Structure-activity relationships in polycyclic aromatic hydrocarbons: induction of microsomal aryl hydrocarbon hydroxylase and its possible importance in chemical carcinogenesis

Aminopyrine-N-demethylase. I. Directed modification of substrates' structure as a way of production of inducer of the monooxygenase isoform P-450b

The effect of the phenobarbital-type monooxygenase inducers is accomplished via the native molecule. This made it possible to transform the typical substrate of cytochrome P-450b aminopyrine into an inducer of this isozyme by blocking the substrate molecule position undergoing monooxygenation. Substitution of two methyl groups in the aminopyrine -N(CH3)2 position by an isopropyl group gave rise to a clear-cut inducive effect. This was registered by spectral, kinetic, radiological and immunochemical methods.

Molecular orbital calculations and quantitative structure-activity relationships for some polyaromatic hydrocarbons

1. Correlations between biological activity and electronic structure for 7 polyaromatic hydrocarbons are reported. 2. Molecular orbital (MO) calculations by the MINDO/3 method indicate that the hydrophobic parameter, log P (the logarithm of the octanol/water partition coefficient) exhibits a parallelism with total electrophilic superdelocalizability, and a similar relationship is shown with protein binding. 3. Inhibition of dimethylnitrosamine (DMN) demethylase activity is linearly related to total nucleophilic superdelocalizability, and other correlations are shown with mutagenicity and benzo[a]pyrene hydroxylase (AHH) activity.

Induction of aryl hydrocarbon hydroxylase in rat tissue following intratracheal instillation of diesel particulate extract and benzo[a]pyrene

The potential for aryl hydrocarbon hydroxylate (AHH) induction by inhaled diesel particles was assessed by intratracheal administration. Benzo[a]pyrene, (B[a]P) a reference compound, or an extract of particles, dissolved in gelatin-saline solution was administered to Fischer 344 rats at several dose levels. Twenty-four hours after administration of B[a]P or diesel particulate extract, the AHH activity increased in a dose-response manner in the lung, but not in liver. The maximum increase in the AHH activity in the lung was observed 12 h after intratracheal instillation of B[a]P (5 mg/kg), and the levels remained elevated for seven days. The AHH activity in the liver reached the maximum 24 h after the administration, and returned rapidly to control values. In contrast, intratracheal instillation of diesel particulate extract resulted in a significant increase of AHH activity in the lungs only after doses greater than 6 mg kg-1. The activity, however, declined rapidly and returned to control values within 75 h. The liver AHH activity in this instance, remained unchanged throughout the experimental period. These data indicate that in the lung, hydrocarbons extracted from diesel particles are weak enzyme inducers and exposure to these compounds by intratracheal administration did not result in AHH induction in the liver. The results suggest that doses higher than those normally expected from occupational exposures will be required to induce AHH activity in organs examined.

Aryl hydrocarbon hydroxylase activity induced by injected diesel particulate extract vs inhalation of diluted diesel exhaust

The effects of long-term inhalation of diluted diesel exhaust on aryl hydrocarbon hydroxylase activity (AHH) and cytochrome P-450 content in lung and liver microsomes were investigated in male Fischer-344 rats and compared with repeated parenteral administration of organic solvent extracts of hydrocarbons adsorbed on the diesel particulate surface during the combustion process. The animals were exposed to concentrations of 750 micrograms m-3 or 1500 micrograms m-3 of diesel particulates from a 5.7L GM diesel engine 20 h per day, 5 1/2 days per week for up to 9 months or treated by repeated IP injections of diesel particulate extract (dissolved in corn oil) from the same engine at several dose levels for 4 days. No significant effects of long-term inhalation exposure were observed in liver microsomal AHH activity. A slight decrease in lung microsomal AHH activity was found in rats following 6 months of exposure to diesel exhaust at the particulate concentration of 1500 micrograms m-3. The total mass of particles deposited in the lung during the inhalation exposure was estimated and an equivalent dose of extractable hydrocarbons was administered intraperitoneally; no increase in AHH activity was observed in the lung or liver microsomes. In contrast, 1.4- to 9-fold increases in AHH activity were observed in liver and lung microsomes of rats pretreated by intraperitoneal doses 10-50 times larger than the most conservative estimate of the deposited lung burden. No changes in cytochrome P-450 content were observed in the microsomes of rat liver after inhalation or injection treatment studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Inducibility of drug-metabolizing enzymes in the rat skin

The influence of systemic application of chemically unrelated drugs and xenobiotics (phenobarbital (PB), rifampicin (Rifa), pregnenolone-16 alpha-carbonitrile (PCN) and 3-methylcholanthrene (MC) on drug metabolizing enzymes was investigated in the skin of rats of both sexes. The results were compared with those obtained in the liver. PB, Rifa and PCN induced the aryl hydrocarbon hydroxylase (AHH) in the skin though to a lesser extent than MC. The basal enzymic activity as well as the inducibility were considerably lower in the skin than in the liver. The activity of cytochrome c reductase was similar in all rats independently from the pretreatment. Only slight sex differences were noted: generally, the activity was higher in males. Cytochrome P-450 content of the skin could not be detected as well as various dealkylation activities.

Regulation of the binding of 7,12-dimethylbenz[a]-anthracene to DNA of cultured murine epidermal cells at metabolic level: competition of the binding by polycyclic aromatic hydrocarbons and by other precarcinogens

The binding of labeled carcinogen [3H]DMBA to murine epidermal cells (MEC) DNA in culture has been studied. The influence of unlabeled noncarcinogenic and carcinogenic polycyclic aromatic hydrocarbons (PAH), several PAH metablites, and various directly and indirectly acting non-PAH carcinogens on the binding of [3H]DMBA to MEC DNA has been examined. All the carcinogenic PAH and some of non-carcinogenic PAH effectively inhibit the binding of [3H]DMBA to MEC DNA. The non-PAH chemical carcinogens requiring metabolic activation also reduce the binding of labeled DMBA to MEC DNA; however, a higher concentration of these compounds is required for 50% inhibition of binding than the concentrations of PAH for the same degree of inhibition of binding of [3H]DMBA to MEC DNA. The directly acting carcinogens do not significantly inhibit the binding of [3H]DMBA to DNA. The relationship between structures of PAH and their ability to inhibit the binding of [3H]DMBA to MEC DNA is also discussed. Thus, it appears that the binding of DMBA to cellular DNA is primarily controlled at a level of metabolism and to some extent at the level of binding of reactive metabolites to DNA.

Michaelis--Menten kinetic analysis of the hepatic microsomal benzpyrene hydroxylase from control, phenobarbital- and methyl-3-cholanthrene-treated rats

The sensitive fluorimetric assay for hydroxy-3-benzpyrene (3-OH-BP) described by Dehnen et al., was used to study the effect of microsomal membrane concentration of the benzpyrene hydroxylase activity. Microsomes from phenobarbital (PB) and methyl-3-cholanthrene (3-MC)-treated rats were used in comparison with the microsomal fraction from control animals. At very low protein concentration, benzpyrene hydroxylase follows as Michaelis--Menten type kinetics. When the concentration of microsomal membrane is higher than a minimal value (+/- 6 mug protein/ml) the Km increases with increasing concentration of protein due to competitive inhibition by reversible and non-specific binding of the substrate. The Ki's for such a binding have been calculated. Pretreatment of rats with 3-MC selectively shortens the time linearity, decreases the Ks value, and has no effect on Vmax, while the administration of PB prolongs the time linearity, decreases Vmax and does not modify the Ks. 3-MC and PB specifically act on cytochrome P-450 and do not modify the physico-chemical properties of the microsomal membrane as measured by the non-specific binding of benzpyrene (BP).