Epoxide hydrolase in the bulb mite Rhizoglyphus robini: properties and induction

Using styrene oxide as substrate, most of the epoxide hydrolase (EH) activity monitored in the bulb mite Rhizoglyphus robini was associated with the microsomal compartment. The microsomal and cytosolic EHs did not display any significant preference in hydrating trans stilbene oxide (TSO) and cis stilbene oxide (CSO). The microsomal EH, which has a Km value of 5×10^-5M and pH optimum of 7.8, was sensitive to ethanol and its activity was inhibited to a moderate extent by 4-fluorochalcone oxide, TSO, CSO and trans-chalcone oxide at a level of 10^-4M. Microsomal EH was considerably induced (4–5-fold) in mites feeding garlic and onion, or ingesting TSO-impregnated filter papers. Other epoxides like CSO, 2,4-dichlorostilbene oxide, methyl chalcone oxide and heptachlor epoxide displayed moderate induction levels (1.4–2.6-fold). Of the toxicants assayed only sodium phenobarbital was a potent inducer. Lindane, malathion and DDT did not stimulate EH activity and 3-methyl-cholanthrene was even inhibitory. A decrease in EH activity was observed with a number of phytochemicals tested such as sinigrin, flavone, menthol, trans--carotene, chalcone, allyl sulphide and trans-cinnamic acid.