Abstract:
Gamma-oryzanol (γ-OR) is a unique phytosterol in rice bran and its oil. It is a mixture of 10 distinct triterpene derived compounds that are esterified to ferulic acid. Hypocholesterolemic of rice bran oil and γ-OR in animal and human studies have been reported. This study aimed to investigate the proposed mechanism of γ-OR on hypocholesterolemic and antioxidant activities of γ-OR against LDL oxidation. The effects of γ-OR on the cholesterol micellarization, cholesterol uptake by Caco-2 intestinal cells, and HMG-CoA reductase activity were determined. The results showed that γ-OR incorporated efficiently into micelles during simulated digestion of rice meal mixed with high γ-OR oil. High concentration of γ-OR at 1500 µM did not decrease the efficiency of micellarization of cholesterol at 400 µM in synthesis micelles. At 10- and 20-fold molar ratios to cholesterol but low concentration (13 and 26 µM), γ-OR did not inhibit the efficiency of cholesterol micellarization during simulated digestion. Nevertheless, γ-OR at 20-fold molar ratio to cholesterol significantly decreased cholesterol uptake by Caco-2 intestinal cells (p<0.05). In addition, γ-OR showed the inhibitory effect on the HMG-CoA reductase activity similar to simvastatin. Furthermore, the antioxidant activities of γ-OR against LDL oxidation were investigated in seven healthy volunteers. Isolated LDL fractions were prepared and the measurement of conjugated dienes, thiobarbituric acid-reactive substances (TBARS) formation, relative electrophoresis mobility (REM), and apo B-100 fragmentation were assessed with varies concentrations of γ-OR compared with ferulic acid and α-tocopherol after initiation of in vitro oxidation for 4 hours. This in vitro study demonstrated that γ-OR was a good antioxidant against LDL oxidation. Gamma-oryzanol provided stronger antioxidative activity than those of ferulic acid and α-tocopherol in decreasing TBARS formation, and the same as that of ferulic acid and α-tocopherol in the REM test (p<0.05). Furthermore, γ-OR was good at prolonging the lag time of conjugated diene formation, and reducing apo B-100 fragmentation, although it was weaker than those of ferulic acid and α-tocopherol (p<0.05). These findings suggest that γ-OR provide the strong antioxidative activity against LDL oxidation. Moreover, hypocholesterolemic activity of γ-OR is supposedly due to the inhibition of cholesterol biosynthesis, rather than the inhibition of cholesterol micellarization. However, further study is still needed in nutrigenomic approach to understand the whole hypocholesterolemic mechanism.
