3-Sulfinoalanine or cysteinesulfinic acid is a N-methyl-D-aspartate agonist. It is a product of cysteine dioxygenase or CDO [EC 1. 13. 11. 20]. In humans cysteine catabolism is tightly regulated via regulation of cysteine dioxygenase (CDO) levels in the liver, with the turnover of CDO protein being dramatically decreased when intracellular cysteine levels increase. This occurs in response to changes in the intracellular cysteine concentration via changes in the rate of CDO ubiquitination and degradation. Expressed at high levels in the liver with lower levels in the kidney, brain, and lung, cysteine dioxygenase catalyzes the addition of molecular oxygen to the sulfhydryl group of cysteine, yielding cysteinesulfinic acid. The oxidative catabolism of cysteine to cysteinesulfinate by CDO represents an irreversible loss of cysteine from the free amino acid pool. Once generated, cysteinesulfinate is shuttled into several pathways including hypotaurine/taurine synthesis, sulfite/sulfate production, and the generation of pyruvate.