Crystal structure of arylamine N-acetyltransferases: insights into the mechanisms of action and substrate selectivity.

INTRODUCTION: Arylamine N-acetyltransferases (NATs) are polymorphic xenobiotic metabolizing enzymes catalyzing the acetylation of aromatic amine chemicals of pharmacological/toxicological relevance (drugs, carcinogens). NATs are primordial determinants of the detoxification and/or bioactivation of these compounds. These enzymes are found in prokaryotes and eukaryotes. Several NAT isoenzymes may be present in one organism, and their substrate specificity profile and pattern of tissue expression suggest distinct functional roles. AREAS COVERED: Many advances in NAT mechanism, substrate specificity, and functional impact of polymorphism have come from crystallographic and NMR studies. To date, the crystal structures of 10 different NAT homologues have been solved, including two human isoforms and several bacterial NATs. The authors present the most recent snapshot in NAT structure differences and similarities. The authors also depict the structural bases of substrate/inhibitor recognition and specificity, cofactor binding, catalytic mechanism, genetic regulation (polymorphism), and enzyme inhibition. EXPERT OPINION: The determination of other NATs structures will help to develop specific inhibitors of NAT enzymes with potential clinical relevance. In addition, it will contribute to the identification of endogenous substrates and novel functions associated to this family of enzymes.

Metabolism by N-acetyltransferase 1 in vitro and in healthy volunteers: a prototype for targeted inhibition.

Inhibition of drug metabolism is generally avoided but can be useful in limited circumstances, such as reducing the formation of toxic metabolites. Acetylation is a major pathway for drug elimination that can also convert substrates into toxic species, including carcinogens. Sulfamethoxazole, a widely used antibiotic, is metabolized via arylamine N-acetyltransferase 1. p-Aminosalicylate, used for antitubercular treatment, is also metabolized by N-acetyltransferase 1 and could potentially inhibit sulfamethoxazole metabolism. Human hepatocytes from 4 donors were incubated in vitro with sulfamethoxazole and paminosalicylate at clinically achievable concentrations. p-Aminosalicylate competitively reduced the acetylation of sulfamethoxazole in vitro by 61% to 83% at 200 microM. Four healthy volunteers were studied following doses of 500 mg sulfamethoxazole either alone or during administration of paminosalicylate (4 g ter in die). Plasma concentrations of paminosalicylate exceeded 100 microM. With each subject as his or her own control, p-aminosalicylate reduced by 5-fold the ratio of plasma concentrations of acetylsulfamethoxazole relative to parent drug (P < .001). Metabolic drug-drug interaction studies in vitro successfully predicted inhibition of acetylation via N-acetyltransferase 1 in vivo. Although no specific toxic species was investigated in this work, the potential was demonstrated for improving the therapeutic index of drugs that have toxic metabolites.

Pharmacogenetics and development: are infants and children at increased risk for adverse outcomes?

Over the past two decades, pharmacokinetic data have clearly demonstrated that development can markedly influence the absorption, distribution, excretion, and metabolism of xenobiotics. With respect to many of the processes that govern drug metabolism, the underlying pharmacogenetic determinants that may control either the affinity or the capacity of a drug or toxicant substrate for the enzymes responsible for its biotransformation appear to be altered as a function of development by mechanisms that are, for the most part, not well defined. Nonetheless, for many xenobiotics, the pharmacogenetic-developmental interface produces a "pattern" for drug metabolism that, when characterized, supports the pharmacokinetic properties (eg, drug clearance) reported for many agents across the pediatric age spectrum. With the exception of a few relatively well-characterized adverse drug effects (eg, toxicity of 6-mercaptopurine in patients with absent thiopurine methyltransferase activity, increased incidence of hepatotoxicity to valproic acid in young infants), the relationship of development and pharmacogenetics to enhanced toxicity risk from xenobiotic exposure is poorly defined. However, failure to adequately appreciate the pharmacokinetic consequences of the pharmacogenetic-developmental interface and to individualize therapy accordingly may lead to a clinically significant risk of drug therapy, namely, over- or underdosing.