methionine

noun

me·​thi·​o·​nine mə-ˈthī-ə-ˌnēn 

: a crystalline sulfur-containing essential amino acid C₅H₁₁NO₂S that occurs in the levorotatory form as a constituent of many proteins

Examples of methionine in a Sentence

Recent Examples on the Web

Nitrous oxide irreversibly disables methionine synthase, the enzyme your body needs to build and maintain the myelin sheath insulating the nerves. — Jesse M. Pines, Forbes.com, 6 Aug. 2026

Researchers highlighted three essential amino acids that are building blocks of protein — methionine, isoleucine and valine. — Khloe Quill, FOXNews.com, 3 Aug. 2026

It’s made using three amino acids—arginine, glycine and methionine. — Jackie Flynn Mogensen, Scientific American, 7 July 2026

The dietary interventions included the Mediterranean diet, ketogenic diet, and omega-3 and methyl-donor supplements, including B12, folate, choline, and methionine. — Michele Laufik, Martha Stewart, 2 Apr. 2026

Word History

Etymology

International Scientific Vocabulary, from methyl + thion- + -ine entry 2

First Known Use

1928, in the meaning defined above

Rhymes for methionine

• acridine
• acrolein
• adenine
• alkylene
• aniline
• anthracene
• aquiline
• asphaltene
• atlantean
• atrazine
• butadiene
• butylene

Medical Definition

methionine

noun

me·​thi·​o·​nine mə-ˈthī-ə-ˌnēn 

: 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 especially 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

—abbreviation Met