Phenolic compounds isolated from Pilea microphylla prevent radiation-induced cellular DNA damage

Six phenolic compounds namely, quercetin-3-O-rutinoside (1), 3-O-caffeoylquinic acid (2), luteolin-7-O-glucoside (3), apigenin-7-O-rutinoside (4), apigenin-7-O-beta-D-glucopyranoside (5) and quercetin (6) were isolated from the whole plant of Pilea microphylla using conventional open silica gel column chromatography and preparative HPLC. Further, these compounds were characterized by 1D, 2D NMR techniques and high-resolution LC MS. Compounds 1-3 and 6 exhibited significant antioxidant potential in scavenging free radicals such as DPPH, ABTS and SOD with IC50 of 3.3-20.4 mol/L. The same compounds also prevented lipid peroxidation with IC50 of 10.4-32.2 imaol/L. The compounds also significantly prevented the Fenton reagent-induced calf thymus DNA damage. Pre-treatment with compounds 1-3 and 6 in V79 cells attenuated radiation-induced formation of reactive oxygen species, lipid peroxidation, cytotoxicity and DNA damage, correlating the antioxidant activity of polyphenols with their radioprotective effects. Compounds 1, 3 and 6 significantly inhibited lipid peroxidation, presumably due to 3',4'-catechol ortho-dihydroxy moiety in the B-ring, which has a strong affinity for phospholipid membranes. Oxidation of flavonoids, with catechol structure on B-ring, yields a fairly stable ortho-semiquinone radical by facilitating electron delocalization, which is involved in antioxidant mechanism. Hence, the flavonoid structure, number and location of hydroxyl groups together determine the antioxidant and radioprotection mechanism. (C) 2011 Institute of Materia Medica, Chinese Academy of Medical Sciences and Chinese Pharmaceutical Association. Production and hosting by Elsevier B.V. All rights reserved.