Marked enhancement of synergistic antioxidative activity of astaxanthin and curcumin by co-encapsulation in a liposomal formulation

Natural antioxidants may help prevent oxidative stress-mediated aging. Astaxanthin (Asx) is a powerful antioxidant that has been used in various dietary supplements and cosmetics. Previously, we reported that combination of Asx with tocotrienol (T3) or capsaicin (Cap) exhibited synergistic antioxidative effects; however, the synergistic increase in activity was unremarkable. Herein, we employed curcumin (Cur) to more effectively enhance the antioxidant activity of Asx. The antioxidative activity of liposomes co-encapsulating Asx and Cur (Asx/Cur-lipo) showed effective synergism (activity increase of ~2). A shift in peak absorption suggested intermolecular interactions between Asx and Cur. Since synthetic Asx is a mixture of various stereoisomers, we examined the effect of Asx stereochemistry on the effective synergism by investigating liposomes prepared with 3<i>R</i>,3'<i>R</i>-form (Asx-<i>R</i>), and 3<i>S</i>,3'<i>S</i>-form (Asx-<i>S</i>). Antioxidative activities of Asx-<i>R</i>/Cur-lipo and Asx-<i>S</i>/Cur-lipo were evaluated. Only Asx-<i>S</i>/Cur-lipo exhibited a synergistic effect; however, the activity increase of Asx-<i>S</i>/Cur-lipo was lower than that of Asx/Cur-lipo. In conclusion, liposomes co-encapsulating a synthetic mixture of stereochemical isomers of Asx and Cur may be most potent in preventing oxidative stress-mediated aging.