Species differences in the metabolism of olefins: implications for risk assessment

Olefinic compounds are commercially valuable because they form useful polymeric substances. The same chemical property (presence of double bonds) that makes the olefins useful may also cause them to be toxic in the body. The double bonds of olefins can be oxidized by cytochrome P450 enzymes to epoxides, which are electrophiles that can react with DNA and may cause alterations in the genetic information carried by that macromolecule. Epoxides can be rendered inactive toward DNA by binding to proteins, by hydrolysis to diols through epoxide hydrolase enzymes (EHs), or by forming conjugates with glutathione via glutathione S-transferase (GST) activities. The balance between the oxidizing enzymatic activities and the hydrolyzing or conjugating enzymatic activities in the livers of different species can influence the potential toxicity of the olefins. The location of the enzymes and the potential for concerted reactions in which epoxides are inactivated immediately after formation will also influence the potential toxicity of the olefins. Cytochrome P450 enzymes and EHs are in microsomes located in the rough endoplasmic reticulum surrounding the nucleus where the DNA is located. GST is in the cytoplasm of the cell. In the case of 1,3-butadiene (BD), such enzymatic differences may strongly influence the toxicity in different species. The mouse, in which BD is a potent multi-site carcinogen, has the lowest microsomal EH activity of any species. This allows the monoepoxides formed in the microsomes by cytochrome P450 enzymes to be further oxidized to the highly genotoxic diepoxide (DEB), and both epoxides can either be released into the blood for distribution throughout the body or can react with DNA in the nucleus. The rat, in which BD is a weak carcinogen, has much higher levels of microsomal EH, and only trace amounts of DEB enter the bloodstream. Major BD metabolites in primates suggest that the hydrolysis pathway is even more prominent in primates than in rats. Data suggest that BD will be much less toxic in primates than in mice. Considering these quantitative differences in metabolism may help to reduce the uncertainties in extrapolating animal data on olefin toxicity to health risk assessments for humans exposed to the compounds.