*3.4. In Vitro Tyrosinase Inhibition Assay by UPLC-MS*/*MS Analysis*

The crude extract of *A. keiskei* root (15 mg) was dissolved in 75 μL of DMSO and 525 μL of 50 mM phosphate buffer (pH 6.8) to form the blank solution. Similarly, another 15 mg of the crude extract was dissolved in 75 μL DMSO and 425 μL of phosphate buffer solution (50 mM, pH 6.8) before 100 μL of tyrosinase (716 units/mL) was added as the test solution. Both solutions were stirred and incubated in the 37 ◦C water bath for 40 minutes in the dark. Subsequently, 900 μL of acetonitrile was added to each solution in order to terminate the enzyme reaction. Both test solutions with the final concentration of 10 mg/mL were filtered through the 0.22 μm filters. The working solutions were then analyzed by UPLC-MS/MS.
