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Metabolite Card

Note

Valine (Val) or L-valine is an alpha-amino acid. These are amino acids in which the amino group is attached to the carbon atom immediately adjacent to the carboxylate group (alpha carbon). Amino acids are organic compounds that contain amino (–NH2) and carboxyl (–COOH) functional groups, along with a side chain (R group) specific to each amino acid. L-valine is one of 20 proteinogenic amino acids, i.e., the amino acids used in the biosynthesis of proteins. Valine is found in all organisms ranging from bacteria to plants to animals. It is classified as a non-polar, uncharged (at physiological pH) aliphatic amino acid. Valine was first isolated from casein in 1901 by Hermann Emil Fischer. The name valine comes from valeric acid, which in turn is named after the plant valerian due to the presence of valine in the roots of the plant. Valine is essential in humans, meaning the body cannot synthesize it, and it must be obtained from the diet. Human dietary sources are foods that contain protein, such as meats, dairy products, soy products, beans and legumes. L-valine is a branched chain amino acid (BCAA). The BCAAs consist of leucine, valine and isoleucine (and occasionally threonine). BCAAs are essential amino acids whose carbon structure is marked by a branch point at the beta-carbon position. BCAAs are critical to human life and are particularly involved in stress, energy and muscle metabolism. BCAA supplementation as therapy, both oral and intravenous, in human health and disease holds great promise. BCAAs have different metabolic routes, with valine going solely to carbohydrates (glucogenic), leucine solely to fats (ketogenic) and isoleucine being both a glucogenic and a ketogenic amino acid. The different metabolism accounts for different requirements for these essential amino acids in humans: 12 mg/kg, 14 mg/kg and 16 mg/kg of valine, leucine and isoleucine respectively. Like other branched-chain amino acids, the catabolism of valine starts with the removal of the amino group by transamination, giving alpha-ketoisovalerate, an alpha-keto acid, which is converted to isobutyryl-CoA through oxidative decarboxylation by the branched-chain α-ketoacid dehydrogenase complex. This is further oxidised and rearranged to succinyl-CoA, which can enter the citric acid cycle. Furthermore, these amino acids have different deficiency symptoms. Valine deficiency is marked by neurological defects in the brain, while isoleucine deficiency is marked by muscle tremors. Many types of inborn errors of BCAA metabolism exist, and are marked by various abnormalities. The most common form is the maple syrup urine disease, marked by a characteristic urinary odor. Other abnormalities are associated with a wide range of symptoms, such as mental retardation, ataxia, hypoglycemia, spinal muscle atrophy, rash, vomiting and excessive muscle movement. Most forms of BCAA metabolism errors are corrected by dietary restriction of BCAA and at least one form is correctable by supplementation with 10 mg of biotin daily. BCAA are decreased in patients with liver disease, such as hepatitis, hepatic coma, cirrhosis, extrahepatic biliary atresia or portacaval shunt. Valine in particular, has been established as a useful supplemental therapy to the ailing liver. Valine, like other branched-chain amino acids, is associated with insulin resistance: higher levels of valine are observed in the blood of diabetic mice, rats, and humans (PMID: 25287287). Mice fed a valine deprivation diet for one day have improved insulin sensitivity and feeding of a valine deprivation diet for one week significantly decreases blood glucose levels (PMID: 24684822). In diet-induced obese and insulin resistant mice, a diet with decreased levels of valine and the other branched-chain amino acids results in reduced adiposity and improved insulin sensitivity (PMID: 29266268). In sickle-cell disease, valine substitutes for the hydrophilic amino acid glutamic acid in hemoglobin. Because valine is hydrophobic, the hemoglobin does not fold correctly. Hypervalinemia is another example of an inborn error of metabolism involving valine.