α-Hydroxytamoxifen Is a Substrate of Hydroxysteroid (Alcohol) Sulfotransferase, Resulting in Tamoxifen DNA Adducts¹

When α-hydroxytamoxifen (α-OHTAM) was incubated with rat liver hydroxysteroid (alcohol) sulfotransferase a (STa) and 3′-phosphoadenosine 5′-phosphosulfate, (E)-α-OHTAM was found to be a better substrate for STa than (Z)-α-OHTAM. To explore the formation of taxoxifen (TAM)-derived DNA adducts, DNA was incubated with STa and either (E)-α-OHTAM or (Z)-α-OHTAM in the presence of 3′-phosphoadenosine 5′-phosphosulfate. Using ³²P-postlabeling analysis, the amount of TAM-DNA adducts resulting from (E)-α-OHTAM was 29 times higher than that observed with (E)-α-OHTAM alone. Using (Z)-α-OHTAM and STa, some TAM-DNA adducts were also detected but at levels 6.5 times lower than that observed with (E)-α-OHTAM and STa. Wehn compared with standards of stereoisomers of 2′-deoxyguanosine 3′-monophosphate-N²-tamoxifen, the major tamoxifen adduct was identified chromatographically as an epimer of the trans form of α-(N²-deoxyguanosinyl)tamoxifen, and the minor adduct was identified as an epimer of the cis form. In the reaction mixture, a conversion from (E)-α-OHTAM to (Z)-α-OHTAM through the carbocation intermediate was also detected. These results show that sulfation of α-OHTAM catalyzed by STa results in the formation of TAM-DNA adducts.