Major inter-species differences in the rates of O-sulphonation and O-glucuronylation of alpha-hydroxytamoxifen in vitro: a metabolic disparity protecting human liver from the formation of tamoxifen-DNA adducts
Dj. Boocock et al., Major inter-species differences in the rates of O-sulphonation and O-glucuronylation of alpha-hydroxytamoxifen in vitro: a metabolic disparity protecting human liver from the formation of tamoxifen-DNA adducts, CARCINOGENE, 21(10), 2000, pp. 1851-1858
Tamoxifen is a hepatic genotoxin in rats and mice but a hepatocarcinogen on
ly in rats, It is not associated with DNA adducts and liver tumours in pati
ents, The proposed major pathway for its bioactivation in rats involves alp
ha -hydroxylation, O-sulphonation and generation of carbocation that reacts
with DNA, Rat liver microsomes catalyse a-hydroxylation at similar to2- an
d 4-fold the rate achieved by human and murine liver microsomes, respective
ly, O-glucuronylation will deactivate alpha -hydroxytamoxifen and compete w
ith sulphonation, Rates of O-sulphonation of alpha -hydroxytamoxifen in hep
atic cytosol have been determined by a HPLC assay of substrate-dependent 3'
-phosphoadenosine 5'-phosphate production, The rank order of O-glucuronylat
ion in hepatic microsomes was estimated by HPLC-mass spectrometry, The rate
of sulphonation of trans-alpha -hydroxytamoxifen (25 muM) in cytosol from
adult female Sprague-Dawley rats and CD1 mice was 5.3 +/- 0.8 and 3.9 +/- 0
.5 pmol/min/mg protein (mean +/- SD, n = 3), respectively, In cytosol fract
ions from women aged 40-65 years, the rate was 1.1 +/- 0.4 pmol/min/mg prot
ein (mean +/- SD, n = 6). The K-m for trans-alpha -hydroxytamoxifen in rat,
mouse and human cytosol was 84.6 +/- 3.8, 81.4 +/- 4.6 and 104.3 +/- 5.6 m
uM (mean +/- SD, n = 3), respectively; the corresponding V-max values were
22.4 +/- 3.4, 17.1 +/- 3.1 and 6.3 +/- 1.9 pmol/min/mgprotein, These K-m we
re similar to a value obtained by others using purified rat liver hydroxyst
eroid sulphotransferase a. Turnover of the cis epimer was too slow for accu
rate determination of rates, Sulphonation of trans-alpha -hydroxytamoxifen
in human uterine cytosol was undetectable, The rank order of O-glucuronylat
ion of trans-alpha -hydroxytamoxifen in liver microsomes was human >> mouse
> rat, In combination, lower rates of alpha -hydroxylation and O-sulphonat
ion and a higher rate of O-glucuronylation in human liver would protect pat
ients from the formation of tamoxifen-DNA adducts.