Akademik

2-Amino-4-(methylthio)butyric acid; the l-isomer is a nutritionally essential amino acid and the most important natural source of “active methyl” groups in the body, hence usually involved in methylations in vivo; the dl-form is used as an adjunct in the treatment of liver diseases.

- active m. SYN: S-adenosyl-l-m..

- m. adenosyltransferase an enzyme catalyzing the condensation of l-m. and ATP, forming S-adenosyl-l-m., orthophosphate, and pyrophosphate; a deficiency of the hepatic enzyme will result in hypermethionemia. SYN: m.-activating enzyme.

- m. sulfoxime a toxic derivative of m. formed when proteins containing it are treated with nitrogen chloride to give –SO(NH)CH₃ in place of –SCH₃.

- m. synthase tetrahydropteroylglutamate methyltransferase; m.-homocysteine methyltransferase; an enzyme that catalyzes the reaction of N⁵-methyltetrahydrofolate with l-homocysteine to form tetrahydrofolate and l-m.; a cobalamin-requiring enzyme; a deficiency of this enzyme results in an accumulation of l-homocysteine and neurological abnormalities. SYN: tetrahydrofolate methyltransferase.

me·thi·o·nine mə-'thī-ə-.nēn n a crystalline sulfur-containing essential amino acid C₅H₁₁NO₂S that occurs in the L-form as a constituent of many proteins (as casein and egg albumin), that is important esp. as a source of sulfur for the biosynthesis of cysteine and as a source of methyl groups for transmethylation reactions (as in the biosynthesis of choline, creatine, and adrenaline), and that is used as a dietary supplement for human beings and their domestic mammals and poultry and in the treatment of fatty infiltration of the liver abbr. Met

a sulphur-containing essential amino acid. See also amino acid.