Tyrosinase inhibitors from bark of talawrinta (Manilkara Kauki (L.) dubard

Abstract

Melanin is essential for protecting human skin from ultraviolet radiation. However, its overproduction in the basal epithelial layer leads to hyperpigmentary disorders of the skin such as melasma, blemish and age spots. Tyrosinase is one of key enzymes in melanogenesis. Manilkara kauki (L.) Dubard or Talawrinta is one of the plants in genus Manilkara of Sapotaceae family. This study attempted to investigate in vitro total phenolic and flavonoid contents, antioxidant and tyrosinase inhibitory activities of crude extract of different parts of M. kauki and purify tyrosinase inhibitors from stem barks of M. kauki. Methanol and aqueous crude extracts of fruits, leaves, seeds, stem barks and woods of M. kauki evaluated on their total phenolic and flavonoid contents, antioxidant and tyrosinase inhibitory activities. Stem barks of M. kauki showed high amount of total phenolic and flavonoid contents and strong antioxidant activity. Methanol crude extracts of stem barks exhibited the highest tyrosinase inhibitory activity (IC₅₀ value of 0.26 ± 0.05 mg/mL). Thus, stem barks of M. kauki was extracted with n-hexane, ethyl acetate, methanol and water, respectively. Both of ethyl acetate and methanol crude extracts of stem barks exhibited high amount of phenolic compounds and flavonoids and strong antioxidant activity. Furthermore, ethyl acetate crude extract of stem bark exhibited higher tyrosinase inhibitory activity than n-hexane, methanol and aqueous crude extracts with IC₅₀ values of 0.24 ± 0.02 and 0.28 ± 0.04 mg/mL for L-tyrosine and L-DOPA as substrates, respectively. Thus, ethyl acetate crude extract was further isolated and purified to afford compound I as taraxerol and compound II as dihydrokaempferol. Compound I exhibited tyrosinase inhibitory activity with IC₅₀ values of 2.32 ± 0.06 and 2.65 ± 0.03 mM for L-tyrosine and L-DOPA as substrates, respectively. Compound II exhibited tyrosinase inhibitory activity with IC₅₀ values of 1.15 ± 0.06 and 1.74 ± 0.05 mM for L-tyrosine and L-DOPA as substrates, respectively. Compounds I and II showed lower tyrosinase inhibitory activity than kojic acid but they were exhibited higher tyrosinase inhibitory activity than α-arbutin. These results suggested that ethyl acetate crude extract of stem barks of M. kauki should be further investigated for others biological activities including anti-allergic, cytotoxicity and in vivo tyrosinase inhibitory activities to evaluate it before using as a potential ingredient in whitening cosmetics.