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Hoffmann, R. A wiki for the life sciences where authorship matters. Nature Genetics (2008)

Acetaminophen is an inhibitor of hepatic N-acetyltransferase 2 in vitro and in vivo.

Slow acetylators of the polymorphic N-acetyltransferase 2 (NAT2, EC suffer more often from side-effects of NAT-substrates than fast acetylators. Since concomitant administration of drugs may inhibit NAT2, we studied the influence of acetaminophen on NAT2 in human hepatic cytosol in vitro and in healthy individuals. In-vitro acetylation was assessed in liver homogenate of one fast and one slow acetylator using sulfamethazine as a test substrate. Acetaminophen competitively inhibited sulfamethazine acetylation in fast and slow acetylator liver samples with Ki values of acetaminophen of 2144 micromol/l and 712 micromol/l, respectively. In additional experiments, exposure of human liver cytosol to p-aminophenol, a putative precursor of acetaminophen in this reaction, revealed production of substantial amounts of acetaminophen, which indirectly suggests that acetaminophen may bind to the active site of NAT2. In-vivo acetylation was quantified with a urinary caffeine assay in 20 healthy volunteers at baseline and after repetitive oral administration of 1000 mg acetaminophen every 6 h for 1 day. The ratio of the acetylated caffeine metabolite acetyl-amino-6-formylamino-3-methyluracil to 1-methylxanthine was reduced by 30.9% (range 11.0-50.1%) in fast acetylators (n = 10) and by 19.3% (range 0.2-36.5%) in slow acetylators (n = 10). Acetaminophen, a widely used over-the-counter drug, which shares structural similarities with acetylated products, inhibits NAT2 both in vitro and in vivo. These findings suggest that even compounds which are not metabolized by NAT2 may inhibit the enzyme and reduce its metabolic capacity.[1]


  1. Acetaminophen is an inhibitor of hepatic N-acetyltransferase 2 in vitro and in vivo. Rothen, J.P., Haefeli, W.E., Meyer, U.A., Todesco, L., Wenk, M. Pharmacogenetics (1998) [Pubmed]
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