OAT is a 49- kDa nucleus-encoded protein imported into mitochondria to give the mature 48- kDa OAT polypeptide. It has been described in humans, animals, insects, plants and microorganisms. Especially OAT have sex-differential expression in the mouse kidney. OAT plays crucial physiological roles in amino acid metabolism. OAT shows a large structural and mechanistic similarity to other enzymes from the subgroup III of aminotransferases, which transfer an amino group from a carbon atom that does not carry a carboxyl function. OAT is essential for nitrogen recycling from arginine but not for the stress-induced proline accumulation.