Cystathionine is an intermediate in the synthesis of cysteine. It is generated from homocysteine and serine by cystathionine beta synthase. It is cleaved into cysteine and α-ketobutyrate by cystathionine gamma-lyase. An excess in the urine is called cystathioninuria. Cystathionine (R-S-(2-amino-2-carboxyethyl)-l-homocysteine) is a non-proteinogenic thioether containing amino acid. In mammals, cystathionine is formed as an intermediate of the transsulfuration pathway by the condensation of serine and homocysteine (Hcy) in a reaction catalyzed by cystathionine β-synthase (CBS). Cystathionine is subsequently converted to cysteine plus ammonia and α-ketobutyrate by the action of cystathionine γ-lyase (CGL). Pathogenic mutations in CBS result in CBS-deficient homocystinuria (HCU) which, if untreated, results in mental retardation, thromboembolic complications and connective tissue disorders.
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