4-Hydroxyestradiol is conjugated with thiols primarily at C-2: evidence from regiospecific displacement of tritium by rat liver microsomes or tyrosinase.

4-Hydroxyestradiol bearing a 3H label specifically at C-2 was prepared chemically and incubated with male rat liver microsomes or mushroom tyrosinase. A very high proportion (80-90%) of the 3H was displaced from the labeled steroid when either glutathione or N-acetylcysteine was present, and tyrosinase was shown not to require NADPH as cofactor for this reaction. In either case, only negligible amounts (less than 3%) of the 3H radioactivity were found associated with water-soluble adducts in contrast to 3H-labeled 2-hydroxyestradiol, which gave rise to about 25% of such products. The effect of ascorbic acid on the microsomal reaction with regiospecifically labeled estradiol, 2-hydroxyestradiol, and 4-hydroxyestradiol was also investigated, and the results are discussed in terms of the reactivity at different carbon atoms in ring A of the catechol estrogens. All the evidence points to conjugation of 4-hydroxyestradiol with glutathione or N-acetylcysteine at C-2 but not C-1 of this highly reactive catechol estrogen. Measuring the displacement of 3H as 3H2O from specific positions in the steroid ring provides a useful and sensitive method to assess the formation of adducts in cases where their isolation and characterization is particularly difficult.