Assay of labile estrogen o-quinones, potent carcinogenic molecular species, by high performance liquid chromatography–electrospray ionization tandem mass spectrometry with phenazine derivatization

A sensitive and selective assay method for labile estrogen o-quinones, estrone (E1)-2,3-quinone (Q), E1-3,4-Q, estradiol (E2)-2,3-Q and E2-3,4-Q, based on the use of phenazine (Phz) derivatization with o-phenylenediamine and high performance liquid chromatography–electrospray ionization tandem mass spectrometry (HPLC–ESI-MS/MS) was described. The Phz derivatives of four estrogen o-quinones were purified by solid phase extraction and analyzed by HPLC–ESI-MS/MS. The protonated molecule was observed as a base peak for all Phz derivatives in their ESI-mass spectra (positive mode). In multiple reaction monitoring, the transition from [M+H]+ to m/z 231 was chosen for quantification. Calibration curves for the o-quinones were obtained using standard catechol estrogens after sodium metaperiodate treatment and Phz derivatization. Using this method, these four estrogen o-quinones were analyzed with the limit of quantification of 5 ng/ml in acetonitrile (MeCN)–blank matrix (1:4, v/v), respectively, on a basis of the weight of catechol estrogens. Assay accuracy and precision for four estrogen o-quinones were 89.6–113.0% and 3.1–12.6% (5, 125 and 2000 ng/ml in MeCN–blank matrix). Applications of this method enabled to determine the catalytic activities on hydroxylation and subsequent oxidation of E1 and E2 of Mushroom tyrosinase and rat liver microsomal fraction. It was confirmed by this method that tyrosinase exhibited 2- and 4-hydroxylation and further oxidation activities for catechols in the ring-A of estrogens. Whereas rat liver microsomal fraction possessed only 2- and 4-hydroxylation activities, and further oxidation activity for catechol estrogens was low.