Influence of silybin on biophysical properties of phospholipid bilayers.

AIM: Silybin (silibinin) is major biologically active flavonolignan extracted from milk thistle (Sylibum marianum). Its biological activities include hepato-protection, anticancer properties, and antioxidant- and membrane-stabilizing functions. Although membranes are postulated to be one of the cellular targets for silybin, little is known about its interaction with phospholipid bilayers. METHODS: In the present work, the interactions of silybin with phosphatidylcholine bilayers were studied in detail using fluorescence spectroscopy, microcalorimetry and electron spin resonance techniques. RESULTS: The results showed that silybin interacted with the surface of lipid bilayers. It affected the generalized polarization of the fluorescent probe Prodan, while not influencing the more deeply located Laurdan. Silybin lowered the main phospholipid phase transition temperature as judged by microcalorimetry, and caused the immobilization of spin probe Tempo-palmitate located on the surface of membranes. The mobility of spin probes 5- and 16-doxyl stearic acid was not affected by silybin. Silybin-induced quenching of 1,6-diphenyl-1,3,5-hexatriene fluorescence indicated that some flavonoid molecules partitioned into the hydrophobic region of membranes, which did not change significantly the biophysical properties of the deeper membrane regions. CONCLUSION: Such a behavior of silybin in membranes is in accordance with its postulated biological functions and neglectable side effects of therapies using silybin.

Modulation of MRP1 protein transport by plant, and synthetically modified flavonoids.

The influence of novel synthetic and plant origin flavonoids on activity of multidrug resistance-associated protein (MRP1) was investigated in human erythrocytes used as a cell model expressing MRP1 in plasma membrane. The fluorescent probe, BCPCF (2', 7'-bis-(3-carboxy-propyl)-5-(and-6)-carboxyfluorescein), was applied as a substrate for MRP1 multidrug resistance transporter. The effect of compounds belonging to different classes of natural flavonoids: flavone, flavonol, isoflavones and flavanolignan was compared with action of new synthetic derivatives of genistein. Most of the flavonoids showed strong or moderate ability to inhibit transport carried out by MRP1. Inhibitory properties of flavonoids were compared to the effects of indomethacin, probenecid and MK-571 known as MRP1 inhibitors. Studying the influence of new synthetic genistein derivatives on BCPCF transport we have found that the presence of hydrophobic groups substituting hydrogen of hydroxyl group at the position 4' in ring B of isoflavone is more important for inhibitory properties than hydrophobic substitution at the position 7 in ring A. In case of naturally occurring isoflavones the replacement of hydrogen at position 4' by hydrophobic ring structure seems also to be favourable for inhibition potency.

Metabolic O-demethylation does not alter the influence of isoflavones on the biophysical properties of membranes and MRP1-like protein transport activity.

Multidrug resistance transporter MRP1 could be effectively inhibited by some flavonoids. The influence of the two pairs of isoflavones: formononetin/daidzein and biochanin A/genistein on the efflux of fluorescent substrate of MRP1-like protein from erythrocytes and biophysical properties of lipid membranes has been compared. Compounds in each pair differ by the substituent in position 4' of B ring of isoflavone molecule. In the process of O-demethylation, CH(3) group (present in formonetin and biochanin A) is replaced by hydrogen (daidzein, genistein). Inhibition of MRP1-like protein transport activity by methylated and demethylated isoflavones was very similar. Their influence on lipid thermotropic properties and fluidity of lipid bilayer was not also significantly different.