59 Mutation Research, 79 (1980) 59--71 © Elsevier/North-Holland Biomedical Press THE EFFECT OF HEPATIC MICROSOMAL AND CYTOSOLIC SUBCELLULAR FRACTIONS ON THE MUTAGENIC ACTIVITY OF EPOXIDE-CONTAINING COMPOUNDS IN THE SALMONELLA ASSAY MOSTAFA A. EL-TANTAWY Zagazig University, Faculty of Agriculture, Plant Protection Department, Zagazig (Egypt) and BRUCE D. HAMMOCK Division of Toxicology and Physiology, Department of Entomology, University of California, Riverside, CA 92521 (U.S.A.) (Received 5 October 1979) (Revision received 24 January 1980) Accepted 1 April 1980) Summary 7 epoxide-containing compounds: allylbenzene oxide, styrene oxide, trans-fi- methylstyrene oxide, 4-chlorophenyl glycidyl ether, vinylcyclohexene dioxide, octene dioxide and hexene dioxide were evaluated for mutagenic activity in 4 histidine-requiring strains of Salmonella typhimurium, namely: TA1535, TA100, TA1537 and TA98. These epoxides, except trans-~-methylstyrene ox- ide, were mutagenic in TA1535 and TA100 but none of the tested compounds caused mutations in strains TA1537 and TA98. Both the cytosolic (100 000 g soluble) and/or microsomal (100 000g pellet) fractions derived from non- induced mouse, guinea pig, and/or rat consistently decreased the mutagenic activity of the 3 most active mutagens: allylbenzene oxide, styrene oxide and 4-chlorophenyl glycidyl ether. This reduction was found to depend on the substrate and the source of the enzyme fraction. Glutathione alone or in com- bination with the mouse cytosolic fraction resulted in negligible suppression in the mutagenic activity of the 3 epoxides under the conditions reported in this paper. The enzyme(s) in the cytosol responsible for the reduction in muta- genicity co-eluted from gel filtration with the epoxide hydrolase activity. These data are not consistent with the assumption that all epoxide hydrolase activity in an "$9" fraction is microsomal. The use of in vitro microbial mutagenesis assays either alone or coupled with metabolic activating enzymes is of great importance since there is a close cor- relation between mutagenicity and carcinogenicity [2--4,27--29]. Electrophilic