Antioxidant, antimicrobial, and α-glucosidase inhibitory activities of saponin extracts from walnut (Juglans regia L.) leaves

Abstract

Objective: To investigate the relationship between triterpenoid saponin content and antioxidant, antimicrobial, and α-glucosidase inhibitory activities of 70% ethanolic, butanolic, aqueous, supernate and precipitate extracts of Juglans regia leaves. Methods: Triterpenoid saponins of different Juglans regia leaf extracts were measured by the vanillin method. Antioxidant activity was evaluated against DPPH and ABTS free radicals. We also assessed α-glucosidase inhibitory and antimicrobial activities of the leaf extracts. Pearson's correlation coefficient was evaluated to determine the correlation between the saponin content and biological activities. Results: The butanolic extract was most effective against DPPH with an IC₅₀ of 6.63 µg/mL, while the aqueous extract showed the highest scavenging activity against ABTS free radical with an IC₅₀ of 42.27 µg/mL. Pearson's correlation analysis indicated a strong negative correlation (r = −0.956) between DPPH radical scavenging activity (IC₅₀) and the saponin content in the samples examined. In addition, the aqueous extract showed the best α-glucosidase inhibitory activity compared with other extracts. All the extracts had fair antibacterial activity against Bacillus subtilis, Escherichia coli, and Klebsiella pneumoniae except for the aqueous extract. Conclusions: Juglans regia extracts show potent antioxidant, antimicrobial, and α-glucosidase inhibitory activities. There is a correlation between saponin levels in Juglans regia leaf extracts and the studied activities. However, additional research is required to establish these relationships by identifying the specific saponin molecules responsible for these activities and elucidating their mechanisms of action.

Keywords

Juglans regia leaves / Triterpenoid saponin / Antioxidant activity / DPPH / ABTS / Antidiabetic activity / α-Glucosidase / Antimicrobial activity

Acknowledgments

The authors would like to thank the Deanship of Scientific Research at Umm Al-Qura University for supporting this work (Grant Code: 22UQU4331128DSR77).

Conflict of interest statement

The authors declare no conflict of interest.

Funding

The study was supported by the Deanship of Scientific Research at Umm Al-Qura University (Grant code: 22UQU4331128DSR77).

Authors’ contributions

MT and LCM were responsible for conceptualization, as well as project administration, and AEY provided supervision. YE, SM, and MH were responsible for data curation and formal analysis, while AEY, HH, and AB conducted investigations. AB and LCM also contributed to the drafting and critical revision of the manuscript. MEAF, AB, KWG, and AK were responsible for validation, and YE, AK, ANA, and KWG contributed to visualization. YE, and MH were responsible for writing the original draft, while AB, HH, AK, ANA, KWG, and MEAF contributed to the review and editing of the manuscript. All authors have read and agreed to the published version of the manuscript.

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