3.2.3. Hydroxyl radical scavenging activity
The hydroxyl radical is the neutral form of the hydroxide ion. Hydroxyl radicals are highly reactive and consequently short-lived. The potentially reactive hydroxyl radicals can damage virtually all types of macromolecules: carbohydrates, nucleic acids, lipids and amino acids. This makes it a very dangerous compound to the organism (Reiter et al., 1995). Hydroxyl radical scavenging activity of astaxanthin and astaxanthin/HPCD complex were assayed and also compared with ascorbic acid (Fig. 7). The scavenging activity of the three compounds all showed a positive correlation with their concentrations. However, the scavenging effects of astaxanthin and astaxanthin/HPCD complex were about dozens of higher than ascorbic acid, ascorbic acid scavenged 32.13% of hydroxyl radicals at 73.86 μmol/L, while 83.16% of hydroxyl radicals were quenched by astaxanthin at 4.55 μmol/L. The result is in agreement with the report that astaxanthin has a higher antioxidant activity than α-tocopherol and β-carotene (Lorenz & Cysewski, 2000). The scavenging activity of the complex was close to astaxanthin; nevertheless, the former became lower at high concentrations. Maybe, the existence of HPCD hindered the reaction between astaxanthin and hydroxyl radical.
4. Conclusions
In this study, the storage stability and antioxidant activity of astaxanthin/HPCD complex were evaluated. Both native astaxanthin and the complex showed high stability under storage condition at 4 °C and 25 °C. Complexation improved the stability of astaxanthin under storage conditions. Both astaxanthin and astaxanthin/HPCD complex had high antioxidant activity in vitro. Compared with ascorbic acid, the reducing power and DPPH radical scavenging activity of astaxanthin were lower in the water due to the poor solubility and self-assembly; the complex was stronger at low concentration for solubility enhancing effect of HPCD. However, the activity decreased at high concentration for the difficult release of astaxanthin in the complex. Astaxanthin and the complex were about dozens of higher than ascorbic acid in the hydroxyl radical scavenging activity, and the complex was a little lower than native astaxanthin. In summary, complexation with HPCD could affect the antioxidant activity of astaxanthin; it should be improved at low concentration aqueous solution, but hindered at high due to the controlled-release effect. More detailed studies are underway in our laboratory to understand the storage stability and antioxidant potential of astaxanthin/HPCD complex in the food system.
Acknowledgments
This work was supported by National Science & Technology Pillar Program during the Eleventh Five-year Plan Period (Grant no. 2006BAD27B06) and Doctoral Fund of Anyang Institute of Technology (Grant no. AYIT2009002). We are grateful to Prof. Yibing Tan (Southern Yangtze University) for providing technical assistance.