<Tr> <Th_colspan="2"> (what is this?) (verify) </Th> </Tr> <P> Carnitine (β - hydroxy - γ - N - trimethylaminobutyric acid, 3 - hydroxy - 4 - N, N, N - trimethylaminobutyrate) is a quaternary ammonium compound involved in metabolism in most mammals, plants and some bacteria . Carnitine may exist in two isomers, labeled D - carnitine and L - carnitine, as they are optically active . At room temperature, pure carnitine is a white powder, and a water - soluble zwitterion with low toxicity . Carnitine only exists in animals as the L - enantiomer, and D - carnitine is toxic because it inhibits the activity of L - carnitine . Carnitine was discovered in 1905 as a result of its high concentration in muscle tissue . It was originally labeled vitamin B; however, because carnitine is synthesized in the human body, it is no longer considered a vitamin . Carnitine can be synthesized by most humans; about 1 in 350 males is unable to synthesize it due to genetic causes on the X chromosome . Carnitine is involved in the oxidation of fatty acids, and involved in systemic primary carnitine deficiency . It has been studied for preventing and treating other conditions, and is used as a purported performance enhancing drug . </P> <P> Many eukaryotes have the ability to synthesize carnitine, including humans . Humans synthesize carnitine from the substrate TML (6 - N - trimethyllysine), which is in turn derived from the methylation of the amino acid lysine . TML is then hydroxylated into hydroxytrimethyllysine (HTML) by trimethyllysine dioxygenase, requiring the presence of ascorbic acid . HTML is then cleaved by HTML aldose, yielding 4 - trimethylaminobutyraldehyde (TMABA) and glycine . TMABA is then dehydrogenated into gamma - butyrobetaine, in an NAD - dependent reaction, catalyzed by TMABA dehydrogenase . Gamma - butyrobetaine is then hydroxylated by gamma butyrobetaine hydroxylase into L - carnitine, requiring iron in the form of Fe . </P> <P> Carnitine is involved in transporting fatty acids across the mitochondrial membrane, by forming a long chain acetylcarnitine ester and being transported by carnitine palmitoyltransferase I and carnitine palmitoyltransferase II . Carnitine also plays a role in stabilizing Acetyl - CoA and coenzyme A levels through the ability to receive or give an acetyl group . </P>

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