Bioassay guided isolation of caffeoylquinic acids from the leaves of Ilex pubescens Hook. et Arn. and investigation of their anti-influenza mechanism

Ilex pubescens Hook. et Arn. (Maodongqing, MDQ) is a common herbal tea ingredient in Southern China for heat clearance and anti-inflammation. Our preliminary screening showed that 50% ethanol extract of its leaves has anti-influenza virus activity. In this report, we proceed to identify the active components and clarify the related anti-influenza mechanisms. We aim to isolate and identify the anti-influenza virus phytochemicals from the extract of the MDQ leaves, and study their anti-influenza virus mechanism. Plaque reduction assay was used to test the anti-influenza virus activity of fractions and compounds. Neuraminidase inhibitory assay was used to confirm the target protein. Molecular docking and reverse genetics were used to confirm the acting site of caffeoylquinic acids (CQAs) on viral neuraminidase. Eight CQAs, 3,5-di-O-caffeoylquinic acid methyl ester (Me 3,5-DCQA), 3,4-di-O-caffeoylquinic acid methyl ester (Me 3,4-DCQA), 3,4,5-tri-O-caffeoylquinic acid methyl ester (Me 3,4,5-TCQA), 3,4,5-tri-O-caffeoylquinic acid (3,4,5-TCQA), 4,5-di-O-caffeoylquinic acid (4,5-DCQA), 3,5-di-O-caffeoylquinic acid (3,5-DCQA), 3,4-di-O-caffeoylquinic acid (3,4-DCQA), and 3,5-di-O-caffeoyl-epi-quinic acid (3,5-epi-DCQA) were identified from the MDQ leaves, in which Me 3,5-DCQA, 3,4,5-TCQA and 3,5-epi-DCQA were isolated for the first time. All these eight compounds were found to inhibit neuraminidase (NA) of influenza A virus. The results of molecular docking and reverse genetics indicated that 3,4,5-TCQA interacted with Tyr100, Gln412 and Arg419 of influenza NA, and a novel NA binding groove was found. Eight CQAs isolated from the leaves of MDQ were found to inhibit influenza A virus. 3,4,5-TCQA was found to interact with Tyr100, Gln412 and Arg419 of influenza NA. This study provided scientific evidence on the use of MDQ for treating influenza virus infection, and laid the foundation for the development of CQA derivatives as potential antiviral agents.