Phosphorylation by Tyrosine Kinases Dictates Endogenous Substrate‐Selection for UDP‐glucuronosyltransferase(UGT)‐2B7

Wide‐tissue‐distributed UGT‐2B7 preferentially detoxifies genotoxic 4‐OH‐estradiol (4‐OHE 2 ) and 4‐OH‐estrone (4‐OHE 1 ) with negligible 17β‐estradiol (E 2 ) conversion following expression in COS‐1 cells. Consistent with UGTs requirement for regulated phosphorylation, UGT‐2B7His expressed in COS‐1 cells incorporated immunoprecipitable [ 33 P] orthophosphate. Microsomal 2B7His, isolated from SYF‐(Src, Yes, Fyn) −/− cells and solubilzed, was highly radiolabeled by Src and [γ 33 PATP], but not [γ 33 PATP] alone. Unexpectedly, the relative in‐vitro rate of 4‐OHE 1 turnover following expression per cell‐system was: SYF −/− > SYF +/− > COS‐1, which was similar to that for E 2 except 2 to 3 orders of magnitude lower activity. Results indicated unidentified tyrosine kinase(s) phosphorylated 2B7 in SYF −/− cells, which was verified by custom‐developed and highly specific anti‐phospho‐Y438‐2B7. Further studies indicated Src down‐regulates 2B7 glucuronidation of 4‐OHE 1 in COS‐1 cells to a level(s) that prevent E 2 metabolism and that the phosphorylating TK(s) dictate substrate selection. While other UGTs are shown to require differential PKC phosphorylation per UGT studied, 2B7 is the first isozyme shown to require tyrosine kinase phosphorylation. This research was supported by the Intramural Research Program of the NIH, NICHD