Detection of Free Radicals Produced from Reactions of Lipid Hydroperoxide Model Compounds with Cu(II) Complexes by ESR Spectroscopy

Using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap, alkoxyl radicals and peroxyl radicals produced from the reactions oftert-butyl hydroperoxide(tBuOOH) and cumene hydroperoxide (PhC(CH3)2OOH) with some copper(Cu)(II) complexes such as Cu(II) complexes of cimetidine (Cim), cyclo(L-histidyl-L-histidyl) (CyHH),L-histidylglycine (HG), andL-histidylglycylglycine (HGG) were detected by electron spin resonance (ESR) spectroscopy. However, Cu(II) complexes of glycyl-L-histidine (GH), glycyl-L-histidylglycine (GHG), glycylglycyl-L-histidine (GGH), and glycylglycyl-L-histidylglycine (GGHG) did not cause the generation of free radicals during the reaction withtert-butyl or cumene hydroperoxide. Addition of a biological reductant such as cysteine or glutathione to the system including these Cu(II) complexes and hydroperoxides gavetert-butoxyl and cumyl alkoxyl (RO·) radicals, respectively. These alkoxyl radicals underwent subsequent β-scission reaction and generated the carbon-centered radical (R·). Although cysteine and glutathione are considered to be cellular antioxidants, our results suggest that these biological reductants facilitate Cu(II) complexes-dependent free radical generation.