Arylamine N-acetyltransferase of Mycobacterium tuberculosis is a polymorphic enzyme and a site of isoniazid metabolism

Arylamine N-acetyltransferases (NATs; E.C 2.3.1.5) N-acetylate arylhydralaz ine and arylamine substrates using acetyl coenzyme A. Human NAT2 acetylates and inactivates the antituberculosis drug, isoniazid (INH), and is polymor phic. We previously demonstrated that there is a homologue of human NAT2 in Mycobacterium tuberculosis, whose product N-acetylates INH in vitro. We no w demonstrate that the nat gene is expressed in M. tuberculosis and M. bovi s Bacille Calmette-Guerin (BCG), using reverse transcription-polymerase cha in reaction and Western blotting, The NAT protein is active in M. bovis BCG in vivo, as detected by the presence of N-acetyl INH in M. bovis BCG lysat es grown in INK Sequence analysis of the M. tuberculosis nat coding region reveals a single nucleotide polymorphism in 18% of a random cohort of M. tu berculosis clinical isolates, conferring a G to R change. The recombinant m utant protein appears less stable than the wild type, and has an apparent a ffinity for INH of 10-fold less than the wild type. Modelling the change in M. tuberculosis NAT shows that the G to R change is close to the active si te, and supports the experimental findings. Minimum inhibitory concentratio n data suggest that this polymorphism in nat is linked to low-level changes in the INH susceptibility of M. tuberculosis clinical isolates.