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 from the condensation of serine and homocysteine and is an intermediate in the transsulfuration pathway, which is essential for synthesizing glutathione, an important antioxidant in the body. In healthy individuals, cystathionine is typically present in low concentrations in the blood and tissues. Elevated levels may indicate a disruption in the metabolism of sulfur amino acids, which can be associated with various health conditions, including cardiovascular diseases and neurodegenerative disorders.