Structural Characterization and Assessment of Anti-Inflammatory and Anti-Tyrosinase Activities of Polyphenols from <i>Melastoma normale</i>

• Main Authors: Rui-Jie He, Jun Li, Yong-Lin Huang, Ya-Feng Wang, Bing-Yuan Yang, Zhang-Bin Liu, Li Ge, Ke-Di Yang, Dian-Peng Li
• Format: Article
• Language: English
• Published: MDPI AG 2021-06-01
• Series: Molecules
• Subjects: <i>Melastoma normale</i>; <i>Melastoma</i>; polyphenols; tyrosinase; anti-inflammatory; ellagitannins
• Online Access: https://www.mdpi.com/1420-3049/26/13/3913

Polyphenols, widely distributed in the genus <i>Melastoma</i> plants, possess extensive cellular protective effects such as anti-inflammatory, anti-tyrosinase, and anti-obesity, which makes it a potential anti-inflammatory drug or enzyme inhibitor. Therefore, the aim of this study is to screen for the anti-inflammatory and enzyme inhibitory activities of compounds from title plant. Using silica gel, MCI, ODS C18, and Sephadex LH-20 column chromatography, as well as semipreparative HPLC, the extract of <i>Melastoma normale</i> roots was separated. Four new ellagitannins, Whiskey tannin C (<b>1</b>), 1-<i>O</i>-(4-methoxygalloyl)-6-<i>O</i>-galloyl-2,3-<i>O</i>-(<i>S</i>)-hexahydroxydiphenoyl-<i>β</i>-<span style="font-variant: small-caps;">d</span>-glucose (<b>2</b>), 1-<i>O</i>-galloyl-6-<i>O</i>-(3-methoxygalloyl)-2,3-<i>O</i>-(<i>S</i>)-hexahydroxydiphenoyl-<i>β</i>-<span style="font-variant: small-caps;">d</span>-glucose (<b>3</b>), and 1-<i>O</i>-galloyl-6-<i>O</i>-vanilloyl-2,3-<i>O</i>-(<i>S</i>)-hexahydroxydiphenoyl-<i>β</i>-<span style="font-variant: small-caps;">d</span>-glucose (<b>4</b>), along with eight known polyphenols were firstly obtained from this plant. The structures of all isolates were elucidated by HRMS, NMR, and CD analyses. Using lipopolysaccharide (LPS)-stimulated RAW2 64.7 cells, we investigated the anti-inflammatory activities of compounds <b>1</b>–<b>4</b>, unfortunately, none of them exhibit inhibit nitric oxide (NO) production, their IC₅₀ values are all > 50 μM. Anti-tyrosinase activity assays was done by tyrosinase inhibition activity screening model. Compound <b>1</b> showed weak tyrosinase inhibitory activity with IC₅₀ values of 426.02 ± 11.31 μM. Compounds <b>2</b>–<b>4</b> displayed moderate tyrosinase inhibitory activities with IC₅₀ values in the range of 124.74 ± 3.12–241.41 ± 6.23 μM. The structure–activity relationships indicate that hydroxylation at C-3′, C-4′, and C-3 in the flavones were key to their anti-tyrosinase activities. The successful isolation and structure identification of ellagitannin provide materials for the screening of anti-inflammatory drugs and enzyme inhibitors, and also contribute to the development and utilization of <i>M. normale</i>.