Species comparison of acrylonitrile epoxidation by microsomes from mice, rats and humans: relationship to epoxide concentrations in mouse and rat blood.

Acrylonitrile (ACN) has been shown to cause tumors of the brain, stomach and Zymbal's gland in rats in several bioassays, but it has not been tested in other species. The carcinogenic risk of humans exposed to ACN is unclear. ACN is metabolized in the liver to 2-cyanoethylene oxide (CEO), which is believed to be the proximate or ultimate carcinogenic species. Therefore, the kinetics of CEO formation were studied with liver and lung microsomes from mice and humans using a GC-MS assay for CEO, and the data were compared with previously obtained kinetic parameters for rat microsomal enzymes. The rate of CEO formation by human liver microsomes was comparable to that of rat liver microsomes, but less than that of mouse liver microsomes. Liver microsomes produced more CEO than lung microsomes with all three species. CEO formation by microsomes from mice was approximately 4 times greater than that by microsomes from rats or humans, suggesting that mice would have higher CEO concentrations in blood than rats after ACN exposure. However, after oral administration of ACN, the concentration of CEO in mouse blood was one-third that in rat blood at all doses and time points examined. These results show that CEO circulates via the blood, providing exposure to distant sites. The blood concentrations of CEO do not appear to correlate with rates of microsomal CEO formation. This suggests that species differences in the detoxication of CEO may play an important role in determining circulating CEO concentrations and distant organ exposure.