Arylamine N-acetyltransferase (NAT) is a cytosolic enzyme of approximately 30 kDa. It facilitates the transfer of an acetyl group from acetyl coenzyme A on to a wide range of arylamine, N-hydroxyarylamines and hydrazines. Acetylation of these compounds generally results in inactivation. NAT is found in many species from Mycobacteria (Mycobacterium tuberculosis, Mycobacterium smegmatis etc) to Homo sapiens. It was the first enzyme to be observed to have polymorphic activity amongst human individuals. NAT is responsible for the inactivation of Isoniazid (a drug used to treat tuberculosis) in humans. The NAT protein has also been shown to be involved in the breakdown of folic acid. NAT catalyses the reaction:

NAT is the target of a common genetic polymorphism of clinical relevance in humans. The N-acetylation polymorphism is determined by low or high NAT activity in liver. NAT has been implicated in the action and toxicity of amine-containing drugs, and in the susceptibility to cancer and systematic lupus erythematosus. Two highly similar human genes for NAT, termed NAT1 and NAT2, encode genetically invariant and variant NAT proteins, respectively.