Antioxidant and scavenging activity of emodin, aloe-emodin, and rhein on free-radical and reactive oxygen species

The objective of this study was to investigate the ability of emodin (1), aloe-emodin (2), and rhein (3) to inhibit free-radical or reactive oxygen species (.OH, O-1(2), H2O2) generated in cell-free systems using isoluminol and luminol-enhanced chemiluminescence and electronic absorption spectra. In the presence of 1, 2, and 3, a dose-dependent inhibition period was observed in this system as assayed by isoluminol-enhanced chemiluminescence (ILCL) with horseradish peroxidase (HRP), as well as by luminol-enhanced chemiluminescence (LCL) with H2O2 or ferrous iron. On the other hand, these hydroxyanthraquinones showed an efficient scavenging activity of galvanoxyl radical in ethanolic solutions. In a separate experiment, we observed the trapping of singlet oxygen (O-1(2)) generated by rose bengal in the presence of 1, 2, or 3. These results suggest that emodin, aloe-emodin, and rhein scavenge reactive oxygen and free-radical species in the following decreasing order: emodin > rhein > aloe-emodin.