Acetyl CoA:arylamine N-acetyltransferase (NAT) enzymes catalyze the N-acetylation of aromatic amines and the O-acetylation of aryl hydroxylamines, reactions that govern the disposition and toxicity of many drugs and carcinogens. The human NAT genes and enzymes NAT1 and NAT2 are highly polymorphic and constitute one of the best studied examples of the genetic control of drug metabolism. Naturally occurring human NAT variants provide limited insight into the relationship between NAT amino acid sequence and enzyme activity. We have shown previously that the expression of recombinant NAT2 in bacterial tester strains results in greatly enhanced sensitivity to mutagenic nitroaromatic compounds (which are reduced to aryl hydroxylamines by bacterial enzymes). We hypothesized that random mutagenesis combined with rapid screening could be used to identify functionally significant amino acid residues in NAT enzymes. Pools of NAT2 variants were generated by polymerase chain reaction-mediated random mutagenesis of the complete coding sequence. Reversion induced by a NAT-dependent mutagen, 3-methyl-2-nitroimidazo[4,5-f]quinoline, was used as the basis for screening these pools to identify variants with altered enzyme activity. Eighteen variants were characterized by quantitative mutagenicity assays and enzyme kinetic measurements. This approach can provide new insight into the biochemistry of enzymes involved in the metabolic activation of mutagens.

In Salmonella typhimurium, a single enzyme catalyzes both the acetyl CoA-dependent O-acetylation of hydroxylamines (a key step in the activation of mutagenic nitroaromatic compounds and related aromatic and heterocyclic amines) and the N-acetylation of aromatic amines. S. typhimurium Ames test mutants lacking this activity are highly resistant to the genotoxic effects of nitro compounds. However, such mutants have not yet been obtained in Escherichia coli. We used a PCR-based method to engineer a null mutation (deletion) of the nhoA gene encoding the enzyme in E. coli and we transduced this mutation into a lacZ strain background suitable for use in mutation assays. In E. coli, as in S. typhimurium, nhoA mutants show marked resistance to nitro compound mutagenicity. The new strains provide a clean background for expression of recombinant N-acetyltransferases.

DS151 is the first 151 amino acids of the Dictyostelium discoidium protein severin, which shares high sequence similarity with segment 1 of the actin-severing protein gelsolin. DS151 is able to mediate the depolymerization of F-actin in a calcium-dependent fashion, much like segment 1 of gelsolin. A structural model of DS151 was obtained by comparative modeling studies with segment 1 of gelsolin. This model was tested by studies of chemical cross linking between DS151 and bound actin, suggesting that Cys residues on DS151 are cross linked with Lys residues of actin. The model suggests that Cys125 of DS151 cross links with either Lys326 or Lys328 of actin. Mutagenesis of DS151 demonstrates that Cys125 of DS151 dominates the cross linking, whereas Cys25 of DS151 makes a minor contribution through a longer-range cross link with Cys374 of actin, which likely involves flexibility of both proteins in that region.