In Vitro Generation of Peroxynitrite by 2- and 4-Hydroxyestrogens in the Presence of Nitric Oxide

Estrogen metabolism is altered in most, if not all, breast cancer tumors. These alterations primarily lead to the formation of the catechol estrogen metabolites, 2- and 4-hydroxyestrogens, which can generate superoxide anion radicals (O2 •-) through the redox cycling of semiquinone/quinone derivatives. In breast cancer cells, the activity of nitric oxide synthase is also frequently elevated, resulting in an increased level of exposure to nitric oxide (•NO). Since •NO rapidly reacts with O2 •- to produce the peroxynitrite anion (ONOO-), this study was undertaken to determine whether ONOO- can be generated when 2- and 4-hydroxyestrogens are incubated in vitro with •NO donor compounds. Using dihydrorhodamine 123 as a specific probe for ONOO- formation, a ratio of 100 μM dipropylenetriamine NONOate (DPTA/NO) to 10 μM 4-hydroxyestradiol (4-OHE2) gave an optimal ONOO- production of 11.9 ± 1.9 μM (mean ± SD). Quantification of ONOO- was not modified by mannitol, supporting the idea that the hydroxyl radical was not involved. This production of ONOO- required the presence of the catechol structure of estrogen metabolites since all methoxyestrogens that were tested were inactive. Hydroxyestrogen metabolites derived from estradiol showed the same efficiency in producing ONOO- as those originating from estrone. With DPTA/NO, the 4-hydroxyestrogens generated 30−40% more ONOO- than the 2-hydroxyestrogens. Optimal production of ONOO- was assessed with DPTA/NO and diethylenetriamine NONOate (initial •NO generation rates of 0.76 and 0.08 μM min-1, respectively). With faster •NO-releasing compounds, such as diethylamine NONOate and spermine NONOate, lower levels of ONOO- were detected. These data suggest that once the optimal concentration of •NO was obtained, the reaction between •NO and 4-OHE2 was saturated. The excess of •NO would probably react with aqueous oxygen to form nitrite (NO2 -). Since the third-order reaction rate for the reaction between 2•NO and O2 is 2 × 106 M-2 s-1, it can therefore be suggested that the reaction between •NO and 4-OHE2 occurs at a faster rate.