Concentration-dependence of prooxidant and antioxidant effects of aloin and aloe-emodin on DNA

To understand the toxic effects and conflicting reports of aloin and aloe-emodin, we investigated their antioxidant and prooxidant effects on free radical-induced DNA breaks over a wide range of concentrations. Aloin, at concentrations ranging from 1.25 to 2.5 mM, prevented OH-induced DNA breaks by 5-30% over control values. In contrast, addition of aloin at lower concentrations (8-300 μM) increased DNA damage by about 24-74%, indicating its prooxidant effect. Aloe-emodin showed little antioxidant effect at any tested concentration, but addition of higher concentrations (1.25-2.5 mM) of aloe-emodin had a prooxidant effect on DNA (about 29-35%). The greater reducing activity of aloin at low concentrations, reducing Fe3+ to Fe2+, which enhanced the generation of OH from the Fenton reaction, may play a key role in its prooxidant effect on DNA. At higher concentrations, however, free radical-scavenging activity of aloin gradually predominated over its reducing power, resulting in the protection of DNA. Predomination of reducing power over scavenging activity resulted in the prooxidant effect of aloe-emodin at high concentrations. The relationship between structures and anti- or pro-oxidant effects of aloin and aloe-emodin is also discussed.