*2.5. Assay of Tyrosinase Activity*

Tyrosinase activity was measured using a Tyrosinase activity assay kit (BC4055, Beijing Solarbio Science & Technology Co., Ltd., Beijing, China). OD was measured at 475 nm.

#### *2.6. Measurement of Melanin Synthesis*

B16F10 cells were seeded in 6-well plates at a density of 2 × 106 cells/well. After incubation for 24 h, the cells were treated with FTGML (0–1.6 mg/mL) or kojic acid (0.75 mg/mL), and incubated at 37 ◦C in 5% CO2 for 48 h. The cells were washed with PBS, digested with trypsin, and centrifuged at 1500 rpm for 10 min. To dissolve the cell precipitates, 1 mL 1 M NaOH containing 10% DMSO was used at 80 ◦C for 1 h. The optical density of each well at 405 nm was measured with a microplate reader, and the content of melanin in the cell was calculated [12]:

$$\text{Melanin content} / \%= \frac{(\text{A}\_{\text{s}} - \text{A}\_{\text{b}})}{(\text{A}\_{\text{c}} - \text{A}\_{\text{b}})} \times 100\% \tag{1}$$

where, As is the absorbance of experimental wells; Ac is the absorbance of control well; Ab is the absorbance of blank wells.

#### *2.7. Assay of Antioxidant Activity*

The centrifuge precipitate in Section 2.6 was dissolved with 1 mL 1 M NaOH containing 10% DMSO at 80 ◦C for 1 h were collected separately for antioxidant activity determination. The content of reduced glutathione (GSH), oxidized glutathione (GSSG), reactive oxygen species (ROS), and malondialdehyde (MDA), and the activity of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPX) were measured using different assay kits that were all from Beijing Solarbio Science & Technology Co., Ltd., Beijing, China, and the item numbers were as follows: GSH by BC1175; GSSG by BC1185; ROS by CA1410; MDA by BC0025; SOD by BC0175; CAT by BC0205; GPX by BC1195.

#### *2.8. Measurement of Intracellular cAMP Concentration*

B16F10 cells were inoculated in 6-well plates at a density of 2 × <sup>10</sup><sup>5</sup> cells/well. After 24 h incubation, the cells were treated with FTGML (0–1.6 mg/mL) or kojic acid (0.75 mg/mL), and incubated at 37 ◦C in 5% CO2 for 48 h. Cells were washed with cold PBS, and dissolved on ice in a RIPA buffer containing protease and phosphatase inhibitors. The samples were centrifuged at 1000× *g* for 20 min to remove impurities and cell debris, and the supernatant was used for cAMP detection. A cAMP immunoassay kit (Jiancheng Bioengineering Institute, Nanjing, China) was used to measure intracellular cAMP concentration [12].
