Dihydrodiol dehydrogenase (EC 1.3.1.20) catalyzes the
NADP+-dependent oxidation of a variety of trans-
dihydrodiol proximate
carcinogens, a reaction that may suppress their carcinogenicity. Using benzenedihydrodiol [(+)-trans-1,2-dihydroxy-3,5-
cyclohexadiene] as a substrate, this
enzyme can be detected spectrophotometrically in rat H-4IIe
hepatoma cells with a specific activity similar to that observed in rat liver cytosol. The
hepatoma cell
enzyme is potently inhibited by 6-medroxy-progesterone
acetate (IC50 = 38 nM) and
indomethacin (IC50 = 3.5 microM). These cells contain
3 alpha-hydroxysteroid dehydrogenase which is also sensitive to inhibition by the same two drugs. Chromatofocusing of
hepatoma cell lysates indicates that both
dihydrodiol dehydrogenase and
3 alpha-hydroxysteroid dehydrogenase activities coelute with a pI = 5.8. Western blot analysis of
hepatoma cell lysates, using rabbit anti-rat 3 alpha-hydroxy-
steroid/
dihydrodiol dehydrogenase serum detects a single immunoreactive species with a Mr 34,000. Using this antiserum it was possible to immunotitrate both these
enzyme activities in H-4IIe lysates. Exposure of confluent cells to either 10 microM
benz[a]anthracene or 10 microM
dexamethasone, which are known inducers in H-4IIe cells of aryl-
hydrocarbon hydroxylase and
tyrosine aminotransferase respectively, failed to elevate
dihydrodiol dehydrogenase activity. The following agents also failed to induce
dihydrodiol dehydrogenase activity:
phenobarbital,
ethoxyquin, phenolic
anti-oxidants,
testosterone,
estradiol-17 beta, and
growth hormone. Since the
hepatoma cell
enzyme has properties in common with the purified rat liver
enzyme (which is identical to
3 alpha-hydroxysteroid dehydrogenase) including, Mr, pI, immunoreactivity, and sensitivity to
drug inhibition, this cell line represents a useful system for studying the role of
dihydrodiol dehydrogenase in the further metabolism of trans-dihydrodiols. Interestingly, the
enzyme does not appear to be under the control of known inducers of phase I and phase II
drug metabolizing
enzymes.