Application Internal standard for amino acid analysis.1 Biochem/physiol Actions L-cysteic acid is an oxidation product of L-cysteine. L-Cysteic acid, an analogue of cysteine sulfinic acid, may be used in studies of excitatory amino acids in the brain, such as those that bind to cysteine sulfinic acid receptors. L-Cysteic acid is a useful agonist at several rat metabotropic glutamate receptors (mGluRs).