Stevia rebaudiana (Asteraceae), a perennial plant, has been used as a low-calorie sweetener and is being developed as a therapeutic agent for diabetes, hypertension, myocardial diseases, and microbial infections. Despite the common use of its leaves and stem, the bioavailability of the components present in S. rebaudiana flowers, when used as ingredients of cosmetics, has not been well investigated. Herein, we investigated the antioxidative and antimelanogenic effects of an aqueous extract of S. rebaudiana flowers (Stevia-F). Total flavonoid and phenolic content in Stevia-F were determined to be 8.64 ± 0.23 ㎎ of quercetin equivalents/100 g and 631.5 ± 2.01 ㎎ of gallic acid equivalents/100 g, respectively. The IC50 values of Stevia-F for reducing power, and 2,2-diphenyl-1-picryl-hydrazyl-hydrate radical, hydrogen peroxide, and nitric oxide scavenging activities were 5541.96, 131.39, 466.34, and 10.44 ㎍/mL, respectively. Stevia-F showed inhibitory effects on the tyrosinase (IC50 = 134.74 ㎍/mL) and α-glucosidase (IC50 = 114.81 ㎍/mL) activities. No significant cytotoxicity of Stevia-F was observed in B16F10 cells, treated with up to 100 ㎍/mL of the extract for 24 and 48 h (p > 0.05). Stevia-F (1–100 ㎍/mL) suppressed α-melanocyte stimulating hormone-induced melanin production in B16F10 cells (p < 0.05) and also inhibited the cellular tyrosinase activity (p < 0.05). Overall, our results show that Stevia-F possesses potential for inhibiting tyrosinase and α-glucosidase activities and has significant antioxidant capacity. The antimelanogenic potential of Stevia-F should extend the usage of S. rebaudiana flowers in the development of skinwhitening products.

Abstract
Introduction
Materials and Methods
    Chemicals
    Preparation of S. rebaudiana flower extract
    Determination of total phenolic and total flavonoid content
    Determination of DPPH radical scavenging activity
    Determination of hydrogen peroxide scavenging activity
    Determination of the reducing power of Stevia-F
    Determination of NO scavenging activity
    Effect of Stevia-F on α-glucosidase activity in cell-free system
    Effect of Stevia-F on tyrosinase inhibitory activity in cellfree system
    Cell culture
    Cell viability
    Effect of Stevia-F on the production of melanin in B16F10 cells
    Effect of Stevia-F on cellular tyrosinase activity
    Statistical analysis
Results
    Total content of phenolics and flavonoids in Stevia-F
    Antioxidant capacity of Stevia-F
    Effect of Stevia-F on the tyrosinase and α-glucosidase activities
    Effect of Stevia-F on the viability of B16F10 cells
    Inhibitory effect of Stevia-F on melanin production in B16F10 cells
    Effect of Stevia-F on tyrosinase activity stimulated by α-MSH
Discussion
References

• Gyeongseop So(Department of Herbal Medicine Resource, Kangwon National University)
• Sung Ryul Lee(Department of Convergence Biomedical Science, Cardiovascular and Metabolic Disease Center, Inje University)
• Sung Hyeok Kim(Department of Herbal Medicine Resource, Kangwon National University)
• Chang Woo Ha(Department of Herbal Medicine Resource, Kangwon National University)
• Yuna Park(Department of Herbal Medicine Resource, Kangwon National University)
• Sohee Jang(Department of Herbal Medicine Resource, Kangwon National University)
• Jong Phil Bak(Department of Herbal Medicine Resource, Kangwon National University)
• Hyun Jung Koo(Department of Medicinal and Industrial Crops, Korea National College of Agriculture and Fisheries)
• Eun-Hwa Sohn(Department of Herbal Medicine Resource, Kangwon National University) Corresponding author