JOURNAL OF COSMETIC SCIENCE 356 stability of soy isofl avones. In this study, a soy isofl avone derivative, 8-hydroxydaidzein, was demonstrated to be unstable in alkaline solutions. In fact, daidzein itself, the bio- transformed precursor of 8-hydroxydaidzein, would be stable in both pH 7 and pH 9 buffers at 25°C (data not shown). Hence, to understand the detailed mechanism causing the instability of 8-hydroxydaidzein in alkaline solutions, more study is needed. Besides being a potent irreversible inhibitor toward mushroom tyrosinase (3–4), 8- hydroxydaidzein also contains strong antioxidant activity (1–2), anti-mutagenic activity (8), anti-aldose reductase activity (9), and anti-proliferation activity toward cancer cell lines (10). Because of the multifunctional bioactivities of 8-hydroxydaidzein, it is ex- pected that broad application and high quality will be needed for storage of the com- pound. Hence, according to the data in the present report, it is recommended that 8-hydroxydaidzein be formulated in an acid solution for its application in whitening cosmetics, foods, or medical agents. REFERENCES (1) H. Esaki, H. Onozaki, Y. Morimitsu, S. Kawakishi, and T. Osawa, Potent antioxidant isofl avones iso- lated from soybeans fermented with Aspergillus saitoi, Biosci. Biotechnol. Biochem., 62, 740–746 (1998). (2) H. Esaki, R. Watanabe, H. Onozaki, and S. Kawakishi, Formation mechanism for potent antioxidative 0-dihydroxyisofl avones in soybeans fermented with Aspergillus saitoi, Biosci. Biotechnol. Biochem., 63, 851–858 (1999). (3) T. S. Chang, H. Y. Ding, S. S. K. Tai, and C. Y. Wu, Tyrosinase inhibitors isolated from soygerm koji fermented with Aspergillus oryzae BCRC 32288, Food Chem., 105, 1430–1438 (2007). (4) T. S. Chang, Two potent suicide substrates of mushroom tyrosinase: 7,8,4′,-trihydroxyisofl avone and 5,7,8,4′,-tetrahydroxyisofl avone, J. Agric. Food Chem., 55, 2010–2015 (2007). (5) Y. Chen, M. Inaba, N. Abe, and A. Hirota, Antimutagenic activity of 8-hydroxyisofl avones and 6- hydroxydaidzein from soybean miso, Biosci. Biotechnol. Biochem., 67, 903–906 (2003). Figure 3. Residual tyrosinase inhibitory activity (vertical lines) and DPPH-radical scavenging activity (hor- izontal lines) of 8-hydroxydaidzein in a series of solutions with different pH values. For the tyrosinase in- hibitory activity analysis, 100 μl of the tested sample was pre-incubated with 400 μl of tyrosinase (20 units) in 50 mM of phosphate buffer (pH 6.8) at 25°C for 30 min. Then, 500 μl of 2.5 mM L-Dopa in the same phosphate buffer was added and the reaction mixture was incubated at 25°C for 10 min. The formation of dopachrome in each reaction was monitored at 475 nm with a spectrophotometer. For the DPPH-radical scavenging activity analysis, 100 μl of the tested sample was mixed with 900 μl of 2 mM DPPH MeOH solu- tion. After standing for 30 min, the absorbance of the mixture at 517 nm was measured. The tested samples included a freshly prepared 8-hydroxydaidzein solution [100 μg/ml in 50 mM of phosphate buffer (pH 6.8)] and the prepared solutions at the end of the stability experiment in Figure 2. The relative activity was calcu- lated by dividing the absorbance of each reaction mixture by that of the control reaction, in which phosphate buffer replaced the tested compound. The residual activity of each sample was calculated by dividing the relative activity of each sample by that of the freshly prepared sample.

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