Isolation and structural elucidation of pentacyclic triterpenoids from Maprounea africana

The Main Protease of SARS-CoV-2 as a Target for Phytochemicals against Coronavirus

In late December 2019, the first cases of COVID-19 emerged as an outbreak in Wuhan, China that later spread vastly around the world, evolving into a pandemic and one of the worst global health crises in modern history. The causative agent was identified as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Although several vaccines were authorized for emergency use, constantly emerging new viral mutants and limited treatment options for COVID-19 drastically highlighted the need for developing an efficient treatment for this disease. One of the most important viral components to target for this purpose is the main protease of the coronavirus (Mpro). This enzyme is an excellent target for a potential drug, as it is essential for viral replication and has no closely related homologues in humans, making its inhibitors unlikely to be toxic. Our review describes a variety of approaches that could be applied in search of potential inhibitors among plant-derived compounds, including virtual in silico screening (a data-driven approach), which could be structure-based or fragment-guided, the classical approach of high-throughput screening, and antiviral activity cell-based assays. We will focus on several classes of compounds reported to be potential inhibitors of Mpro, including phenols and polyphenols, alkaloids, and terpenoids.

Sterols and Triterpenes: Antiviral Potential Supported by In-Silico Analysis

The acute respiratory syndrome caused by the novel coronavirus (SARS-CoV-2) caused severe panic all over the world. The coronavirus (COVID-19) outbreak has already brought massive human suffering and major economic disruption and unfortunately, there is no specific treatment for COVID-19 so far. Herbal medicines and purified natural products can provide a rich resource for novel antiviral drugs. Therefore, in this review, we focused on the sterols and triterpenes as potential candidates derived from natural sources with well-reported in vitro efficacy against numerous types of viruses. Moreover, we compiled from these reviewed compounds a library of 162 sterols and triterpenes that was subjected to a computer-aided virtual screening against the active sites of the recently reported SARS-CoV-2 protein targets. Interestingly, the results suggested some compounds as potential drug candidates for the development of anti-SARS-CoV-2 therapeutics.

Structure-Activity-Relationship and Mechanistic Insights for Anti-HIV Natural Products

Acquired Immunodeficiency Syndrome (AIDS), which chiefly originatesfroma retrovirus named Human Immunodeficiency Virus (HIV), has impacted about 70 million people worldwide. Even though several advances have been made in the field of antiretroviral combination therapy, HIV is still responsible for a considerable number of deaths in Africa. The current antiretroviral therapies have achieved success in providing instant HIV suppression but with countless undesirable adverse effects. Presently, the biodiversity of the plant kingdom is being explored by several researchers for the discovery of potent anti-HIV drugs with different mechanisms of action. The primary challenge is to afford a treatment that is free from any sort of risk of drug resistance and serious side effects. Hence, there is a strong demand to evaluate drugs derived from plants as well as their derivatives. Several plants, such as Andrographis paniculata, Dioscorea bulbifera, Aegle marmelos, Wistaria floribunda, Lindera chunii, Xanthoceras sorbifolia and others have displayed significant anti-HIV activity. Here, weattempt to summarize the main results, which focus on the structures of most potent plant-based natural products having anti-HIV activity along with their mechanisms of action and IC50 values, structure-activity-relationships and important key findings.

3α,19-Dihydroxyl-ent-pimara-8(14),15-diene, a new diterpenoid from the rhizomes of Ricinus communis

A new natural product, 3α,19-dihydroxyl-ent-pimara-8(14),15-diene (1), which possesses an α-orientation hydroxymethyl at C-4 and ∆^8,14 groups, as well as eight known compounds, was isolated from the rhizomes of Ricinus communis. The structure of 1 was elucidated by extensive spectroscopic methods and its absolute configurations were confirmed by X-ray crystallographic analysis. The inhibitory rate of 1 against protein tyrosine phosphatase 1B (PTP1B) was 49.49% at the concentration of 6.58 × 10^-5 mol/L.

Discovery of anti-inflammatory terpenoids from Mallotus conspurcatus croizat

ETHNOPHARMACOLOGICAL RELEVANCE

Mallotus conspurcatus croizat (Euphorbiaceae), a plant native to Jinxiu in Guangxi, is popularly used in folk medicine to treat pelvic inflammatory disease. The anti-inflammatory activities of the compounds obtained from M. conspurcatus root were evaluated in this study.

AIM OF THE STUDY

This study explored the major anti-inflammatory components of this plant.

MATERIALS AND METHODS

The ethyl acetate fraction of the ethanol extract from M. conspurcatus was separated using chromatographic techniques. The structures of the isolates were elucidated from NMR, MS and X-ray data as well as from ECD. The anti-inflammatory activities of the isolates from M. conspurcatus were evaluated using LPS-stimulated RAW 264.7 cell models. The production of NO, TNF-α and PGE-2 was determined by ELISA and Griess tests. The expression levels of COX-2, NF-κB/p65 and iNOS were measured by western blotting.

RESULTS

Two new diterpenoids, malloconspur A (1) and malloconspur B (2), and sixteen known terpenoids (3-18) were identified by comprehensive spectroscopic analyses and comparison with literature data. Malloconspur B (2) and 17-hydroxycleistantha-12,15-dien-3-one (3) substantially inhibited the release of NO with IC₅₀ values of 10.47 μM and 9.32 μM, respectively. Compounds 1, 2 and 3 markedly decreased the secretion of PGE₂ and TNF-α (P < 0.01) by LPS-induced RAW264.7 cells. Compounds 2 and 3 markedly decreased iNOS, NF-κB/p65 and COX-2 protein expression.

CONCLUSIONS

Our identification of these diterpenoids provides strong evidence for the use of M. conspurcatus among the Yao people as a medicinal plant for the treatment of inflammation. The dramatic differences in the chemical structures of the active diterpenoids of this plant from those on the market suggest these compounds have potential as anti-inflammatory lead compounds for follow-up research.

Two novel prenylated kaempferol derivatives from fresh bud's fur of Platanus acerifolia and their anti-proliferative activities

Two novel prenylated kaempferol derivatives (1, 2), together with seven known metabolites were isolated from ethanol extract of fresh Platanus acerifolia bud's fur by multistep chromatographic processing. Structure of compounds 1 and 2 was confirmed by 1D, 2D NMR spectra and HR-ESI-MS. In addition, compound 1 was further analysed by X-ray crystallography. Anti-proliferative activities in vitro against human breast carcinoma (MCF-7) and human hepatocellular carcinoma (Hep-G2) cell lines for compound 1, 2 and 8 were evaluated. Compound 1 exhibited cytotoxic activity towards MCF-7 and Hep-G2 cell lines with the IC₅₀ values 38.2 and 39.5 μM, respectively. Moreover, compound 2 showed weak cytotoxic activities against the two cell lines.

Anti-HIV and NO production inhibition activities of epi-aleuritolic acid derivatives

Fifteen epi-aleuritolic acid derivatives were synthesized and evaluated for anti-HIV activity in 293 T cells and NO production inhibition activity. Of the derivatives, 1, 2, 3, 4, 11, and 13 showed relatively potent anti-HIV activity with EC50 values ranging from 5.80 to 13.30 μM. The most potent compound, 3α-2',2'-dimethylsuccinic acyl epi-aleuritolic acid (11), displayed significant anti-HIV activity with an EC50 value of 5.80 μM. Compounds 1, 3, 4, and 11 showed NO inhibition activity, with IC50 values ranging from 3.40 to 7.10 μM and compound 1 inhibited NO production with an IC50 value of 3.40 μM.

NAD(+)-dependent DNA ligase: a novel target waiting for the right inhibitor

DNA ligases (EC.6.5.1.1) are key enzymes that catalyze the formation of phosphodiester bonds at single stranded or double stranded breaks between adjacent 5' phosphoryl and 3' hydroxyl groups of DNA. These enzymes are important for survival because they are involved in major cellular processes like DNA replication/repair and recombination. DNA ligases can be classified into two groups on the basis of their cofactor specificities. NAD(+)-dependent DNA ligases are present in bacteria, some entomopox viruses and mimi virus while ATP-dependent DNA ligases are ubiquitous. The former have recently been drawing a lot of attention as novel targets for antibiotics to overcome current drug resistance issues. Currently a diverse range of inhibitors have been identified. There are several issues to be addressed in the quest for optimized inhibitors of the enzyme. In the first part of the review we summarize current structural work on these enzymes. Subsequently we describe the currently available classes of inhibitors. We also address modalities to improve the specificity and potencies of new inhibitors identified using protein structure based rational approaches. In conclusion, NAD(+)-dependent ligases show great promise and represent a novel drug target whose time has come.

Natural-product inhibitors of human DNA ligase I

Enzymatic activity mediated by recombinant human DNA ligase I (hLI), in conjunction with tannin removal procedures, has been applied to a natural-product screen involving approximately 1000 plant extracts and various pure compounds. The primary hLI activity assay involved the measurement of the amount of radiolabelled phosphate in a synthetic nucleic acid hybrid that becomes resistant to alkaline phosphatase as a result of ligation. A bioactivity-guided fractionation scheme resulted in the isolation of ursolic [IC50=100 micrograms/ml (216 microM)] and oleanolic [IC50=100 micrograms/ml (216 microM)] acids from Tricalysia niamniamensis Hiern (Rubiaceae), which demonstrated similar DNA ligase inhibition profiles to other triterpenes such as aleuritolic acid. Protolichesterinic acid [IC50=6 micrograms/ml (20 microM)], swertifrancheside [IC50 = 8 micrograms/ml(11)microM)] and fulvoplumierin [IC50=87 micrograms/ml (357 microM)] represent three additional natural-product structural classes that inhibit hLI. Fagaronine chloride [IC50=10 micrograms/ml (27 micronM] and certain flavonoids are also among the pure natural products that were found to disrupt the activity of the enzyme, consistent with their nucleic acid intercalative properties. Further analyses revealed that some of the hLI-inhibitory compounds interfered with the initial adenylation step of the ligation reaction, indicating a direct interaction with the enzyme protein. However, in all cases, this enzyme-inhibitor interaction did not disrupt the DNA relaxation activity mediated by hLI. These results indicate that, although the same enzyme active site may be involved in both enzyme adenylation and DNA relaxation, inhibitors may exert allosteric effects by inducing conformational changes that disrupt only one of these activities. Studies with inhibitors are important for the assignment of specific cellular functions to these enzymes, as well as for their development into clinically useful antitumour agents.