Carbon black nanoparticles impair acetylation of aromatic amine carcinogens through inactivation of arylamine N-acetyltransferase enzymes.

Carbon black nanoparticles (CB NPs) and their respirable aggregates/agglomerates are classified as possibly carcinogenic to humans. In certain industrial work settings, CB NPs coexist with aromatic amines (AA), which comprise a major class of human carcinogens. It is therefore crucial to characterize the interactions of CB NPs with AA-metabolizing enzymes. Here, we report molecular and cellular evidence that CB NPs interfere with the enzymatic acetylation of carcinogenic AA by rapidly binding to arylamine N-acetyltransferase (NAT), the major AA-metabolizing enzyme. Kinetic and biophysical analyses showed that this interaction leads to protein conformational changes and an irreversible loss of enzyme activity. In addition, our data showed that exposure to CB NPs altered the acetylation of 2-aminofluorene in intact lung Clara cells by impairing the endogenous NAT-dependent pathway. This process may represent an additional mechanism that contributes to the carcinogenicity of inhaled CB NPs. Our results add to recent data suggesting that major xenobiotic detoxification pathways may be altered by certain NPs and that this can result in potentially harmful pharmacological and toxicological effects.

Cadmium alters the biotransformation of carcinogenic aromatic amines by arylamine N-acetyltransferase xenobiotic-metabolizing enzymes: molecular, cellular, and in vivo studies.

BACKGROUND: Cadmium (Cd) is a carcinogenic heavy metal of environmental concern. Exposure to both Cd and carcinogenic organic compounds, such as polycyclic aromatic hydrocarbons or aromatic amines (AAs), is a common environmental problem. Human arylamine N-acetyltransferases (NATs) are xenobiotic-metabolizing enzymes that play a key role in the biotransformation of AA carcinogens. Changes in NAT activity have long been associated with variations in susceptibility to different cancers in relation with exposure to certain AAs. OBJECTIVE: We explored the possible interactions between Cd and the NAT-dependent biotransformation of carcinogenic AAs. METHODS: We exposed purified enzymes, lung epithelial cells, and mouse models to Cd and subsequently analyzed NAT-dependent metabolism of AAs. RESULTS: We found that Cd, at biologically relevant concentrations, impairs the NAT-dependent acetylation of carcinogenic AAs such as 2-aminofluorene (2-AF) in lung epithelial cells. NAT activity was strongly impaired in the tissues of mice exposed to Cd. Accordingly, mice exposed to Cd and 2-AF displayed altered in vivo toxicokinetics with a significant decrease (~ 50%) in acetylated 2-AF in plasma. We found that human NAT1 was rapidly and irreversibly inhibited by Cd [median inhibitory concentration (IC50) ≈ 55 nM; rate inhibition constant (k(inact)) = 5 × 104 M⁻¹ • sec⁻¹], with results of acetyl coenzyme A (acetyl-CoA) protection assays indicating that Cd-mediated inhibition was due to the reaction of metal with the active-site cysteine residue of the enzyme. We found similar results for human NAT2, although this isoform was less sensitive to inactivation (IC50 ≈ 1 µM; k(inact) = 1 × 104 M⁻¹ • sec⁻¹). CONCLUSIONS: Our data suggest that Cd can alter the metabolism of carcinogenic AAs through the impairment of the NAT-dependent pathway, which may have important toxicological consequences.

The human xenobiotic-metabolizing enzyme arylamine N-acetyltransferase 1 (NAT1) is irreversibly inhibited by inorganic (Hg2+) and organic mercury (CH3Hg+): mechanism and kinetics.

Human arylamine N-acetyltransferase 1 (NAT1) is a xenobiotic-metabolizing enzyme that biotransforms aromatic amine chemicals. We show here that biologically-relevant concentrations of inorganic (Hg2+) and organic (CH3Hg+) mercury inhibit the biotransformation functions of NAT1. Both compounds react irreversibly with the active-site cysteine of NAT1 (half-maximal inhibitory concentration (IC50)=250 nM and kinact=1.4x10(4) M(-1) s(-1) for Hg2+ and IC50=1.4 microM and kinact=2x10(2) M(-1) s(-1) for CH3Hg+). Exposure of lung epithelial cells led to the inhibition of cellular NAT1 (IC50=3 and 20 microM for Hg2+ and CH3Hg+, respectively). Our data suggest that exposure to mercury may affect the biotransformation of aromatic amines by NAT1.