title: An electron spin resonance study of o-semiquinones formed during the enzymatic and autoxidation of catechol estrogens.
abstract: Electron spin resonance spectroscopy has been used to demonstrate production of semiquinone-free radicals from the oxidation of the catechol estrogens 2- and 4-hydroxyestradiol and 2,6- and 4,6-dihydroxyestradiol. Radicals were generated either enzymatically (using horseradish peroxidase-H2O2 or tyrosinase-O2) or by autoxidation, and were detected as their complexes with spin-stabilizing metal ions (Zn2+ and/or Mg2+). In the peroxidase system, radicals are produced by one-electron oxidation of the catechol estrogen and their decay is by a second-order pathway, consistent with their disproportionation to quinone and catechol products. With tyrosinase-O2, radical generation occurs indirectly. Initial hydroxylation of phenolic estrogen (at either the 2- or 4-position) gives a catechol estrogen in situ; subsequent two-electron oxidation of the catechol to the quinone, followed by reverse disproportionation, leads to the formation of radicals. A competing mechanism for radical production involves autoxidation of the catechol. Results obtained from the estrogen systems have been compared with those from the model compound 5,6,7,8-tetrahydro-2-naphthol.
mesh_terms: D016227:Benzoquinones; D004578:Electron Spin Resonance Spectroscopy; D004958:Estradiol / Q000031:analogs & derivatives / Q000378:metabolism; D002393:Estrogens, Catechol / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism; D014442:Monophenol Monooxygenase / Q000378:metabolism; D009284:Naphthols / Q000378:metabolism; D011809:Quinones / Q000378:metabolism*; D015032:Zinc / Q000378:metabolism
chemical_list: D016227:Benzoquinones; D002393:Estrogens, Catechol; D009284:Naphthols; D011809:Quinones; C025232:semiquinone radicals; D004958:Estradiol; C001390:2-hydroxyestradiol; C014036:4-hydroxyestradiol; D014442:Monophenol Monooxygenase; D008274:Magnesium; D015032:Zinc; C043496:5,6,7,8-tetrahydro-2-naphthol