Melanogenesis inhibitory activity of a 7-O-9'-linked neolignan from Alpinia galanga fruit.

ABSTRACT

An aqueous acetone extract from the fruit of Alpinia galanga (Zingiberaceae) demonstrated inhibitory effects on melanogenesis in theophylline-stimulated murine B16 melanoma 4A5 cells (IC50=7.3µg/mL). Through bioassay-guided separation of the extract, a new 7-O-9'-linked neolignan, named galanganol D diacetate (1), was isolated along with 16 known compounds including 14 phenylpropanoids (2-15). The structure of 1, including its absolute stereochemistry in the C-7 position, was elucidated by means of extensive NMR analysis and total synthesis. Among the isolates, 1 (IC50=2.5µM), 1'S-1'-acetoxychavicol acetate (2, 5.0µM), and 1'S-1'-acetoxyeugenol acetate (3, 5.6µM) exhibited a relatively potent inhibitory effect without notable cytotoxicity at effective concentrations. The following structural requirements were suggested to enhance the inhibitory activity of phenylpropanoids on melanogenesis (i) compounds with 4-acetoxy group exhibit higher activity than those with 4-hydroxy group; (ii) 3-methoxy group dose not affect the activity; (iii) acetylation of the 1'-hydroxy moiety enhances the activity; and (iv) phenylpropanoid dimers with the 7-O-9'-linked neolignan skeleton exhibited higher activity than those with the corresponding monomer. Their respective enantiomers [1' (IC50=1.9µM) and 2' (4.5µM)] and racemic mixtures [(±)-1 (2.2µM) and (±)-2 (4.4µM)] were found to exhibit melanogenesis inhibitory activities equivalent to those of the naturally occurring optical active compounds (1 and 2). Furthermore, the active compounds 1-3 inhibited tyrosinase, tyrosine-related protein (TRP)-1, and TRP-2 mRNA expressions, which could be the mechanism of melanogenesis inhibitory activity.