Evaluation of antioxidant activities and contents in bran extracts of Thai pigmented rice using chemical and cellular assays

Abstract

Due to adverse effects of synthetic antioxidants, pigmented rice has become attractive as potential sources of natural antioxidants to prevent oxidative stress implicated in many chronic diseases. In the present study, antioxidant activities of ethanol-water (70:30, v/v) bran extracts of 11 Thai pigmented rice (red and purple) and 2 non-pigmented rice varieties were evaluated by chemical assays (DPPH and ABTS radical scavenging and ferrous ion chelating assays) and cellular assays (intracellular free radical scavenging activity-NBT and DCF assays). Also, contents of total phenolics and their subtypes (flavonoids, proanthocyanidins, and anthocyanins) were assessed using colorimetric assays. Correlations between these contents and antioxidant activities were further analyzed. The results revealed that the pigmented rice bran extracts exhibited higher scavenging activity in chemical assays (IC50= 5.3 – 46.9 µg/ml) and in NBT (% inhibition = 12.3 – 62.6%) and DCF (% inhibition = 31.4 – 70.2%) assays at 30 µg/ml as compared to the non-pigmented rice extracts. High correlations between such activity and contents of phenolics and flavonoids indicated their contribution to the strong scavenging activity in red rice followed by purple rice bran extracts. In particular, noticeably high contents of proanthocyanidins in red bran and anthocyanins in purple bran as well as their positive correlations with the scavenging activity strongly suggest that they are the phenolic subtype mainly accounting for such strong activity in these rice varieties. In addition, the pigmented rice bran extracts demonstrated modulate ferrous ion chelating activity (IC50= 0.62 – 0.87 mg/ml) and modulate correlations of such activity with contents of phenolics and flavonoids. In particular for the red rice bran extracts, this activity was partly due to the presence of proanthocyanidins. Thus, pigmented rice bran extracts are abundant, safe, and inexpensive sources of potent natural antioxidants.