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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).

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.

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.

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.

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.

2-Ethyl-2-Hydroxybutyric acid, also known as 2-ethyl-2-hydroxybutanoate or 2-et-2-hba, belongs to the class of organic compounds known as hydroxy fatty acids. These are fatty acids in which the chain bears a hydroxyl group. 2-Ethyl-2-Hydroxybutyric acid is a very hydrophobic molecule, practically insoluble in water, and relatively neutral.

2-Hydroxy-3-methylpentanoic acid or 2-hydroxy-3-methylvaleric acid (HMVA) is an organic acid generated by L-isoleucine metabolism. It is derived from the reduction of 2-Keto-3-methylvaleric acid (KMVA), possibly through the action of a lactate dehydrogenase (PMID: 1429566). There are 4 stereoisomers of HMVA (2S,3S-HMVA, 2R,3R-HMVA, 2S,3R-HMVA and 2R,3S-HMVA), of which the 2S,3S and 2S,3R derivatives are generally separable. HMVA is found in the urine and blood of normal individuals but in very elevated levels in patients with maple syrup urine disease (MSUD) (PMID: 1429566). Maple syrup urine disease (MSUD) is an inherited metabolic disease predominantly characterized by neurological dysfunction including psychomotor/delay/mental retardation.

DL-Acetylcarnitine is an acylcarnitine. More specifically, it is an acetic acid ester of carnitine. Acylcarnitines were first discovered more than 70 year ago (PMID: 13825279). It is believed that there are more than 1000 types of acylcarnitines in the human body. The general role of acylcarnitines is to transport acyl-groups (organic acids and fatty acids) from the cytoplasm into the mitochondria so that they can be broken down to produce energy. This process is known as beta-oxidation. According to a recent review [Dambrova et al. 2021, Physiological Reviews], acylcarnitines (ACs) can be classified into 9 different categories depending on the type and size of their acyl-group: 1) short-chain ACs; 2) medium-chain ACs; 3) long-chain ACs; 4) very long-chain ACs; 5) hydroxy ACs; 6) branched chain ACs; 7) unsaturated ACs; 8) dicarboxylic ACs and 9) miscellaneous ACs. Short-chain ACs have acyl-groups with two to five carbons (C2-C5), medium-chain ACs have acyl-groups with six to thirteen carbons (C6-C13), long-chain ACs have acyl-groups with fourteen to twenty once carbons (C14-C21) and very long-chain ACs have acyl groups with more than 22 carbons. DL-Acetylcarnitine is therefore classified as a long chain AC. As a long-chain acylcarnitine DL-Acetylcarnitine is generally formed through esterification with long-chain fatty acids obtained from the diet. The main function of most long-chain acylcarnitines is to ensure long chain fatty acid transport into the mitochondria (PMID: 22804748). Altered levels of long-chain acylcarnitines can serve as useful markers for inherited disorders of long-chain fatty acid metabolism. In particular DL-Acetylcarnitine is elevated in the blood or plasma of individuals with very long chain acyl-CoA dehydrogenase deficiency (PMID: 9034211), colorectal cancer (PMID: 25105552), short-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (PMID: 11489939), paclitaxel induced neuropathy (PMID: 29946863), type 2 diabetes Mellitus (PMID: 28726959; PMID: 25964024, PMID: 28726959, PMID: 19369366, PMID: 24358186), chronic heart failure (PMID: 22622056), ornithine transcarbamylase (PMID: 3346778), pre-diabetes (PMID: 23010998, PMID: 24358186), type 1 diabetes mellitus (PMID: 16789638), methylmalonic acidemia (PMID: 8214594), myeloma (PMID: 30096165), and diastolic heart failure (PMID: 26010610). It is also decreased in the blood or plasma of individuals with 3-methyl-crotonyl-glycinuria (PMID: 25732994), antiviral drug induced neuropathy (PMID: 9030365, PMID: 11364244), Alzheimer Disease (PMID: 27196316), major depressive disorder (PMID: 30061399), carnitine palmitoyltransferase 2 deficiency (PMID: 20543534), Familial Mediterranean Fever (PMID: 29900937), chronic fatigue syndrome (PMID: 9854142), methylmalonic acidemia (PMID: 15164354), hepatocellular carcinoma (PMID: 26976432, PMID: 31662827), and coronary artery disease (PMID: 20173117). DL-Acetylcarnitine is elevated in the urine of individuals with colorectal cancer (25105552), uterine fibroids (32590215), heart failure (8070147), diabetes mellitus (10221661), and hepatocellular carcinoma (24923488). It is also decreased in the urine of individuals with carnitine palmitoyltransferase 2 deficiency (20543534). Carnitine palmitoyltransferase I (CPT I, EC:2.3.1.21) is involved in the synthesis of long-chain acylcarnitines (more than C12) on the mitochondrial outer membrane. Elevated serum/plasma levels of long-chain acylcarnitines are not only markers for incomplete FA oxidation but also are indicators of altered carbohydrate and lipid metabolism. High serum concentrations of long-chain acylcarnitines in the postprandial or fed state are markers of insulin resistance and arise from insulin's inability to inhibit CPT-1-dependent fatty acid metabolism in muscles and the heart (PMID: 19073774). Increased intracellular content of long-chain acylcarnitines is thought to serve as a feedback inhibition mechanism of insulin action (PMID: 23258903). In healthy subjects, increased concentrations of insulin effectively inhibits long-chain acylcarnitine production. Several studies have also found increased levels of circulating long-chain acylcarnitines in chronic heart failure patients (PMID: 26796394). The study of acylcarnitines is an active area of research and it is likely that many novel acylcarnitines will be discovered in the coming years. It is also likely that many novel roles in health and disease will be uncovered. An excellent review of the current state of knowledge for acylcarnitines is available at [Dambrova et al. 2021, Physiological Reviews].

Diacetyl, also known as 2,3-butadione or dimethylglyoxal, belongs to the class of organic compounds known as alpha-diketones. These are organic compounds containing two ketone groups on two adjacent carbon atoms. Thus, diacetyl is considered to be an oxygenated hydrocarbon lipid molecule. Diacetyl is a very hydrophobic molecule, practically insoluble in water, and relatively neutral. Diacetyl exists in all living species, ranging from bacteria to humans. Diacetyl is a strong, sweet, and butter tasting compound. Outside of the human body, diacetyl is found, on average, in the highest concentration in kohlrabis. diacetyl has also been detected, but not quantified in several different foods, such as nances, tartary buckwheats, tamarinds, pineapples, and celeriacs. This could make diacetyl a potential biomarker for the consumption of these foods. Diacetyl is a potentially toxic compound. Diacetyl has been found to be associated with several diseases such as crohn's disease, ulcerative colitis, and nonalcoholic fatty liver disease; also diacetyl has been linked to the inborn metabolic disorders including celiac disease.

3-Methylbutan-2-one

A butynediol that is but-2-yne substituted by hydroxy groups at positions 1 and 4." []

Cyclopropanecarboxylic acid is a monocarboxylic acid and a member of cyclopropanes. It is a conjugate acid of a cyclopropanecarboxylate.

Beta-Lactose is the beta-pyranose form of the compound lactose [CCD].

Raffinose is a complex carbohydrate. It is a trisaccharide composed of galactose, fructose, and glucose. It can be found in beans, cabbage, brussels sprouts, broccoli, asparagus, other vegetables, and whole grains. Raffinose is hydrolyzed to D-galactose and sucrose by D-galactosidase (D-GAL). D-GAL also hydrolyzes other D-galactosides such as stachyose, verbascose, and galactinol [1-O-(D-galactosyl)-myoinositol], if present. The enzyme does not cleave linked galactose, as in lactose. Raffinose is also known as melitose and may be thought of as galactose and sucrose connected via an alpha(1->6) glycosidic linkage. Thus, raffinose can be broken down into galactose and sucrose via the enzyme alpha-galactosidase. Human intestines do not contain this enzyme. Raffinose is a trisaccharide occurring in Australian manna (from Eucalyptus spp, Myrtaceae) and in cottonseed meal.

Isocrotonic acid is used in food preservatives.Isocrotonic acid (or quartenylic acid) is the cis analogue of crotonic acid. It is an oil, possessing a smell similar to that of brown sugar. (Wikipedia

Galactaric acid, also known as mucic acid or galactarate, belongs to the class of organic compounds known as glucuronic acid derivatives. Glucuronic acid derivatives are compounds containing a glucuronic acid moiety (or a derivative), which consists of a glucose moiety with the C6 carbon oxidized to a carboxylic acid. Technically, galactaric acid is an aldaric acid obtained by oxidation of galactose. Galactaric acid exists as a white crystalline powder, which melts at 210 - 230 oC. It is insoluble in alcohol, and nearly insoluble in cold water (1 g/300 mL) but more soluble in hot water (1 g/60 mL).. Galactaric acid exists in all living organisms, ranging from bacteria to plants to humans. In plants, galactaric acid is commonly produced or utilized as an osmorgulator (PMID: 31505987). Galactaric acid has been detected, but not quantified in, several different foods, such as fruits, vegetables and bovine milk. A recent large-scale dietary study found that galactaric acid can serve as a biomarker for long-term dairy intake and for the consumption of carotenoid-rich vegetables (PMID: 33566801). In food production, galactaric acid can be used to replace tartaric acid in self-rising flour or fizzies.

Dehydroascorbic acid (DHA) is an oxidized form of ascorbic acid (vitamin C). It is actively imported into the endoplasmic reticulum of cells via glucose transporters. It is trapped therein by reduction back to ascorbate by glutathione and other thiols. Dehydroascorbic acid, also known as L-dehydroascorbate or DHAA, belongs to the class of organic compounds known as gamma butyrolactones. Gamma butyrolactones are compounds containing a gamma butyrolactone moiety, which consists of an aliphatic five-member ring with four carbon atoms, one oxygen atom, and bears a ketone group on the carbon adjacent to the oxygen atom. Dehydroascorbic acid has similar biological activity as ascorbic acid. Currently dehydroascorbic acid is an experimental drug with no known approved indications. Dehydroascorbic acid may be a unique E. coli metabolite. Norepinephrine and dehydroascorbic acid can be biosynthesized from dopamine and ascorbic acid through its interaction with the enzyme dopamine beta-hydroxylase. In humans, dehydroascorbic acid is involved in the metabolic disorder called tyrosinemia type I. Concerning dehydroascorbic acid's antiviral effect against herpes simplex virus type 1, it is suggested that dehydroascorbic acid acts after replication of viral DNA and prevents the assembly of progeny virus particles. This is important because one study has found that after an ischemic stroke, dehydroascorbic acid has neuroprotective effects by reducing infarct volume, neurological deficits, and mortality. This reaction is reversible, but dehydroascorbic acid can instead undergo irreversible hydrolysis to 2,3-diketogulonic acid. In addition, unlike ascorbic Dehydroascorbic acid acid can cross the blood brain barrier and is then converted to ascorbic acid to enable retention in the brain. Dehydroascorbic acid is made from the oxidation of ascorbic acid. The exact mechanism of action is still being investigated, but some have been elucidated. Both compounds have been shown to have antiviral effects against herpes simplex virus type 1, influenza virus type A and poliovirus type 1 with dehydroascorbic acid having the stronger effect. In the body, both dehydroascorbic acid and ascorbic acid have similar biological activity as antivirals but dehydroascorbic acid also has neuroprotective effects. Even though dehydroascorbic acid and ascorbic acid have similar effects, their mechanism of action seems to be different.

Acetyl-L-carnitine

2-Methylbutyroylcarnitine is an acylcarnitine. More specifically, it is an 2-methylbutanoic acid ester of carnitine. Acylcarnitines were first discovered more than 70 year ago (PMID: 13825279). It is believed that there are more than 1000 types of acylcarnitines in the human body. The general role of acylcarnitines is to transport acyl-groups (organic acids and fatty acids) from the cytoplasm into the mitochondria so that they can be broken down to produce energy.  This process is known as beta-oxidation. According to a recent review [Dambrova et al. 2021, Physiological Reviews], acylcarnitines (ACs) can be classified into 9 different categories depending on the type and size of their acyl-group: 1) short-chain ACs; 2) medium-chain ACs; 3) long-chain ACs; 4) very long-chain ACs; 5) hydroxy ACs; 6) branched chain ACs; 7) unsaturated ACs; 8) dicarboxylic ACs and 9) miscellaneous ACs. Short-chain ACs have acyl-groups with two to five carbons (C2-C5), medium-chain ACs have acyl-groups with six to thirteen carbons (C6-C13), long-chain ACs have acyl-groups with fourteen to twenty once carbons (C14-C21) and very long-chain ACs have acyl groups with more than 22 carbons. 2-Methylbutyroylcarnitine is therefore classified as a short chain AC. As a short-chain acylcarnitine 2-methylbutyroylcarnitine is a member of the most abundant group of carnitines in the body, comprising more than 50% of all acylcarnitines quantified in tissues and biofluids (PMID: 31920980). Some short-chain carnitines have been studied as supplements or treatments for a number of diseases, including neurological disorders and inborn errors of metabolism. In particular 2-methylbutyroylcarnitine is elevated in the blood or plasma of individuals with exudative age-related macular degeneration (PMID: 32120889), type 2 Diabetes Mellitus (PMID: 31782507, PMID: 20111019), obesity (PMID: 20111019), acute cerebral infarction (PMID: 29265114), diastolic heart failure (PMID: 26010610), systolic heart failure (PMID: 26010610). It is also decreased in the blood or plasma of individuals with pregnancy (PMID: 24704061 - in serum of pregnant women with fetus with CHD). Carnitine acetyltransferase (CrAT, EC:2.3.1.7) is responsible for the synthesis of all short-chain and short branched-chain acylcarnitines (PMID: 23485643). The study of acylcarnitines is an active area of research and it is likely that many novel acylcarnitines will be discovered in the coming years. It is also likely that many novel roles in health and disease will be uncovered. An excellent review of the current state of knowledge for acylcarnitines is available at [Dambrova et al. 2021, Physiological Reviews].

Butyrylcarnitine, also known as (3R)-3-(butyryloxy)-4-(trimethylammonio)butanoate or L-carnitine butyryl ester, is classified as a member of the acylcarnitines. Acylcarnitines are organic compounds containing a fatty acid with the carboxylic acid attached to carnitine through an ester bond. Butyrylcarnitine is considered to be practically insoluble (in water) and acidic. Butyrylcarnitine is elevated in patients with short-chain acyl-CoA dehydrogenase (SCAD) deficiency, in infants with acute acidosis and generalized muscle weakness, and in middle-aged patients with chronic myopathy localized in muscle (OMIM: 201470).

Glucaric acid, also known as glucarate or D-saccharic acid, belongs to the class of organic compounds known as glucuronic acid derivatives. Glucuronic acid derivatives are compounds containing a glucuronic acid moiety (or a derivative), which consists of a glucose moiety with the C6 carbon oxidized to a carboxylic acid. Glucaric acid is a sugar acid derived from D-glucose in which both the aldehydic carbon atom and the carbon atom bearing the primary hydroxyl group are oxidized to carboxylic acid groups. D-glucaric acid is found in fruits, vegetables, and mammals. The highest concentrations of glucaric acid are found in grapefruits, apples, oranges, and cruciferous vegetables (PMID: 18772850). Glucaric acid is produced through the oxidation of glucose. Cytochrome P450 is thought to be responsible for the production of D-glucaric acid in vivo (PMID: 3779687). In mammals, D-glucaric acid and D-glucaro-l,4-lactone are also known end-products of the D-glucuronic acid pathway (PMID: 18772850). Glucaric is available as a dietary supplement in the form of calcium D-glucarate and has been studied for therapeutic purposes including cholesterol reduction and cancer chemotherapy (PMID: 9101079). D-Glucaric acid has a potential use as a building block for a number of polymers, including new nylons and hyperbranched polyesters. D-glucaric acid produced from D-glucose has been successfully utilized to produce a hydroxylated nylon.

A glycosylglucose consisting of two D-glucopyranose units connected by a beta-(1->6)-linkage." []

O-propanoyl-carnitine

Amygdalin is found in almond. Bitter glycoside of the Rosaceae, found especially in kernels of cherries, peaches and apricots. Amygdalin is present in cold pressed bitter almond oil from the above sources prior to enzymic hydolysis and steam distillation for food use Amygdalin , C20H27NO11, is a glycoside initially isolated from the seeds of the tree Prunus dulcis, also known as bitter almonds, by Pierre-Jean Robiquet and A. F. Boutron-Charlard in 1803, and subsequently investigated by Liebig and Wohler in 1830, and others. Several other related species in the genus of Prunus, including apricot (Prunus armeniaca) and black cherry (Prunus serotina), also contain amygdalin. It was promoted as a cancer cure by Ernst T. Krebs under the name "Vitamin B17", but studies have found it to be ineffective. Amygdalin is sometimes confounded with laevomandelonitrile, also called laetrile for short; however, amygdalin and laetrile are different chemical compounds