Phytoconstituents and bioactivities of Cassia fistula leaf and stem bark: Antioxidant capacity and α-glucosidase inhibition via molecular docking and in vitro approaches

Cassia fistula is a traditional medicinal plant especially used in Ayuverdic and Traditional Chinese Medicine (TCM) for various diseases. This study aims to evaluate the antioxidant and α-glucosidase inhibition activities of extracts and fractions from C. fistula leaves and stem bark. C. fistula leaves and stem barks were extracted using methanol, and then stem barks extract (BME) further fractionated with hexane (BHF), ethyl acetate (BEAF), and methanol (BMF) due to its higher α-glucosidase inhibition than leaves extract (LME). Extracts and fractions were analyzed TPC, TFC, antioxidant capacity (DPPH, FRAP, and ABTS), α-glucosidase inhibition, LC-HRMS, and molecular docking of identified compounds. BME exhibited higher TPC and TFC values at 38.23 mgGAE/g and 98.41 mgQE/g compared to LME at 3.12 mgGAE/g and 4.46 mgQE/g, while stem bark fractions contain TPC values ranging from 8.44 to 39.44 mgGAE/g and TFC from 33.62 to 121.86 mgQE/g. BME exhibited higher antioxidant capacity than LME, while BEAF exhibited the highest antioxidant capacity compared to other fractions. Similarly, BME also exhibited higher α-glucosidase inhibition than LME, with IC₅₀ values of 2.81 μg/mL and 60.48 μg/mL, respectively, while BME fractions α-glucosidase inhibition ranging from 5.86 to 12.15 μg/mL. LC-HRMS identified 32 unique compounds consisting of phenolics, flavonoids, steroids, terpenoids, anthraquinones, and coumarins. Among the identified compounds, quercetin 3-O-rhamnoside-7-O-glucoside demonstrated the highest binding affinity to α-glucosidase (PDB ID: 3A4A) trough molecular docking. These findings suggest BME possesses higher antioxidant capacity and α-glucosidase than LME, while BEAF showed the highest compared to other fractions.