In vivo oestrogenicity and binding characteristics of alpha-zearalanol (P-1496) to different classes of oestrogen binding proteins in rat liver.

It is now well established that the mycotoxin zearalenone and some of its derivatives possess oestrogenic activity. In the present study, the binding characteristics of [3H]zearalanol (P-1496) to different classes of sites including [1] the oestrogen receptor, [2] the higher capacity lower affinity (HCLA) sites, [3] the antioestrogen sites and [4] a new class of binding sites apparently specific for P-1496 were examined in rat liver. Analysis of the binding by sucrose density gradient centrifugation confirmed that P-1496 binds to the oestrogen receptor but not to the higher capacity lower affinity sites for oestradiol-17 beta. Furthermore, saturation experiments using partially-purified fractions showed that P-1496 binds to the oestrogen receptor with an affinity very similar to that of oestradiol-17 beta (apparent dissociation constants ranged from 0.1-0.3 nM). Competition studies using partially purified cytosolic oestrogen receptor suggested that P-1496 binds to a second high affinity site distinct from the oestrogen receptor. This binding site was further characterized as selective for P-1496 by saturation analysis following the complete occupancy of oestrogen receptor by oestradiol-17 beta. The in vitro binding characteristics of P-1496 were then compared with in vivo effects on concentrations of serum triglycerides. Treatment of ovariectomized female rats daily with 1.5 or 2 mg P-1496/kg body weight resulted in marked increases in the concentrations of serum triglycerides associated with the very low density lipoprotein (VLDL) fraction. Dose-response studies indicated that there was no sex difference with respect to the dose necessary to produce significant increases in serum triglycerides. The present study shows striking similarities between the binding of P-1496 and oestradiol-17 beta to liver oestrogen receptor in vitro. However, differences are observed with respect to their binding to other cytoplasmic components of liver. In addition, although P-1496 is capable of eliciting in vivo oestrogenic effects in liver, it is much less potent than oestradiol-17 beta.