A crystalline, sulphur-containing amino acid produced by the hydrolysis of penicillins, used pharmacologically as a chelating agent to remove heavy metals such as copper from the body, and as an anti-inflammatory agent, especially for rheumatoid arthritis; 2-amino-3-methyl-3-mercaptobutanoic acid, (CH₃)₂ C(SH)CH(NH₂)COOH.