Microdontin A and B: Two New Aloin Derivatives from Aloe microdonta

New opportunities and perspectives on biosynthesis and bioactivities of secondary metabolites from Aloe vera

Historically, the genus Aloe has been an indispensable part of both traditional and modern medicine. Decades of intensive research have unveiled the major bioactive secondary metabolites of this plant. Recent pandemic outbreaks have revitalized curiosity in aloe metabolites, as they have proven pharmacokinetic profiles and repurposable chemical space. However, the structural complexity of these metabolites has hindered scientific advances in the chemical synthesis of these compounds. Multi-omics research interventions have transformed aloe research by providing insights into the biosynthesis of many of these compounds, for example, aloesone, aloenin, noreugenin, aloin, saponins, and carotenoids. Here, we summarize the biological activities of major aloe secondary metabolites with a focus on their mechanism of action. We also highlight the recent advances in decoding the aloe metabolite biosynthetic pathways and enzymatic machinery linked with these pathways. Proof-of-concept studies on in vitro, whole-cell, and microbial synthesis of aloe compounds have also been briefed. Research initiatives on the structural modification of various aloe metabolites to expand their chemical space and activity are detailed. Further, the technological limitations, patent status, and prospects of aloe secondary metabolites in biomedicine have been discussed.

Identification of Potential SARS-CoV-2 Main Protease and Spike Protein Inhibitors from the Genus Aloe: An In Silico Study for Drug Development

Severe acute respiratory syndrome coronavirus (SARS-CoV-2) disease is a global rapidly spreading virus showing very high rates of complications and mortality. Till now, there is no effective specific treatment for the disease. Aloe is a rich source of isolated phytoconstituents that have an enormous range of biological activities. Since there are no available experimental techniques to examine these compounds for antiviral activity against SARS-CoV-2, we employed an in silico approach involving molecular docking, dynamics simulation, and binding free energy calculation using SARS-CoV-2 essential proteins as main protease and spike protein to identify lead compounds from Aloe that may help in novel drug discovery. Results retrieved from docking and molecular dynamics simulation suggested a number of promising inhibitors from Aloe. Root mean square deviation (RMSD) and root mean square fluctuation (RMSF) calculations indicated that compounds 132, 134, and 159 were the best scoring compounds against main protease, while compounds 115, 120, and 131 were the best scoring ones against spike glycoprotein. Compounds 120 and 131 were able to achieve significant stability and binding free energies during molecular dynamics simulation. In addition, the highest scoring compounds were investigated for their pharmacokinetic properties and drug-likeness. The Aloe compounds are promising active phytoconstituents for drug development for SARS-CoV-2.

Vacillantins A and B, New Anthrone C-glycosides, and a New Dihydroisocoumarin Glucoside from Aloe vacillans and Its Antioxidant Activities

A new dihydroisocoumarin glucoside, vacillanoside (3), and two new anthrone C-glycosides microdantin derivatives; vacillantin A (10) and B (11), together with nine known compounds belonging to the anthraquinone, anthrone and isocoumarin groups were isolated from the leaves of Aloe vacillans. The structures were determined based on spectroscopic evidence including 1D and 2D nuclear magnetic resonance (NMR) spectroscopy and high resolution mass spectrometry (HRESIMS) data, along with comparisons to reported data. The leaves were used to extract compounds with different solvents. The extracts were tested for antioxidant activity with a variety of in vitro tests including 2,2-diphenyl-1-picrylhydrazyl (DPPH^•), 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonate (ABTS^•+), ferric reducing antioxidant power assay (FRAP), superoxide, and nitric oxide radical scavenging assays. The dichloromethane fraction was most active, displaying significant free radical scavenging activity. The n-butanol fraction also showed notable activity in all assays. Therefore, these findings support the potential use of A. vacillans leaves as an antioxidant medication due to the presence of polyphenolic compounds.

Characterization and Evaluation of Antioxidant Activity of Aloe schelpei Reynolds

Background

The role of free radical reactions in disease pathology is well known to be involved in many acute and chronic disorders in human beings, such as diabetes, atherosclerosis, aging, immunosuppression and neurodegeneration. The search for new drugs of plant origin becomes increasingly urgent due to drug resistance. Aloe schelpei is an endemic Aloe species traditionally used for the treatment of infectious and chronic diseases.

Aim

This study was conducted to evaluate free radical scavenging activities of leaf latex of Aloe schelpei and its isolated compounds.

Methods

The leaf latex of A. schelpei was subjected to preparative thin-layer chromatography to afford three compounds. Free radical scavenging activities of the leaf latex and its constituents was carried out using a 2, 2-diphenyl-1-picrylhydrazyl method.

Results

Phytochemical investigation of the leaf latex Aloe schelpei by prepartive thin layer chromatography led to the isolation of three compounds, identified as microdontin A/B (1), aloin A/B (2) and aloinoside A/B (3). The results showed that the leaf latex had a strong free radical scavenging activity reaching a maximum of 84.3% at a concentration of 100 μg/mL, and with an IC₅₀ value of 25.3 ± 2.45 μg/mL (p < 0.05). Among the isolated compounds, microdontin A/B (1) was found to have the strongest free radical scavenging activity with an IC₅₀ value of 0.07 ± 0.005 mμ, followed by aloinoside A/B (IC₅₀ = 0.13 ± 0.01 mM) and aloin A/B (IC₅₀ = 0.15 ± 0.02 mM).

Conclusion

The traditional medicinal practice of the leaf latex may be due to the antioxidant activities of the leaf latex of A. schelpei and the isolated compounds.

Phytochemistry and antileishmanial activity of the leaf latex of Aloe calidophila Reynolds

Leishmaniasis is a major protozoal disease threatening the lives of 350 million people throughout the world. However, the therapeutic options for the disease are limited. In the present study, the antiprotozoal activity of the latex obtained from the Ethiopian plant Aloe calidophila Reynolds was evaluated by in vitro testing against Leishmania aethiopica and Leishmania major. It was found that the latex possesses moderate activity against both parasites with IC50 values of 64.05 and 82.29 µg/mL, respectively. Phytochemical investigation resulted in the isolation of three anthrones identified as aloinoside, aloin, and microdontin on the basis of IR, MS, (1) H NMR, and (13) C NMR spectral data. The isolated compounds showed strong antileishmanial activity with IC50 values ranging from 1.76 to 6.32 µg/mL against L. aethiopica and from 2.09 to 8.85 µg/mL against L. major. Although these values were higher than those of amphotericin B (IC50  = 0.109 and 0.067 µg/mL), the selectivity indices (813.35 and 694.90, respectively, against L. aethiopica and L. major) of aloinoside were much better than those of the standard drug (423.49 and 688.96). The results indicate that the isolated compounds have the potential to be used as a scaffold for the development of safe and cost-effective antileishmanial agents.

A New Antimicrobial Anthrone from the Leaf Latex of Aloe trichosantha

Phytochemical investigation of the leaf latex of Aloe trichosantha by preparative TLC gave two closely related anthrones, aloin A/B (1) and aloin-6'-O-acetate A/B (2). The identity of the compounds was established from HRESI-MS, 1H, 13C, DEPT, HMQC and HMBC spectral and chemical data. Whilst aloin A/B occurs in several Aloe species, aloin-6'-O-acetate A/B was isolated for the first time. The isolated compounds inhibited growth of several bacterial and fungal pathogens with minimum inhibitory concentration (MIC) from 10 to 400 μg/mL and 800 to 1000 μg/mL, respectively.

An oligomer from flaxseed composed of secoisolariciresinoldiglucoside and 3-hydroxy-3-methyl glutaric acid residues

A straight-chain oligomeric structure composed of five secoisolariciresinoldiglucoside (SDG) residues interconnected by four 3-hydroxy-3-methyl glutaric acid (HMGA) residues (molecular weight ca. 4000 Da) was assigned to the main lignan of flaxseed on the basis of nuclear magnetic resonance spectroscopy (NMR).

The occurrence and taxonomic distribution of the anthrones aloin, aloinoside and microdontin in Aloe

A chemotaxonomic survey of 380 species of Aloe indicated the presence of the anthrone isomers aloin A and B together with the aloinoside isomers and microdontin A and B in 36 (10%) species of Aloe. This group, referred to as the microdontin chemotype, is thus characterised by a combination of exudate compounds and not merely a single phytochemical marker, implying taxonomic significance of leaf exudate compounds. The 36 representatives of the group occupy disparate taxonomic positions in the largely artificial hierarchy of the present classification system. Although many of the species have previously been considered as related (based on macromorphology only), a large number of species have not been associated with one another before. The chemical profiles and leaf exudate compositions of the species are presented, followed by a brief summary of the morphological diversity. Whilst conceding the possibility of convergent evolution, the geographical distribution of the species and thoughts on possible relationships between the taxa are discussed.

Saponins and phenolic content in plant dietary additives of a traditional subsistence community, the Batemi of Ngorongoro District, Tanzania

Reports of plants added to milk and meat-based soups by the Maasai and Batemi in East Africa support a role for phenolic antioxidants and hypocholesterolemic agents in the diet, and provide explanation of the low incidence of cardiovascular disease of populations that traditionally consume high levels of dietary fat and cholesterol. Plant food additives used by the Batemi of Ngorongoro District, Tanzania, were tabulated, based on interviews with 22 informants, while 17 specimens were collected in the field and analyzed for saponin and phenolic content. A total of 81% of the Batemi additives and 82% of those known to be used by the Maasai contain potentially hypocholesterolemic saponins and/or phenolics.