Beneficial effect of the unsaponifiable matter from rice bran on oxidative stress in vitro compared with α-tocopherol

Unsaponifiable matter (UM) was prepared from rice bran using n-hexane extraction followed by removal of its fatty acid methyl ester with supercritical CO₂ under heat-stable conditions. The UM was made up of 1% of vitamin E isomers, 28% of γ-oryzanol and 71% of uncharacterized compounds. The aim of this study was to determine the antioxidant activities of the UM, using α-tocopherol (α-T) as a positive control, by measuring the Fe³⁺-reducing antioxidant power (FRAP), 2,2-diphenyl-1-picrylhydrazyl (DPPH^•) free-radical-scavenging property and lipid peroxidation in rat liver microsomes. In addition, the effects of the UM on the tert-butyl hydroperoxide (t-BOOH)-induced cytotoxicity in cultured rat hepatocytes were also investigated. In FRAP assay and DPPH^• free-radical- scavenging assay, the results were expressed g^-1 α-T or g ^-1 UM. The amount of UM used in lipid peroxidation assay and cytotoxicity assay was the amount required to have equal amounts of total vitamin E isomers in the sample and the control α-T. The UM, as well as α-T, exhibited significant antioxidant activities in FRAP, radical-scavenging and membrane-lipid oxidation. The FRAP value for total vitamin E isomers in the UM (TVEIUM) was 9.1 times higher than that for α-T. In terms of their capacities to perform radical-scavenging and lipid peroxidation, both TVEIUM and α-T showed similar antioxidant activities. In experiments using cultured rat hepatocytes, the t-BOOH-induced lactate dehydrogenase release was significantly inhibited by the addition of 63.5 and 160 μg ml^-1 of TVEIUM treatments (84 and 89%, respectively), and that of 63.5 and 160 μg ml^-1 of α-T treatment (88 and 93%, respectively). The antioxidant function against oxidative stress of the UM prepared from rice bran may extend its use to being a potential dietary supplement.