Genistein (5,7,4'-trihydroxyisoflavone) modulates the function of several transmembrane ion-channel proteins by mechanisms that are unrelated to phosphorylation events. Daidzein (7,4'-dihydroxy-isoflavone) typically exhibits modest effects, whereas genistin (7-O-glucosyl-genistein) usually exhibits no effect on ion-channel activities. Genistein appears to modulate gramicidin A ion channels by alteration of bilayer mechanical properties, but the associated molecular interactions have not been defined. The incorporation of daidzein into phosphatidylcholine liposomes promotes aggregation and precipitate formation which is problematic for structural studies based on NMR spectroscopy. In the present study, daidzein was incorporated into sodium dodecyl sulfate (SDS) micelles to provide a stable system with no evidence of micelle aggregation. For this reason genistein, daidzein, genistin, and osajin (a diprenyl-genistein derivative) were incorporated into SDS micelles (in D(2)O) to evaluate differences in position and orientation within micelle structures. The (1)H NMR line widths, as a function of Mn(2+) concentration, indicate that genistein is quite mobile and buried within the hydrophobic micelle core. Daidzein and genistin also are mobile but exhibit average positions near the micelle/aqueous interface, with polar groups oriented toward the aqueous compartment. These results demonstrate that daidzein, with only two hydroxyl substituents, has a greater affinity for a polar environment than genistein with three hydroxyl substituents. The 5-hydroxyl group of genistein forms an intramolecular hydrogen bond with the 4-carbonyl group, which diminishes the molecular affinity for a polar matrix. These results suggest an explanation for the relative abilities of these compounds to increase gramicidin channel lifetimes and modulate other ion-channel types.
Distribution coefficients (D) were measured for flavone, monohydroxyflavones, and monomethoxyflavones equilibrated between 1-octanol and aqueous buffer (50 mM MOPS, pH=7.4). The values of LogD were used to determine substitution constants referred to as π values. Hydroxyl groups at the 3 or 5 position of flavone had positive π values (increased hydrophobicity) while hydroxyl groups at other positions had negative π values (increased hydrophilicity). For each monohydroxyflavone, chromatographic capacity factors (k') were determined for both reverse phase (C-18) and immobilized artificial membrane (IAM) columns. For the IAM column, relatively large k' values were observed for both 3-hydroxyflavone and 5-hydroxyflavone indicating that hydroxyl groups at positions 3 and 5 of flavones promote high affinities for phospholipid structures. These results should aid in refinement of quantitative structure activity relationships (QSAR's) that are useful for drug development based on flavonoids as lead compounds.
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