Bioactive Flavonoids from Paulownia tomentosa Flowers: Extraction Optimization and α-Glucosidase Inhibitory Kinetics

Paulownia tomentosa flowers are rich in flavonoids with promising biological activities. However, few studies have investigated their potential for α-glucosidase inhibition. This study compared ultrasound-assisted cellulase extraction and ultrasound-assisted aqueous two-phase extraction for the recovery of flavonoids from Paulownia tomentosa flowers. The aqueous two-phase extraction method demonstrated superior performance, with optimal conditions determined as 17.80% (NH4)2SO4, 12 min ultrasonication, and 44 °C. Purification was efficiently achieved using NKA-9 macroporous resin. Scanning electron microscopy revealed that ultrasonic treatment disrupted the cell walls, facilitating flavonoid release. Ultra-performance liquid chromatography–tandem mass spectrometry identified apigenin-7-glucuronide and scutellarin as the predominant flavonoids. Notably, several compounds—including scutellarin, ombuin, robinetin, and astragalin—were reported for the first time in this plant. The extracted flavonoids exhibited significant α-glucosidase inhibitory activity, with an IC50 value of 0.412 mg/mL, and showed mixed-competitive inhibition. Luteolin 7-O-glucuronide was identified as a major active constituent, exhibiting stronger inhibition than the total flavonoids while sharing the same mechanism. These findings establish a theoretical foundation for the efficient and sustainable extraction of P. tomentosa flavonoids and support their further development for pharmaceutical applications.