Comparison of the biotransformation of 1,3-butadiene and its metabolite, butadiene monoepoxide, by hepatic and pulmonary tissues from humans, rats and mice

Abstract

1,3-Butadiene (BD), a widely used monomer in the production of synthetic rubber and other resins, is one of the 189 hazardous air pollutants identified in the 1990 Clean Air Act Amendments. BD induces tumors at multiple organ sites in B6C3F1 mice and Sprague-Dawley rats; mice are much more susceptible to the carcinogenic action of BD than are rats. Previous in vivo studies have indicated higher circulating blood levels of butadiene monoepoxide (BMO), a potential carcinogenic metabolite of BD, in mice compared to rats, suggesting that species differences in the metabolism of BD may be responsible for the observed differences in carcinogenic susceptibility. The metabolic fate of BD in humans is unknown. The objective of these studies was to quantitate in vitro species differences in the oxidation of BD and BMO by cytochrome P450-dependent monooxygenases and the inactivation of BMO by epoxide hydrolases and glutathione S-transferases using microsomal and cytosolic preparations of livers and lungs obtained from Sprague-Dawley rats, B6C3F1 mice and humans. Maximum rates for BD oxidation (V_max) were highest for mouse liver microsomes (2.6 nmol/mg protein/min) compared to humans (1.2) and rats (0.6). The V_max for BD oxidation by mouse lung microsomes was similar to that of mouse liver but > 10-fold higher than the V_max for the reaction in human or rat lung microsomes. Correlation analysis revealed that P450 2E1 is the major P450 enzyme responsible for oxidation of BD to BMO. Only mouse liver microsomes displayed quantifiable rates for metabolism of BMO to butadiene diepoxide (V_max = 0.2 nmol/mg protein/min), a known rodent carcinogen. Human liver microsomes displayed the highest rate of BMO hydrolysis by epoxide hydrolases. The V_max in human liver microsomes ranged from 9 to 58 nmol/mg protein/min and was at least 2-fold higher than the V_max observed in mouse and rat liver microsomes. The V_max for glutathione S-transferase-catalyzed conjugation of BMO with glutathione was highest for mouse liver cytosol (500 nmol/mg protein/mm) compared to human (45) or rat (241) liver cytosol. In general, the K_Ms for the detoxication reactions were 1000-fold higher than the K_Ms for the oxidation reaction. Because of the low solubility of the BD and the relatively high K^M for oxidation, it is likely that the V_max/K_M ratio will be important for BD and BMO metabolism in vivo. In vivo clearance constants were calculated from in vitro data for BD oxidation and BMO oxidation, hydrolysis and GSH conjugation. The overall activation/detoxication ratio was markedly different for mice (72), rats (5.8) and humans (5.9) and suggest that at concentrations below the K_M for the reactions, mice have a significantly higher ratio of activation/detoxication than do either rats or humans. The differences in the activation/detoxication ratios between mice and rats correlate with the higher carcinogenic sensitivity of mice than rats to BD.