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Reply to Yan et al.: Quercetin possesses a fluorescence quenching effect but is a weak inhibitor against SARS-CoV-2 main protease

Xu et al., Proceedings of the National Academy of Sciences, doi:10.1073/pnas.2309870120

Sep 2023

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Quercetin for COVID-19

22nd treatment shown to reduce risk in July 2021

^*, now known with p = 0.0031 from 11 studies.

Lower risk for ICU admission, hospitalization, recovery, cases, and viral clearance.

No treatment is 100% effective. Protocols combine complementary and synergistic treatments. ^* >10% efficacy in meta analysis with ≥3 clinical studies.

4,100+ studies for 60+ treatments. c19early.org

In Vitro study Yan and associated response from the original authors Xu, collectively showing that quercetin and echinatin had weak SARS-CoV-2 protease inhibition in SDS-PAGE assays Xu, despite false positive FRET results from MCA-AVLQ quenching Xu, Yan. Authors note that the compounds may act via other targets to achieve reported anti-COVID-19 effects Xu, and underscore the importance of meticulous validation with multiple assays when identifying SARS-CoV-2 protease inhibitors Xu, Yan.

54 preclinical studies support the efficacy of quercetin for COVID-19:

33 In Silico studies Ahmed, Akinwumi, Alanzi, Alavi, Alkafaas, Azmi, Chandran, Chellasamy, Corbo, El-Megharbel, Hasanah, Ibeh, Kandeil, Mandal, Moschovou, Nalban, Nguyen, Pan, Qin, Rehman, Sai Ramesh, Sekiou, Shaik, Singh, Singh (B), Thapa, Wang, Waqas, Yang, Yang (B), Zhang, Zhou, Şimşek

16 In Vitro studies Aguado, Bahun, DiGuilio, El-Megharbel, Fang, Goc, Kandeil, Munafò, Pan, Roy, Singh (C), Waqas, Wu, Xu (B), Yang, Zhang (B)

5 In Vivo animal studies Aguado, El-Megharbel, Shaker, Wu, Wu (B)

In Silico studies predict inhibition of SARS-CoV-2, or minimization of side effects, with quercetin or metabolites via binding to the spike Note A, Alavi, Azmi (B), Chandran, Kandeil, Mandal, Moschovou, Nguyen, Pan, Thapa (B), Şimşek, M^pro Note B, Akinwumi, Alanzi, Ibeh, Kandeil, Mandal, Moschovou, Nguyen, Qin, Rehman, Sekiou (B), Singh, Thapa (B), Wang, Zhang, Shaik, Waqas, Nalban, RNA-dependent RNA polymerase Note C, Corbo, PLpro Note D, Ibeh, Zhang, ACE2 Note E, Chandran, Ibeh, Qin, Thapa (B), Şimşek, Alkafaas, TMPRSS2 Note F, Chandran, helicase Note G, Alanzi, Singh (B), endoribonuclease Note H, Alavi, cathepsin L Note I, Ahmed, Wnt-3 Note J, Chandran, FZD Note K, Chandran, LRP6 Note L, Chandran, ezrin Note M, Chellasamy, ADRP Note N, Nguyen, NRP1 Note O, Şimşek, EP300 Note P, Hasanah, PTGS2 Note Q, Qin, HSP90AA1 Note R, Qin, Hasanah, matrix metalloproteinase 9 Note S, Sai Ramesh, IL-6 Note T, Yang, Yang (B), IL-10 Note U, Yang, VEGFA Note V, Yang (B), and RELA Note W, Yang (B) proteins. In Vitro studies demonstrate efficacy in Calu-3 Note X, DiGuilio, A549 Note Y, Yang, HEK293-ACE2+ Note Z, Singh (C), Huh-7 Note AA, Pan, Caco-2 Note AB, Roy, Vero E6 Note AC, Kandeil, El-Megharbel, Roy, mTEC Note AD, Wu, and RAW264.7 Note AE, Wu cells. Animal studies demonstrate efficacy in K18-hACE2 mice Note AF, Aguado, db/db mice Note AG, Wu, Wu (B), BALB/c mice Note AH, Shaker, and rats El-Megharbel (B). Quercetin reduced proinflammatory cytokines and protected lung and kidney tissue against LPS-induced damage in mice Shaker.

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1. Aguado et al., Senolytic therapy alleviates physiological human brain aging and COVID-19 neuropathology, bioRxiv, doi:10.1101/2023.01.17.524329.biorxiv.org, doi.org.

2. Ahmed et al., Evaluation of the Effect of Zinc, Quercetin, Bromelain and Vitamin C on COVID-19 Patients, International Journal of Diabetes Management, doi:10.61797/ijdm.v2i2.259.researchlakejournals.com, doi.org.

3. Akinwumi et al., Evaluation of therapeutic potentials of some bioactive compounds in selected African plants targeting main protease (Mpro) in SARS-CoV-2: a molecular docking study, Egyptian Journal of Medical Human Genetics, doi:10.1186/s43042-023-00456-4.springeropen.com, doi.org.

4. Alanzi et al., Structure-based virtual identification of natural inhibitors of SARS-CoV-2 and its Delta and Omicron variant proteins, Future Virology, doi:10.2217/fvl-2022-0184.futuremedicine.com, doi.org.

5. Alavi et al., Interaction of Epigallocatechin Gallate and Quercetin with Spike Glycoprotein (S-Glycoprotein) of SARS-CoV-2: In Silico Study, Biomedicines, doi:10.3390/biomedicines10123074.mdpi.com, doi.org.

6. Alkafaas et al., A study on the effect of natural products against the transmission of B.1.1.529 Omicron, Virology Journal, doi:10.1186/s12985-023-02160-6.biomedcentral.com, doi.org.

7. Azmi et al., Utilization of quercetin flavonoid compounds in onion (Allium cepa L.) as an inhibitor of SARS-CoV-2 spike protein against ACE2 receptors, 11th International Seminar on New Paradigm and Innovation on Natural Sciences and its Application, doi:10.1063/5.0140285.scitation.org, doi.org.

8. Azmi (B) et al., The role of vitamin D receptor and IL‐6 in COVID‐19, Molecular Genetics & Genomic Medicine, doi:10.1002/mgg3.2172.wiley.com, doi.org.

9. Bahun et al., Inhibition of the SARS-CoV-2 3CLpro main protease by plant polyphenols, Food Chemistry, doi:10.1016/j.foodchem.2021.131594.sciencedirect.com, doi.org.

10. Chandran et al., Molecular docking analysis of quercetin with known CoVid-19 targets, Bioinformation, doi:10.6026/973206300191081.bioinformation.net, doi.org.

11. Chellasamy et al., Docking and molecular dynamics studies of human ezrin protein with a modelled SARS-CoV-2 endodomain and their interaction with potential invasion inhibitors, Journal of King Saud University - Science, doi:10.1016/j.jksus.2022.102277.sciencedirect.com, doi.org.

12. Corbo et al., Inhibitory potential of phytochemicals on five SARS-CoV-2 proteins: in silico evaluation of endemic plants of Bosnia and Herzegovina, Biotechnology & Biotechnological Equipment, doi:10.1080/13102818.2023.2222196.tandfonline.com, doi.org.

13. DiGuilio et al., Quercetin improves and protects Calu-3 airway epithelial barrier function, Frontiers in Cell and Developmental Biology, doi:10.3389/fcell.2023.1271201.frontiersin.org, doi.org.

14. El-Megharbel et al., Chemical and spectroscopic characterization of (Artemisinin/Quercetin/ Zinc) novel mixed ligand complex with assessment of its potent high antiviral activity against SARS-CoV-2 and antioxidant capacity against toxicity induced by acrylamide in male rats, PeerJ, doi:10.7717/peerj.15638.peerj.com, doi.org.

15. El-Megharbel (B) et al., Chemical and spectroscopic characterization of (Artemisinin/Quercetin/ Zinc) novel mixed ligand complex with assessment of its potent high antiviral activity against SARS-CoV-2 and antioxidant capacity against toxicity induced by acrylamide in male rats, PeerJ, doi:10.7717/peerj.15638.peerj.com, doi.org.

16. Fang et al., Development of nanoparticles incorporated with quercetin and ACE2-membrane as a novel therapy for COVID-19, Journal of Nanobiotechnology, doi:10.1186/s12951-024-02435-2.biomedcentral.com, doi.org.

17. Goc et al., Inhibitory effects of specific combination of natural compounds against SARS-CoV-2 and its Alpha, Beta, Gamma, Delta, Kappa, and Mu variants, European Journal of Microbiology and Immunology, doi:10.1556/1886.2021.00022.akjournals.com, doi.org.

18. Hasanah et al., Decoding the therapeutic potential of empon-empon: a bioinformatics expedition unraveling mechanisms against COVID-19 and atherosclerosis, International Journal of Applied Pharmaceutics, doi:10.22159/ijap.2024v16i2.50128.innovareacademics.in, doi.org.

19. Ibeh et al., Computational studies of potential antiviral compounds from some selected Nigerian medicinal plants against SARS-CoV-2 proteins, Informatics in Medicine Unlocked, doi:10.1016/j.imu.2023.101230.sciencedirect.com, doi.org.

20. Kandeil et al., Bioactive Polyphenolic Compounds Showing Strong Antiviral Activities against Severe Acute Respiratory Syndrome Coronavirus 2, Pathogens, doi:10.3390/pathogens10060758.mdpi.com, doi.org.

21. Mandal et al., In silico anti-viral assessment of phytoconstituents in a traditional (Siddha Medicine) polyherbal formulation – Targeting Mpro and pan-coronavirus post-fusion Spike protein, Journal of Traditional and Complementary Medicine, doi:10.1016/j.jtcme.2023.07.004.sciencedirect.com, doi.org.

22. Moschovou et al., Exploring the Binding Effects of Natural Products and Antihypertensive Drugs on SARS-CoV-2: An In Silico Investigation of Main Protease and Spike Protein, International Journal of Molecular Sciences, doi:10.3390/ijms242115894.mdpi.com, doi.org.

23. Munafò et al., Quercetin and Luteolin Are Single-digit Micromolar Inhibitors of the SARS-CoV-2 RNA-dependent RNA Polymerase, Research Square, doi:10.21203/rs.3.rs-1149846/v1.researchsquare.com, doi.org.

24. Nalban et al., Targeting COVID-19 (SARS-CoV-2) main protease through phytochemicals of Albizia lebbeck: molecular docking, molecular dynamics simulation, MM–PBSA free energy calculations, and DFT analysis, Journal of Proteins and Proteomics, doi:10.1007/s42485-024-00136-w.springer.com, doi.org.

25. Nguyen et al., The Potential of Ameliorating COVID-19 and Sequelae From Andrographis paniculata via Bioinformatics, Bioinformatics and Biology Insights, doi:10.1177/11779322221149622.sagepub.com, doi.org.

26. Pan et al., Quercetin: A promising drug candidate against the potential SARS-CoV-2-Spike mutants with high viral infectivity, Computational and Structural Biotechnology Journal, doi:10.1016/j.csbj.2023.10.029.sciencedirect.com, doi.org.

27. Qin et al., Exploring the bioactive compounds of Feiduqing formula for the prevention and management of COVID-19 through network pharmacology and molecular docking, Medical Data Mining, doi:10.53388/MDM202407003.tmrjournals.com, doi.org.

28. Rehman et al., Natural Compounds as Inhibitors of SARS-CoV-2 Main Protease (3CLpro): A Molecular Docking and Simulation Approach to Combat COVID-19, Current Pharmaceutical Design, doi:10.2174/1381612826999201116195851.doi.org, doi.org.

29. Roy et al., Quercetin inhibits SARS-CoV-2 infection and prevents syncytium formation by cells co-expressing the viral spike protein and human ACE2, Virology Journal, doi:10.1186/s12985-024-02299-w.biomedcentral.com, doi.org.

30. Sai Ramesh et al., Computational analysis of the phytocompounds of Mimusops elengi against spike protein of SARS CoV2 – An Insilico model, International Journal of Biological Macromolecules, doi:10.1016/j.ijbiomac.2023.125553.sciencedirect.com, doi.org.

31. Sekiou et al., In-Silico Identification of Potent Inhibitors of COVID-19 Main Protease (Mpro) and Angiotensin Converting Enzyme 2 (ACE2) from Natural Products: Quercetin, Hispidulin, and Cirsimaritin Exhibited Better Potential Inhibition than Hydroxy-Chloroquine Against COVID-19 Main Protease Active Site and ACE2, ChemRxiv, doi:10.26434/chemrxiv.12181404.v1.chemrxiv.org, doi.org.

32. Sekiou (B) et al., In-Silico Identification of Potent Inhibitors of COVID-19 Main Protease (Mpro) and Angiotensin Converting Enzyme 2 (ACE2) from Natural Products: Quercetin, Hispidulin, and Cirsimaritin Exhibited Better Potential Inhibition than Hydroxy-Chloroquine Against COVID-19 Main Protease Active Site and ACE2, ChemRxiv, doi:10.26434/chemrxiv.12181404.v1.chemrxiv.org, doi.org.

33. Shaik et al., Computational identification of selected bioactive compounds from Cedrus deodara as inhibitors against SARS-CoV-2 main protease: a pharmacoinformatics study, Indian Drugs, doi:10.53879/id.61.02.13859.indiandrugsonline.org, doi.org.

34. Shaker et al., Anti-cytokine Storm Activity of Fraxin, Quercetin, and their Combination on Lipopolysaccharide-Induced Cytokine Storm in Mice: Implications in COVID-19, Iranian Journal of Medical Sciences, doi:10.30476/ijms.2023.98947.3102.ac.ir, doi.org.

35. Şimşek et al., In silico identification of SARS-CoV-2 cell entry inhibitors from selected natural antivirals, Journal of Molecular Graphics and Modelling, doi:10.1016/j.jmgm.2021.108038.sciencedirect.com, doi.org.

36. Singh et al., Unlocking the potential of phytochemicals in inhibiting SARS-CoV-2 M Pro protein - An in-silico and cell-based approach, Research Square, doi:10.21203/rs.3.rs-3888947/v1.researchsquare.com, doi.org.

37. Singh (B) et al., Flavonoids as Potent Inhibitor of SARS-CoV-2 Nsp13 Helicase: Grid Based Docking Approach, Middle East Research Journal of Pharmaceutical Sciences, doi:10.36348/merjps.2023.v03i04.001.kspublisher.com, doi.org.

38. Singh (C) et al., The spike protein of SARS-CoV-2 virus induces heme oxygenase-1: Pathophysiologic implications, Biochimica et Biophysica Acta (BBA) - Molecular Basis of Disease, doi:10.1016/j.bbadis.2021.166322.sciencedirect.com, doi.org.

39. Thapa et al., In-silico Approach for Predicting the Inhibitory Effect of Home Remedies on Severe Acute Respiratory Syndrome Coronavirus-2, Makara Journal of Science, doi:10.7454/mss.v27i3.1609.ac.id, doi.org.

40. Thapa (B) et al., In-silico Approach for Predicting the Inhibitory Effect of Home Remedies on Severe Acute Respiratory Syndrome Coronavirus-2, Makara Journal of Science, doi:10.7454/mss.v27i3.1609.ac.id, doi.org.

41. Wang et al., Computational Analysis of Lianhua Qingwen as an Adjuvant Treatment in Patients with COVID-19, Society of Toxicology Conference, 2023, www.researchgate.net/publication/370491709_Y_Wang_A_E_Tan_O_Chew_A_Hsueh_and_D_E_Johnson_2023_Computational_Analysis_of_Lianhua_Qingwen_as_an_Adjuvant_Treatment_in_Patients_with_COVID-19_Toxicologist_1921_507.researchgate.net.

42. Waqas et al., Discovery of Novel Natural Inhibitors Against SARS-CoV-2 Main Protease: A Rational Approach to Antiviral Therapeutics, Current Medicinal Chemistry, doi:10.2174/0109298673292839240329081008.eurekaselect.com, doi.org.

43. Wu et al., SARS-CoV-2 N protein induced acute kidney injury in diabetic db/db mice is associated with a Mincle-dependent M1 macrophage activation, Frontiers in Immunology, doi:10.3389/fimmu.2023.1264447.frontiersin.org, doi.org.

44. Wu (B) et al., Treatment with Quercetin inhibits SARS-CoV-2 N protein-induced acute kidney injury by blocking Smad3-dependent G1 cell cycle arrest, Molecular Therapy, doi:10.1016/j.ymthe.2022.12.002.sciencedirect.com, doi.org.

45. Xu et al., Reply to Yan et al.: Quercetin possesses a fluorescence quenching effect but is a weak inhibitor against SARS-CoV-2 main protease, Proceedings of the National Academy of Sciences, doi:10.1073/pnas.2309870120.pnas.org, doi.org.

46. Xu (B) et al., Bioactive compounds from Huashi Baidu decoction possess both antiviral and anti-inflammatory effects against COVID-19, Proceedings of the National Academy of Sciences, doi:10.1073/pnas.2301775120.pnas.org, doi.org.

47. Yan et al., Reframing quercetin as a promiscuous inhibitor against SARS-CoV-2 main protease, Proceedings of the National Academy of Sciences, doi:10.1073/pnas.2309289120.pnas.org, doi.org.

48. Yang et al., Active ingredient and mechanistic analysis of traditional Chinese medicine formulas for the prevention and treatment of COVID-19: Insights from bioinformatics and in vitro experiments, Medicine, doi:10.1097/MD.0000000000036238.lww.com, doi.org.

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a. The trimeric spike (S) protein is a glycoprotein that mediates viral entry by binding to the host ACE2 receptor, is critical for SARS-CoV-2's ability to infect host cells, and is a target of neutralizing antibodies. Inhibition of the spike protein prevents viral attachment, halting infection at the earliest stage.

b. The main protease or M^pro, also known as 3CL^pro or nsp5, is a cysteine protease that cleaves viral polyproteins into functional units needed for replication. Inhibiting M^pro disrupts the SARS-CoV-2 lifecycle within the host cell, preventing the creation of new copies.

c. RNA-dependent RNA polymerase (RdRp), also called nsp12, is the core enzyme of the viral replicase-transcriptase complex that copies the positive-sense viral RNA genome into negative-sense templates for progeny RNA synthesis. Inhibiting RdRp blocks viral genome replication and transcription.

d. The papain-like protease (PLpro) has multiple functions including cleaving viral polyproteins and suppressing the host immune response by deubiquitination and deISGylation of host proteins. Inhibiting PLpro may block viral replication and help restore normal immune responses.

e. The angiotensin converting enzyme 2 (ACE2) protein is a host cell transmembrane protein that serves as the cellular receptor for the SARS-CoV-2 spike protein. ACE2 is expressed on many cell types, including epithelial cells in the lungs, and allows the virus to enter and infect host cells. Inhibition may affect ACE2's physiological function in blood pressure control.

f. Transmembrane protease serine 2 (TMPRSS2) is a host cell protease that primes the spike protein, facilitating cellular entry. TMPRSS2 activity helps enable cleavage of the spike protein required for membrane fusion and virus entry. Inhibition may especially protect respiratory epithelial cells, buy may have physiological effects.

g. The helicase, or nsp13, protein unwinds the double-stranded viral RNA, a crucial step in replication and transcription. Inhibition may prevent viral genome replication and the creation of new virus components.

h. The endoribonuclease, also known as NendoU or nsp15, cleaves specific sequences in viral RNA which may help the virus evade detection by the host immune system. Inhibition may hinder the virus's ability to mask itself from the immune system, facilitating a stronger immune response.

i. Cathepsin L is a host lysosomal cysteine protease that can prime the spike protein through an alternative pathway when TMPRSS2 is unavailable. Dual targeting of cathepsin L and TMPRSS2 may maximize disruption of alternative pathways for virus entry.

k. The frizzled (FZD) receptor is a host transmembrane receptor that binds Wnt ligands, initiating the Wnt signaling cascade. FZD serves as a co-receptor, along with ACE2, in some proposed mechanisms of SARS-CoV-2 infection. The virus may take advantage of this pathway as an alternative entry route.

l. Low-density lipoprotein receptor-related protein 6 is a cell surface co-receptor essential for Wnt signaling. LRP6 acts in tandem with FZD for signal transduction and has been discussed as a potential co-receptor for SARS-CoV-2 entry.

m. The ezrin protein links the cell membrane to the cytoskeleton (the cell's internal support structure) and plays a role in cell shape, movement, adhesion, and signaling. Drugs that occupy the same spot on ezrin where the viral spike protein would bind may hindering viral attachment, and drug binding could further stabilize ezrin, strengthening its potential natural capacity to impede viral fusion and entry.

n. The Adipocyte Differentiation-Related Protein (ADRP, also known as Perilipin 2 or PLIN2) is a lipid droplet protein regulating the storage and breakdown of fats in cells. SARS-CoV-2 may hijack the lipid handling machinery of host cells and ADRP may play a role in this process. Disrupting ADRP's interaction with the virus may hinder the virus's ability to use lipids for replication and assembly.

o. Neuropilin-1 (NRP1) is a cell surface receptor with roles in blood vessel development, nerve cell guidance, and immune responses. NRP1 may function as a co-receptor for SARS-CoV-2, facilitating viral entry into cells. Blocking NRP1 may disrupt an alternative route of viral entry.

p. EP300 (E1A Binding Protein P300) is a transcriptional coactivator involved in several cellular processes, including growth, differentiation, and apoptosis, through its acetyltransferase activity that modifies histones and non-histone proteins. EP300 facilitates viral entry into cells and upregulates inflammatory cytokine production.

q. Prostaglandin G/H synthase 2 (PTGS2, also known as COX-2) is an enzyme crucial for the production of inflammatory molecules called prostaglandins. PTGS2 plays a role in the inflammatory response that can become severe in COVID-19 and inhibitors (like some NSAIDs) may have benefits in dampening harmful inflammation, but note that prostaglandins have diverse physiological functions.

r. Heat Shock Protein 90 Alpha Family Class A Member 1 (HSP90AA1) is a chaperone protein that helps other proteins fold correctly and maintains their stability. HSP90AA1 plays roles in cell signaling, survival, and immune responses. HSP90AA1 may interact with numerous viral proteins, but note that it has diverse physiological functions.

s. Matrix metalloproteinase 9 (MMP9), also called gelatinase B, is a zinc-dependent enzyme that breaks down collagen and other components of the extracellular matrix. MMP9 levels increase in severe COVID-19. Overactive MMP9 can damage lung tissue and worsen inflammation. Inhibition of MMP9 may prevent excessive tissue damage and help regulate the inflammatory response.

t. The interleukin-6 (IL-6) pro-inflammatory cytokine (signaling molecule) has a complex role in the immune response and may trigger and perpetuate inflammation. Elevated IL-6 levels are associated with severe COVID-19 cases and cytokine storm. Anti-IL-6 therapies may be beneficial in reducing excessive inflammation in severe COVID-19 cases.

u. The interleukin-10 (IL-10) anti-inflammatory cytokine helps regulate and dampen immune responses, preventing excessive inflammation. IL-10 levels can also be elevated in severe COVID-19. IL-10 could either help control harmful inflammation or potentially contribute to immune suppression.

v. Vascular Endothelial Growth Factor A (VEGFA) promotes the growth of new blood vessels (angiogenesis) and has roles in inflammation and immune responses. VEGFA may contribute to blood vessel leakiness and excessive inflammation associated with severe COVID-19.

w. RELA is a transcription factor subunit of NF-kB and is a key regulator of inflammation, driving pro-inflammatory gene expression. SARS-CoV-2 may hijack and modulate NF-kB pathways.

x. Calu-3 is a human lung adenocarcinoma cell line with moderate ACE2 and TMPRSS2 expression and SARS-CoV-2 susceptibility. It provides a model of the human respiratory epithelium, but many not be ideal for modeling early stages of infection due to the moderate expression levels of ACE2 and TMPRSS2.

y. A549 is a human lung carcinoma cell line with low ACE2 expression and SARS-CoV-2 susceptibility. Viral entry/replication can be studied but the cells may not replicate all aspects of lung infection.

z. HEK293-ACE2+ is a human embryonic kidney cell line engineered for high ACE2 expression and SARS-CoV-2 susceptibility.

aa. Huh-7 cells were derived from a liver tumor (hepatoma).

ab. Caco-2 cells come from a colorectal adenocarcinoma (cancer). They are valued for their ability to form a polarized cell layer with properties similar to the intestinal lining.

ac. Vero E6 is an African green monkey kidney cell line with low/no ACE2 expression and high SARS-CoV-2 susceptibility. The cell line is easy to maintain and supports robust viral replication, however the monkey origin may not accurately represent human responses.

ad. mTEC is a mouse tubular epithelial cell line.

ae. RAW264.7 is a mouse macrophage cell line.

af. A mouse model expressing the human ACE2 receptor under the control of the K18 promoter.

ag. A mouse model of obesity and severe insulin resistance leading to type 2 diabetes due to a mutation in the leptin receptor gene that impairs satiety signaling.

ah. A mouse model commonly used in infectious disease and cancer research due to higher immune response and susceptibility to infection.

Xu et al., 5 Sep 2023, peer-reviewed, 7 authors. Contact: huangluqi01@126.com.

In Vitro studies are an important part of preclinical research, however results may be very different in vivo.

This Paper Quercetin All

Reply to Yan et al.: Quercetin possesses a fluorescence quenching effect but is a weak inhibitor against SARS-CoV-2 main protease

Haiyu Xu, Yute Zhong, Jinyue Yang, Lifeng Fu, Yi Shi, Luqi Huang, George F Gao

Proceedings of the National Academy of Sciences, doi:10.1073/pnas.2309870120

Recently, we published an article (1) showing that several bioactive compounds were identified from Huashi Baidu decoction (Q-14) that showed both antiviral activity and antiinflammatory effects against COVID-19. The main protease (M pro ) of SARS-CoV-2 is an attractive target for the development of antiviral drugs. Our results showed that quercetin and echinatin from Q-14 displayed moderate inhibition activities against SARS-CoV-2 M pro at submicromolar levels by the fluorescence resonance energy transfer (FRET)-based protease activity assay. However, Yan et al. suggested that quercetin is a promiscuous M pro inhibitor because of its fluorescence quenching effect (2). We took this suspicion very seriously. Therefore, we repeated the FRET-based protease activity assay and tested the fluorescence quenching effects with the MCA-AVLQ-Lys (Dnp)-Lys-NH2 (MCA-AVLQ) fragment. The FRET assay results showed that both quercetin and echinatin have M pro inhibition activity (Fig. 1 A and E ), but both quercetin and echinatin displayed a fluorescence quenching effect on MCA-AVLQ fragment (Fig. 1 B and F ). The inhibition ratio and quenching ratio were further calculated (Fig. 1 C and G). We found that the inhibition ratio is significantly greater than the quenching rate, especially at low concentrations, which indicates that quercetin and echinatin should possess certain M pro inhibition activity. To further validate the M pro inhibition activity of

References

Xu, Bioactive compounds from Huashi Baidu decoction possess both antiviral and anti-inflammatory effects against COVID-19, Proc. Natl. Acad. Sci. U.S.A

Yan, Production of a versatile SARS-CoV-2 main protease biosensor based on a dimerization-dependent red fluorescent protein, J. Med. Virol

Yan, Zhang, Liu, Wang, Chen, Reframing quercetin as a promiscuous inhibitor against SARS-CoV-2 main protease, Proc. Natl. Acad. Sci. U.S.A

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