Molecular characteristics of carbaryl, a CYP1A1 gene inducer.

Carbaryl belongs to a series of compounds that activate the CYP1A1 gene. This study demonstrates the inability of carbaryl to compete with 2,3,7,8-tetrachlorodibenzo-p-dioxin for binding to the rat aryl hydrocarbon (dioxin) receptor. Structural and physicochemical properties of this insecticide, in relation to the requirements for binding to the aryl hydrocarbon receptor, are described. The crystal structure was determined experimentally using X-ray diffraction. A conformational search using molecular mechanics was performed by means of a Monte-Carlo-type method and a stochastic dynamics simulation. Lipophilicity calculations, log P, and molecular lipophilicity potential are also presented. Common and discriminating properties of carbaryl and aryl hydrocarbon receptor ligands are discussed.

Hepatic Ah receptor binding affinity for 2,3,7,8-tetrachlorodibenzo-p-dioxin: similarity between beagle dog and cynomolgus monkey.

Hepatic AhR binding affinity for [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin ([3H]TCDD) was compared between two species widely used as laboratory animals: beagle dog and cynomolgus monkey (Macaca fascicularis). The enriched 9S fractions from both species were obtained by sucrose gradient sedimentation. After incubation with [3H]TCDD, dextran-coat charcoal treatment (10 mg/ml) revealed that dog and monkey possess an AhR with a low binding affinity for [3H]TCDD. Saturation experiments were then achieved according to the method developed in experiments on human samples. The binding characteristics were determined after analysis of the data by Scatchard and Woolf plots. Receptor concentrations were quite similar in dog and monkey liver (26.6 and 14.4 pmol/mg, respectively) as well as the affinity (Kd) for [3H]TCDD (17.1 and 16.5 nM, respectively). The low binding affinity of dog and monkey AhRs appeared to be similar to those observed in human.

Effects of carbaryl and naphthalene on rat hepatic CYP1A1/2: potential binding to Ah receptor and 4S benzo(a)pyrene-binding protein.

Activation of the CYP1A1 gene has been described to be mediated by the cytosolic Ah receptor (AhR) and a possible cooperative role of the 4S benzo(a)pyrene-binding protein (4S protein). Carbaryl (CAR) has been shown to induce human CYP1A1 gene expression without binding to the human AhR. In this study, Sprague-Dawley rats received a single i.p. dose of 20, 80, 150 micromol/kg CAR or NAPn (naphthalene, the aromatic part of CAR) and were sacrificed after 24 h. CAR increased ethoxyresorufin-O-deethylase and methoxyresorufin-O-demethylase activities, the level of CYP1A1, 1A2 proteins, and CYP1A1 mRNA at the highest dose, whereas NAPn showed no effects. Moreover, CAR, naphthol (its major metabolite) and NAPn were not ligands in vitro of the TCDD binding site of AhR or the benzo(a)-pyrene binding site of 4S protein in rat, neither was CAR a ligand of these two binding sites in mice, dog, monkey or human. Molecular properties of CAR were evaluated and showed that this molecule is far from the structural characteristics of CYP 1A1 specific inducers although a planar conformation can be achieved with an energy < 5 kJ x mol(-1). The data demonstrated that CAR could also modulate the AhR-mediated responses, even though it did not meet the structural requirements to be ligand of AhR.