Cystathionine is a non-proteinogenic amino acid that plays a crucial role in the metabolism of sulfur-containing amino acids, particularly in the conversion of homocysteine to cysteine. It is formed through a reaction involving serine and homocysteine, catalyzed by the enzyme cystathionine β-synthase. This compound is important for maintaining proper levels of homocysteine in the body, as elevated levels can be associated with cardiovascular diseases. Cystathionine is further broken down into cysteine, which is essential for the synthesis of proteins and the production of glutathione, a key antioxidant.