Endogenous role of microsomal epoxide hydrolase. Ontogenesis, induction inhibition, tissue distribution, immunological behaviour and purification of microsomal epoxide hydrolase with 16 alpha, 17 alpha-epoxyandrostene-3-one as substrate.

The specific activities of microsomal epoxide hydrolase with 16 alpha, 17 alpha-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 16 alpha, 17 alpha-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 greater than microsomes approximately equal to rough endoplasmic reticulum much greater than 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 phenobarbital, 3-methylcholanthrene. Arochlor 1254 and trans-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.