Phytochemical constituents and antidiabetic properties of Asystasia gangetica (L.) T. Anderson subsp. micrantha and Bouea macrophylla Griff

The phytochemical investigations were conducted on two plants: Asystasia gangetica (L.) T. Anderson subsp. micrantha (Acanthaceae) and Bouea macrophylla Griff. (Anacardiaceae). This study aims to isolate and elucidate the secondary metabolites from the two plants as mentioned, propose the plausible biosynthetic pathway of the isolates, and assess the isolated compounds' cytotoxicity and glucose uptake activities. The isolation process was conducted by using chromatographic techniques such as Vacuum Liquid Chromatography (VLC), Thin Layer Chromatography (TLC), column chromatography (CC), and High-Performance Liquid Chromatography (HPLC). The structures of the isolated compounds were elucidated employing various spectroscopic analyses, namely infrared (IR), ultraviolet-visible (UV-Vis), mass spectrometer (MS) and nuclear magnetic resonance (NMR) spectroscopies, and comparison with the previous literature studies. Eleven known compounds were isolated from the leaves of A. gangetica, including three monoterpenes, which are loliolide, dehydrololiolide and dehydrovomifoliol; two cinnamic acid derivatives including ferulic acid and methyl caffeates; two flavones namely chrysoeriol and chrysoeriol-4’-glycoside; a glycosidic tyrosol namely salidroside; two phenylethanoids namely verbascoside and forsythiaside as well as two iridoids namely 3’’-O-caffeoyl-6-O-rhamnopyranosyl catalpol and 4’’-O-caffeoyl-6-O-rhamnopyranosyl catalpol. Ten flavonoids were obtained from the twigs of B. macrophylla, which include six flavanonols: garbanzol, fustin, taxifolin, astilbin, isoastilbin and neoastilbin; one flavonol, which is resokaempferol; one flavanol characterized as catechin; and two flavandiol known as mollisacacidin and guibourtacacidin. Meanwhile, the purification steps on the leaves of B. macrophylla have successfully yielded six pure compounds, including three flavonols, which are myricitrin and mearnsitrin, as well as one newly elucidated compound namely myricetin-3-O-(2’’-O-galloyl)-a-rhamnopyranoside. A tannic acid, penta-O-galloyl glucoside, and triterpene, lupeol, were also isolated. This is the first report describing and elucidating the stated compounds from B. macrophylla. Selected isolated compounds were tested for cytotoxicity against C2C12 muscle cell lines and glucose uptake assay. None of the compounds exhibited toxicity. Significant glucose uptake activities (p < 0.05) were observed for salidroside (2.12-fold at 6.25 µg/mL), chrysoeriol (1.79-fold at 1.562 µg/mL), and verbascoside (1.79-fold at 1.562 µg/mL) isolated from A. gangetica. From B. macrophylla, significant activities were found for myricitrin (3.10-fold at 1.56 µg/mL), mearnisitrin (1.58-fold at 1.56 µg/mL), and astilbin (1.28-fold at 3.13 µg/mL). Among all tested compounds, myricitrin exhibited the highest glucose uptake activity. All compounds showed comparable activities to the standard rosiglitazone. Additionally, the study proposed biosynthetic pathways for the newly identified compounds and thoroughly discussed their Structure-Activity Relationship (SAR). The SAR analysis revealed that antidiabetic activity is influenced by the number of hydroxyl groups, the presence of a catechol moiety, and a C-4 ketonic group. Flavanols showed higher activity compared to other flavonoids. The position of methoxy groups affects electron-donating properties and interactions with biological targets. In conclusion, this study isolated several bioactive compounds with significant glucose uptake activity, providing insights into their potential as antidiabetic agents and laying the groundwork for further drug development.