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Theanine , also known as L-γ-glutamylethylamide and ''N5''-ethyl-L-glutamine (see box for more synonyms), is an amino acid analogue of the proteinogenic amino acids L-glutamate and L-glutamine and is found primarily in particular plant and fungal species. It was discovered as a constituent of green tea in 1949 and in 1950 was isolated from gyokuro leaves, which have high theanine content. Appearance of the name "theanine" without a prefix is understood to imply the L-enantiomer, which is the form found in fresh teas and in some, but not all dietary supplements. The opposite D-enantiomer has far less studied pharmacologic properties, but is present in racemic chemical preparations, and substantially in some studied theanine supplements. As an analogue of glutamate and glutamine, the theanine in common preparations (teas, enantiomer supplements, etc.) is absorbed in the small intestine after oral ingestion; its hydrolysis is to L-glutamate and ethylamine and occurs both in the intestine and liver. It can also cross the blood–brain barrier intact, and register pharmacological effects directly. In Japan, L-Theanine has been approved for use in all foods, including herb teas, soft drinks, desserts, etc. with some restrictions applying to infant foods.〔SAKATO Y. J. Agri. Chem. Soc. 1949, 23, 262-7〕〔MASON R. Altern. & Complementary Ther. 2001, 7, 91-5〕 It provides a unique brothy or savory (umami) flavor to green tea infusions. It is generally recognized as safe (GRAS) as an ingredient by the Food and Drug Administration (FDA), and is sold as a dietary supplement in the US. However, the German Federal Institute for Risk Assessment, an agency of their Federal Ministry of Food and Agriculture, has objected to the addition of isolated theanine to beverages. The European Food Safety Authority EFSA advised negatively on health claims related to L-theanine and cognitive function, alleviation of psychological stress, maintenance of normal sleep and reduction of menstrual discomfort.〔http://www.efsa.europa.eu/en/efsajournal/pub/2238〕 Therefore, health claims for L-Theanine are prohibited in the European Union. ==Structure and properties== The chemical name ''N''5-ethyl-L-glutamine,〔 and other synonyms (see box) for theanine reflect its chemical structure. The name theanine, without prefix, is generally understood to imply the L- (''S-'') enantiomer, derived from the related proteinogenic L-amino acid glutamic acid. Theanine is an analog of this amino acid, and its primary amide, L-glutamine (also a proteinogenic amino acid). Theanine is a derivative of glutamine that is ethylated on the amide nitrogen (as the name ''N''5-ethyl-L-glutamine describes), or alternatively, to the amide formed from ethylamine and L-glutamic acid at its γ- (5-) side chain carboxylic acid group (as the name γ-L-glutamylethylamide describes). Relative to theanine, the opposite (D-, ''R-'') enantiomer is largely absent from the literature, except inexplicitly. While natural extracts that are not harshly treated are presumed to contain only the biosynthetic L- enantiomeric form, mishandled isolates and racemic chemical preparations of theanines necessarily contain both theanine and its D-enantiomer (and from racemic syntheses, in equal proportion), and studies have suggested that the D-isomer may actually ''predominate'' in some commercial supplement preparations.〔Q.V. Vuong, M.C. Bowyer & P.D. Roach (2011) J Sci Food Agric. L-Theanine: properties, synthesis and isolation from tea, 91(11):1931-1939, DOI 10.1002/jsfa.4373, see (), accessed 17 August 2014.〕〔See also, e.g., M.J. Desai & D.W. Armstrong (2004) Analysis of derivatized and underivatized theanine enantiomers by high-performance liquid chromatography/atmospheric pressure ionization-mass spectrometry, Rapid Commun Mass Spectrom. 18(3):251-256, and references therein, see (), accessed 17 August 2014.〕 Amino acid racemization in aqueous media is a well-established chemical process promoted by elevated temperature and non-neutral pH values; prolonged heating of ''Camellia'' extracts—possible for oversteeped teas and in undisclosed commercial preparative processes—has been reported to result in increasing racemization of theanine to give increasing proportions of the nonnatural D-theanine, up to equal proportions of each enantiomer.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Theanine」の詳細全文を読む スポンサード リンク
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