Methionine is an organic compound and one of the essential amino acids that constitute the human body. Its molecular formula is C5H11O2NS, it is optically active, and it participates in protein synthesis.

Because it cannot be produced by the body itself, it must be obtained externally. A deficiency in methionine can hinder protein synthesis in the body, leading to physical damage. Excessive oxidation of membrane lipids caused by oxygen free radicals in the body is a cause of various types of damage. Lipid peroxides can damage the membranes of primary and secondary lysosomes, causing the release of acidic phosphatase—a hydrolytic enzyme contained within lysosomes—which harms important organelles such as cell and mitochondrial membranes. Methionine combats these damages through multiple pathways.