Reduction of membrane fluidity by antibacterial sophoraflavanone G isolated from Sophora exigua

Sophoraflavanone G (5,7,2',4'-tetrahydroxy-8-lavandulylflavanone) has been referred as a phytochemical with the intensive antibacterial activity. To elucidate the pharmacological mechanism underlying an antibacterial action, sophoraflavanone G was isolated from Sophora exigua, thereafter its effect on membrane fluidity was studied using model membranes and compared with less active naringenin lacking 8-lavandulyl and 2'-hydroxyl groups. Highly purified sophoraflavanone G of 0.05-5 microg/ml, corresponding to the minimum growth inhibitory concentrations against various bacteria, significantly increased fluorescence polarization of the liposomes prepared from 1,2-dipalmitoyl-L-alpha-phosphatidylcholine and 1-palmitoyl-2-oleoyl-L-alpha-phosphatidylcholine. Such increases were found in both liposomes measured with two fluorescent probes to indicate an alteration of membrane fluidity in hydrophilic and hydrophobic regions, suggesting that sophoraflavanone G reduces the fluidity of outer and inner layers of membranes. Although naringenin also showed the membrane effect, it needed concentrations over 2.5 microg/ml to induce a significant reduction of membrane fluidity. Sophoraflavanone G is considered to exert an antibacterial effect by reducing the fluidity of cellular membranes.