Cysteine sulfinic acid is an amino acid containing a sulfinic acid functional group. It is a white solid that is soluble in water. It is an intermediate in cysteine metabolism. It is not a coded amino acid, but is produced post-translationally. Its formation is catalyzed by the enzyme cysteine dioxygenase.
Proteins containing this residue are found at the active site of some nitrile hydratases.