Optimization of Different Extraction Methods for Phenolic Compound Verbascoside from Chinese Olea europaea Leaves Using Deep Eutectic Solvents: Impact on Antioxidant and Anticancer Activities

Abstract

Chinese Olea europaea leaves, rich in verbascosides, were extracted using ultrasound-assisted extraction (UAE) and wall-breaking extraction (WBE) with deep eutectic solvents (Optimal UAE: 55 min, 200 mL/g liquid–solid ratio, 20% moisture, yielding 206.23 ± 0.58 mg GAE/g total phenolic content (TPC) and 1.59 ± 0.04% verbascoside yield (VAY); Optimal WBE: 140 s, 210 mL/g, 30% moisture, giving 210.69 ± 0.97 mg GAE/g TPC and 1.33 ± 0.2% VAY). HPLC analysis showed that young leaves accumulated higher TPC and phenolic compounds. Among the five olive varieties, Koroneiki and Chemlal showed the highest TPC in UAE, while Arbosana and Chemlal excelled in WBE. WBE yielded a higher TPC and rutin, whereas UAE marginally increased other phenolics. Additionally, the DPPH• assay showed that WBE-extracted verbascoside-rich extracts (VREs) of Chemlal exhibited high antioxidant activity (EC50 of 57 mg/mL), but Koroneiki-VREs exhibited lower activity against the ABTS•+ radical (EC50 of 134 mg/mL). Remarkably, the UAE/WBE-extracted Chemlal-VREs promoted the normal esophageal Het-1A cell line at 25 μg/mL for 24 h; yet, the esophageal cancer Eca-109 cells were sensibly inhibited, especially at 50 μg/mL; and the cell viability decreased dramatically. The results confirmed WBE as a relatively efficient method, and the Chemlal variety may be an excellent source of verbascoside.