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The tyrosinase inhibitory activity of methanol extracts of the leaves of 39 plant species growing on the seashore of Iriomote island (Okinawa, Japan) was investigated. The extracts of Hibiscus tiliaceus, Carex pumila, and Garcinia subelliptica showed potent activity among them. The inhibitors in the extract of Garcinia subelliptica were purified by assay-guided fractionation to give two biflavonoids. These were known compounds (2R,3S-5,7,4',5'',7'',3''',4'''-heptahydroxy flavanone3-8'' flavone and 5,7,4',5'',7'',3''',4'''-heptahydroxy3-8'' biflavanone), although their strong inhibitory activity toward tyrosinase is revealed for the first time in this work. One of these biflavonoids (2R,3S-5,7,4',5'',7'',3''',4'''-heptahydroxy flavanone3-8'' flavone) showed much stronger activity (IC50 2.5 microM) than that of kojic acid (IC50 9.1 microM) when L-tyrosine was used as the substrate.
Masuda et al. (Sat,) studied this question.