Determination of phenolic compounds, flavonoids, and antioxidant activities in water extracts of Thai red and white rice cultivars.

BACKGROUND Free radical-induced oxidative stress damages cellular components leading to many human diseases. Plant-derived antioxidant compounds have become a profitable alternative to prevent oxidative stress in cells. OBJECTIVE To determine and compare total phenolic and flavonoid contents as well as antioxidant activity using both chemical and cell assays in the water extracts of brown rice and rice bran from two Thai rice cultivars: Sangyod, a red pigmented rice typically grown in Southern Thailand and Dawk Mali 105, a commercial white-colored rice. MATERIAL AND METHOD All the rice water extracts were analyzed for their total phenolic and flavonoid contents using the colorimetric assays, as well as for their antioxidant activity through two chemical assays: DPPH radical-scavenging and inhibition of lipid peroxidation assays, as well as through cell-based assays: scavenging capacity of intracellular ROS in HL60 cells using the fluorescent DCF and the NBT reduction. RESULTS The two chemical assays detected free radical scavenging and free radical chain breaking activities in all the rice extracts with EC50 values ranging from 26 to 357 microg/ml. Moreover, the cell-based assays detected ROS scavenging activities of these extracts with EC50 values in the range of 0.6-5 mg/ml. All these assays indicated that the water extracts of Sangyod exerted significantly higher antioxidant activity than those of Dawk Mali 105, which exhibited only moderate to low activity. Furthermore, high levels of antioxidant activity of the water extracts of Sangyod were closely correlated to their flavonoid and phenolic contents, which were approximately 2.5 and 3 times higher, respectively, than those of Dawk Mali 105. CONCLUSION These findings suggest that water extracts from colored brown rice or colored rice bran can be promising sources of potential natural antioxidants.