Catechol formation of fluoro- and bromo-substituted estradiols by hamster liver microsomes. Evidence for dehalogenation.

We have examined the validity of using fluorine-substituted estrogens as probes to assess the significance of 2- and 4-hydroxylation in estrogen-induced carcinogenesis in the hamster. Liver microsomes from castrated hamsters were incubated with 2-fluoro-, 4-fluoro-, or 2,4-difluoroestradiols and analogous bromo-substituted estradiols to determine the extent of 2- and 4-hydroxylation with these substrates. Estrogen 2- and 4-hydroxylase activity was determined by radioenzymatic assay, and the 3H-labeled monomethyl ether products were identified by high performance liquid chromatography. With unsubstituted 17 beta-estradiol as substrate, 97% of the product formed was 2-hydroxylated, and 3% was 4-hydroxylated. The monosubstituted fluoroestradiols exhibited more than a 2-fold enhanced ability to form catechol estrogens compared with their corresponding bromoestradiols. Data presented herein indicate substantial defluorination when 2-fluoroestradiol was the substrate, which amounted to 36% of the total product formed, and 32% of the rate of 2-hydroxylation found with unsubstituted 17 beta-estradiol as substrate. Interestingly, the rate of 4-hydroxylation was elevated 20- and 6.7-fold, respectively, when 2-fluoroestradiol and 2,4-difluoroestradiol were the substrates compared to the rate with 17 beta-estradiol. Moreover, both 4-fluoroestradiol and 2,4-difluoroestradiol exhibited at least a 1.6-fold greater rate of 2-hydroxylation compared with 17 beta-estradiol. In contrast, the rate of dehalogenation with 2-bromoestradiol was only 12% of that found with 2-fluoroestradiol. No debromination was obtained with 4-bromoestradiol, and essentially no catechols were formed using 2,4-dibromoestradiol as substrate with these hamster liver microsomes. These data clearly provide evidence for defluorination of these substituted estrogens, particularly at the C-2 position, and seriously hamper the use of fluoroestrogens in studies of hormonal carcinogenicity.