Anti-Tyrosinase and Radical Scavenging Activities of Selected Cassipourea Plants and Isolated 7-Methoxygeranin A: Traditional Use as Skin Lighteners.

Background: In rural areas of KwaZulu-Natal and Eastern Cape Provinces, South Africa, women have traditionally used bark extracts from Cassipourea flanaganii, C. malosana, and C. gummiflua for skin lightening and sun protection. This study investigates the anti-tyrosinase and antioxidant activities of methanolic bark extracts from these species, aiming to validate their traditional use and identify potential lead compounds for the treatment of skin hyperpigmentation.

Methods: Anti-tyrosinase activity was evaluated using half-maximal inhibitory concentration (IC₅₀) values, and antioxidant capacity was measured through FRAP, DPPH, and TEAC assays. Polyphenol and flavanol contents were quantified using Folin-Ciocalteu method. Potential lead compounds were identified through molecular docking, pharmacokinetic analysis, and molecular dynamics (MD) simulations. Statistical analyses, including ANOVA and post-hoc tests, compared extract activities.

Results: C. flanaganii exhibited the most potent anti-tyrosinase activity (IC₅₀: 37.10 µg/mL), though statistical differences among species were non-significant. C. gummiflua showed the highest polyphenol (143.68 mg GAE/g) and flavanol (14.67 mg QE/g) content, correlating with superior antioxidant activity (FRAP: 526.07 µmol AAE/g; DPPH: 390.26 µmol TE/g; TEAC: 596.98 µmol TE/g). The isolated compound 7-methoxygeranin A demonstrated lower anti-tyrosinase activity (IC₅₀: 45.16 µg/mL) compared to C. flanaganii extract, suggesting the presence of more potent metabolites. Molecular docking and MD simulations identified emodin 6,8-dimethyl ether as a thermodynamically stable lead compound (binding free energy: -39.88 kcal/mol), interacting with key catalytic residues over 150 ns. The compound demonstrated prolonged residence at the active site of tyrosinase, indicating strong-binding stability.

Conclusion: While C. flanaganii demonstrated the strongest anti-tyrosinase activity, C. gummiflua showed the highest antioxidant potential. Emodin 6,8-dimethyl ether emerged as a promising candidate for skin-lightening applications, warranting further in vitro and in vivo validation. These findings support the traditional use of Cassipourea species and highlight their potential for developing natural skin health products. Further studies are needed to explore the pharmacokinetics, safety, and efficacy of these compounds in clinical settings.