2-METHOXYESTRADIOL, AN ENDOGENOUS MAMMALIAN METABOLITE, INHIBITS TUBULIN POLYMERIZATION BY INTERACTING AT THE COLCHICINE SITE

A metabolite of estradiol, 2-methoxyestradiol (2ME), inhibits angiogen esis in the chicken embryo chorioallantoic membrane assay. Since 2ME c auses mitotic perturbations, we examined its interactions with tubulin . In our standard 1.0 M glutamate system (plus 1.0 mM MgCl2 at 37 degr ees C), superstoichiometric concentrations (relative to tubulin) of 2M E inhibited the nucleation and propagation phases of tubulin assembly but did not affect the reaction extent. Although polymer formed in the presence of 2ME was more cold-stable than control polymer, morphology was little changed. Under suboptimal reaction conditions (0.8 M gluta mate/no MgCl2 at 26 degrees C), substoichiometric 2ME totally inhibite d polymerization. No other estrogenic compound was as effective as 2ME as an inhibitor of polymerization or of the binding of colchicine to tubulin. Inhibition of colchicine binding was competitive (K-i, 22 mu M). Thus, a mammalian metabolite of estradiol binds to the colchicine site of tubulin and, depending on reaction conditions, either inhibits assembly or seems to be incorporated into a polymer with altered stab ility properties.