Fg. Zhang et al., Synthesis and reactivity of the catechol metabolites from the equine estrogen, 8,9-dehydroestrone, CHEM RES T, 14(6), 2001, pp. 754-763
The risk factors for women developing breast and endometrial cancers are al
l associated with a lifetime of estrogen exposure. Estrogen replacement the
rapy in particular has been correlated with an increased cancer risk. Previ
ously, we showed that the equine estrogens equilin and equilenin, which are
major components of the widely prescribed estrogen replacement formulation
Premarin, are metabolized to highly cytotoxic quinoids which caused oxidat
ive stress and alkylation of DNA in vitro [Bolton, J. L., Pisha, E., Zhang,
F., and Qiu, S. Chem. Res. Toxicol. 1998, 11, 1113-1127]. In this study, w
e have synthesized 8,9-dehydroestrone (a third equine estrogen component of
Premarin) and its potential catechol metabolites, 4-hydroxy-8,9-dehydroest
rone and 2-hydroxy-8,9-dehydroestrone. Both 2-hydroxy-8,9-dehydroestrone an
d 4-hydroxy-8,9-dehydroestrone were oxidized by tyrosinase or rat liver mic
rosomes to o-quinones which reacted with GSH to give one mono-GSH conjugate
and two di-GSH conjugates. Like endogenous estrogens, 8,9-dehydroestrone w
as primarily converted by rat liver microsomes to the 2-hydroxylated rather
than the 4-hydroxylated o-quinone GSH conjugates; the ratio of 2-hydroxy-8
,9-dehydroestrone versus 4-hydroxy-8,9-dehydroestrone was 6:1. Also in cont
rast to experiments with equilin, 4-hydroxyequilenin was not observed in mi
crosomal incubations with 8,9-dehydroestrone or its catechols. The behavior
of 2-hydroxy-8,9-dehydroestrone was found to be more complex than 4-hydrox
y-8,9-dehydroestrone as GSH conjugates resulting from 2-hydroxy-8,9-dehydro
estrone were detected even without oxidative enzyme catalysis. Under physio
logical conditions, 2-hydroxy-8,9-dehydroestrone isomerized to 8-hydroxyequ
ilenin to form the very stable 2-hydroxyequilenin catechol; however, 4-hydr
oxy-8,9-dehydroestrone was found to be stable under similar conditions. Fin
ally, preliminary studies conducted with the human breast tumor S-30 cell l
ines demonstrated that the catechol metabolites of 8,9-dehydroestrone were
much less toxic than 4-hydroxyequilenin (20-40-fold). These results suggest
that the catechol metabolites of 8,9-dehydroestrone may have the ability t
o cause cytotoxicity in vivo primarily through formation of o-quinones; how
ever, most of the adverse effects of Premarin estrogens are likely due to f
ormation of 4-hydroxyequilenin o-quinone from equilin and equilenin.