Synthesis and structure elucidation of estrogen quinones conjugated with cysteine, N-acetylcysteine, and glutathione

Catechol estrogen quinones (CE-Q) have been implicated as ultimate carcinogenic metabolites in estrogen-induced carcinogenesis. CE-Q may covalently bind to DNA to initiate cancer. These quinones can also be conjugated with glutathione, a reaction that prevents damage to DNA by CE-Q. The glutathione conjugates are then catabolized through mercapturic acid biosynthesis to cysteine and N-acetylcysteine conjugates. This may be the most important detoxification pathway of CE-Q. The chemical synthesis and characterization of these conjugates are the first essential steps to better understand their function in biological systems. Eighteen conjugates were synthesized by reaction of estrone-3,4-quinone (E-1-3,4-Q), estradiol-3,4-quinone (E-2-3,4-Q), estrone-2,3-quinone (E-1-2,3-Q), or estradiol-2,3-quinone (E-2-2,3-Q) with various sulfur: nucleophiles, RSH, in which R is the cysteine, N-acetylcysteine, or glutathione moiety. Reactions of E-1-3,4-Q and E-2-3,4-Q produce regiospecifically 4-OHE1-2-SR and 4-OHE2-2-SR, respectively, in almost quantitative yield. E-1-2,3-Q and E-2-2,3-Q react regioselectively and quantitatively to form S-OHE1(E-2)-1-SR and 8-OHE1(E-2)-4-SR, in which the l-isomers are always the major products. The ratio between 1 and 4 isomers is 3.5 for cysteine, 2.7 for N-acetylcysteine, and 2.5 for glutathione. The synthesized conjugates will be used as standards in the identification of these compounds formed in biological systems.