Endogenous Role of Microsomal Epoxide Hydrolase
Authors: VOGEL‐BINDEL Ulla , BENTLEY Philip , OESCH Franz 
Published: July 2010

The specific activities of microsomal epoxide hydrolase with 16α,17α‐epoxyandrosten‐3‐one (androstene oxide) as substrate were measured in various metabolically important and in various steroidogenic organs of the male and female rat and compared with the activities of 16α,17α‐epoxyestratrienol (estroxide) and benzo[a]pyrene 4,5‐oxide. Androstene oxide was an exceptionally good substrate. The specific activities differed widely between organs but the ratio of the activities towards these substrates was constant in all organs investigated. The ratios compared to benzo[a]pyrene 4,5‐oxide were 2.5 for estroxide, and 8.6 for androstene oxide. The ontogenetic development of specific epoxide hydrolase activity in the livers of both sexes reached a maximum at about day 40 and descended to the adult enzyme level at about 45 days in males and clearly later in females. While in the livers and ovaries significant increases of the enzyme activity with increasing age took place before day 28, the specific activity remained very low in the testis until day 28 and then rose suddenly. During all these differential developments no significant changes in the ratios of activities towards the three substrates were observed.

The specific activity of epoxide hydrolase towards these substrates in subcellular fractions of the rat liver was smooth endoplasmic reticulum > microsomes ∼ rough endoplasmic reticulum ≫ mitochondria, no activity was detectable in cytosol. The ratio of the activities in the different fractions was similar when measured with androstene oxide, estroxide and styrene oxide as substrates. Microsomal hydrolysis responded to pretreatment of animals with phenobarbetal, 3‐methylcholanthrene, Arochlor 1254 andtrans‐stilbene oxide in a manner which was characteristically different for the various agents but similar for the three substrates.

Microsomal epoxide hydrolase which was purified to apparent homogeneity was able to hydrolyse the steroid epoxides, but the apparent purification factors were different for the different substrates: 77 for styrene oxide, 45 for estroxide, and 10 for androstene oxide. The three substrates mutually inhibited their hydrolysis by the microsomal fraction. Some differences in the extent of their effect and in the inhibition of the activities by known epoxide hydrolase inhibitors were observed. Similarly, hydrolysis of the steroid epoxides but not of styrene oxide was inhibited by nonionic detergents (Cutscum, Triton X‐100 and Emulgen 911). These differences could be due to the presence of different enzymes or a single enzyme, the conformational requirements of which are much more demanding for steroid epoxides than for xenobiotic epoxides.

Mono‐specific antiserum precipitated epoxide hydrolase activity from solubilized microsomes with dose‐response curves which were not distinguishable for androstene oxide, estroxide, benzo[a]pyrene 4,5‐oxide and styrene oxide as substrates. At higher concentrations precipitation of activity towards all four substrates was complete.

Thus, the hydrolysis of these endogenous and xenobiotic substrates is catalyzed by a single enzyme or by several enzymes which cannot be distinguished by any of the criteria tested. This implies that in either case modulationsin vivoof microsomal epoxide hydrolase must be expected to be accompanied by changes in the activity towards these steroid epoxides.