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-b-D-glucopyranoside(5)and quercetin(6)were isolated from the whole plant of Pilea microphylla using conventional opensilica 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 IC_(50) of 3.3–20.4 mmol/L.The same compounds also prevented lipid peroxidation with IC_(50) of 10.4–32.2 mmol/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 30,40-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.