-furancarboxaldehyde has been detected, but not quantified, in several different foods, such as green bell peppers, red bell peppers, pepper (c. frutescens), orange bell peppers, and pepper (c. annuum). This could make 5-methyl-2-furancarboxaldehyde a potential biomarker for the consumption of these foods.