Preliminary assessment of luteolin as an affinity ligand for type II estrogen--binding sites in rat uterine nuclear extracts.

Naturally occurring bioflavonoids such as luteolin compete for [3H]estradiol binding to nuclear type II sites and mimic methyl p-hydroxyphenyllactate (MeHPLA) as ligands for this cell regulatory protein. More importantly, luteolin (3',4',5,7-tetrahydroxyflavone) contains catechol hydroxyl groups on the A and B rings that may form quinones capable of binding covalently to proteins; therefore, we evaluated luteolin as a potential affinity ligand for rat uterine nuclear type II sites. The preliminary experiments presented in this manuscript demonstrate that luteolin and a related bioflavonoid, 4,7-dihydroxyflavone (DHF), are competitive inhibitors of [3H]estradiol binding to type II sites in ammonium sulfate (AmSO4) extracts of rat uterine nuclei. This high affinity (Kd 5-10 nM) interaction is specific for type II sites, and neither compound binds to the estrogen receptor (ER). More importantly, the interaction of luteolin with nuclear type II sites was irreversible, whereas DHF readily exchanged with [3H]estradiol for type II sites in these preparations. These findings suggest that this nonexchangable occupancy of type II sites by luteolin is likely to involve covalent attachment. Spectrophotometric analysis of type II site preparations pretreated with luteolin also confirmed the [3H]estradiol exchange assay data, demonstrating that the ligand attachment is irreversible. Because luteolin did not affect [3H]estradiol binding to the ER in uterine cytosol, we suspect that this bioflavonoid may not be simply randomly interacting with a multiplicity of proteins to generate covalent complexes. These preliminary findings suggest that high-affinity binding of luteolin by type II sites is prerequisite to covalent attachment and that this bioflavonoid may be a suitable affinity ligand for the purification of this protein.