The mutagenicity of r-8,t-9-dihydroxy-t-10, 11-oxy-8,9,10,11-tetrahydrobenz[a]anthracene (BA-8,9-diol 10, 11-oxide) toward Salmonella typhimurium TA 100 is not decreased by the presence of large amounts of highly purified microsomal or cytosolic epoxide hydrolase. However, highly purified dihydrodiol dehydrogenase inactivates this diol epoxide, which is a major DNA-binding metabolite of benz[a]anthracene. The K-region epoxide, benz[a]anthracene 5,6-oxide (BA 5,6-oxide) is efficiently inactivated by microsomal epoxide hydrolase, is much less readily inactivated by cytosolic epoxide hydrolase, and is not inactivated by dihydrodiol dehydrogenase. This inactivation of a diol epoxide by dihydrodiol dehydrogenase points to a new significance of this enzyme and a new level of control for diol epoxides.

A method for the extensive purification of rabbit liver cytoplasmic epoxide hydrolase is described. The endproduct, which was purified 550-fold with respect to the cytosolic fraction, appeared to be more than 85 ' %, pure. Results indicate that the enzyme is a dimer of molecular weight I10000 and consists of two subunits,which are identical or very similar ( M , 57000) and possess N-terminal serine. Evidence for the existence of aggregates of higher molecular weight was also obtained. The catalytic properties of the cytoplasmic enzyme with styrene oxide as substrate ( K , = 3.4 m M ; V = 3480 nmol mg-' min-') differed markedly from the values published for the microsomal hydrolase.