Arylamine N-acetyltransferases*

Abstract

Arylamine N -acetyltransferases (NATs) are cytosolic conjugating enzymes which add an acetyl group from acetyl Coenzyme A (CoA) to arylamine and arylhydrazines which in general are detoxification reactions. Acetylation of arylhydroxylamines and the transfer of an acetyl group from the O to N group of arylacetohydroxates generally result in activation, particularly of arylamine carcinogens to produce N -acetoxyesters. The polymorphic NAT enzymes were very important in establishing the basics of pharmacogenetics through the metabolism of the antitubercular hydrazine isoniazid. There are now known to be two human isoenzymes: NAT2 responsible for isoniazid metabolism and NAT1, also polymorphic, which is more specific for p -aminosalicylate (pAS) and p -aminobenzoic acid (p-aba) and the folate catabolite, p -aminobenzoylglutamate (p-abaglu). The polymorphism in NAT1 and NAT2 is primarily through a series of SNPs which occur in haplotypes in the single exon coding region of these genes. Amino acid substitutions result in destabilized protein with mutant versions being degraded in the proteasome following ubiquitination. There are NAT enzymes in mammalian (apart from canids), nonmammalian, and also bacterial species. Transgenic mice are helping to unravel the endogenous role of human NAT1 which is widespread in tissues, expressed very early in development and overexpressed in estrogen-receptor positive breast cancer. It is likely that the NAT1 enzyme has a role in acetylating a folate catabolite. Control of expression of NAT genes is beginning to be understood in relation to the splicing patterns of the noncoding exons (NCEs) which appear to be tissue-specific for human NAT1 . In all NATs which have been studied the reaction mechanism is through an acetylated cysteine intermediate, with the cysteine being activated through a catalytic triad with histidine and aspartate. The structures of the bacterial and the human enzymes have identified the acetyl CoA binding sites which are subtly different. While the catalytic triads of the various NAT enzymes are superimposable, as is the three-domain core structure, the C-terminus and an interdomain loop in the human enzymes occlude the active-site cleft and this may allow a different range of functions for the bacterial enzymes, particularly from the mycobacteria which, from gene deletion studies, have a role in the formation of cell wall lipids. The availability of structural studies and in silico screening, the potential for using recombinant proteins to identify small molecule inhibitors and substrates, and the availability of transgenic model organisms will allow the role of NAT in toxicology to be addressed at many levels.