Objectives: Human N-acetyltransferase 2 (NAT2) plays a significant role in the clearance and biotransformation of many drugs and carcinogens. A TC (Ile114Thr) single nucleotide polymorphism (SNP) of NAT2 is commonly found in slow acetylators, leading to altered drug response and toxicity and possibly cancer susceptibility from carcinogens. The objective of this study was to investigate the mechanism by which this SNP causes slow acetylator phenotype.
Methods: A cDNA expression system was used to compare the NAT2*4 reference allele with an identical one possessing the TC SNP in COS-1 cells. The recombinant human NAT2 enzymes were compared in regard to catalytic activity, kinetic parameters, thermostability, immunoreactive protein level, mRNA level and in-vivo protein degradation.
Results: The TC (Ile114Thr) SNP significantly reduced enzyme activity without changing the apparent kinetic parameters Km and Vmax (normalized for NAT2 protein), indicating that Ile114Thr did not change substrate or cofactor binding affinities or catalytic efficiency. Furthermore, no significant difference in NAT2 mRNA level was observed, indicating no impairment of transcription. The TC (Ile114Thr) SNP did not alter thermostability of NAT2 at either 37 or 50 degrees C. However, this SNP significantly reduced cytosolic NAT2 immunoreactive protein through enhanced protein degradation.
Conclusion: This is the first report indicating that protein degradation is an important mechanism of human NAT2 slow acetylator phenotype.