Hydroxylation of the diterpenes ent-kaur-16-en-19-oic and ent-beyer-15-en-19-oic acids by the fungus Aspergillus niger

Antimicrobial Activity and Molecular Docking Studies of the Biotransformation of Diterpene Acanthoic Acid Using the Fungus Xylaria sp

Biotransformations are reactions mediated by microorganisms, such as fungi. These bioreactions have high chemo- and stereoselectivity on organic substrates and can be applied in the search for new bioactive compounds. In this study, acanthoic acid (AA) was biotransformed using the fungus Xylaria sp., giving the novel compound 3β,7β-dihydroxyacanthoic acid (S1). Both the AA and the product S1 were tested against Gram-positive and Gram-negative bacteria. To identify and validate possible biological targets as enzymes or proteins involved in the activity observed in vitro, we used the molecular docking method. Hydroxylation at the C-3 and C-7 positions of the biotransformation product enhanced its activity against Escherichia coli as well as its binding affinity and interactions with superoxide dismutase 1 (SOD1; PDB ID 4A7G). Based on our results, the SOD1 enzyme was suggested to be a possible target for the antioxidant activity of product S1.

Diterpenoids from Leaves of Cultivated Plectranthus ornatus

Two new diterpenoid derivatives 7α,12β,17-triacetoxy-6β,19-dihydroxy-13β,16-spirocicloabiet-8-ene-11,14-dione ( 1: ) and 6β-acetoxy-3β,7α,12α-trihydroxy-13β,16-spirocicloabiet-8-ene-11,14-dione ( 2: ) along with 11 ( 3: - 13: ) miscellaneous compounds were isolated from the leaves of Plectranthus ornatus Codd. Their structures were elucidated by spectroscopic analysis and gauge independent atomic orbitals ¹³C NMR calculations. The isolated compounds were screened for their effects on intestinal motility using guinea-pig ileum and duodenum and by their cytotoxicity against 4 human cancer cell lines (HCT-116, SF-295, PC-3, and HL-60). Compounds 6: and 9: were moderately cytotoxic against HL-60, whereas 6: and 13: were more active on SF-295 and HCT-116.

New Derivatives from Microbial Transformation of ent-Kaur-16-en-19-oic Acid by Cunninghamella echinulata

Biotransformation of ent-kaur-16-en-19-oic acid using fungus Cunninghamella echinulata resulted in two novel hydroxylated metabolites together with five known compounds. Their structures were elucidated by means of extensive NMR and HR-ESI-MS data analysis. The eight compounds were measured for their cytotoxicity against the human breast carcinoma (MCF-7) and human hepatoblastoma (HepG-2) cell lines. Seven compounds showed no cytotoxicity to the two cell lines. One compound displayed moderate cytotoxicity against HepG-2 and MCF-7 with the IC₅₀ values of 12.6 and 27.1 μM, respectively.

In vitro cytotoxicity and structure-activity relationship approaches of ent-kaurenoic acid derivatives against human breast carcinoma cell line

In this study, ent-kaurenoic acid derivatives were obtained by microbial transformation methodologies and tested against breast cancer cell lines (MCF-7). A multivariate quantitative-structure activity relationship (QSAR) analysis was performed taking into account both microbial transformation derivatives and other analogues previously reported in literature to give some insight into the main features behind the cytotoxic activity displayed by kaurane-type diterpenes against MCF-7 cells. The partial least square regression (PLS) method was employed in the training set and the best PLS model was built with a factor describing 69.92% of variance and three descriptors (logP, ε_HOMO and ε_HOMO-1) selected by the Ordered Predictors Selection (OPS) algorithm. The QSAR model provided reasonable regression (Q² = 0.64, R² = 0.72, SEC = 0.29 and SEV = 0.33). The model was validated by leave-N-out cross-validation, y-randomization and external validation (R²_pred = 0.89 and SEP = 0.27). The selected descriptors indicated that the activity was mainly related to electronic parameters (HOMO and HOMO-1 molecular orbital energies), as well as to logP. These findings suggest that higher activity values are directly related with both higher logP and frontier orbital energy values. The positive relationship between these orbitals and the activity suggests that the ent-kaurenoic acid analogues interaction with the target involves charge displacement, which is entirely consistent with the literature. Based on these findings, three compounds were proposed and one of them was synthesized and tested. The experimental result confirmed the activity predicted by the model.

The remarkable structural diversity achieved in ent-Kaurane Diterpenes by fungal biotransformations

The use of biotransformations in organic chemistry is widespread, with highlights of interesting applications in the functionalization of natural products containing unactivated carbons, like the kaurane diterpenes. A number of compounds with kaurane skeletons can be isolated in large amounts from several plant species and a myriad of biological activities has been related to these compounds. Studies on structure versus activity have showed that, in most cases, in kaurane diterpenes, activity increases with the increase of functionalization. Since naturally occurring kaurane diterpenes usually have limited functional groups to be used as targets for semi-synthetic modifications, production of more polar derivatives from kaurane diterpenes have been achieved mostly through the use of fungal biotransformations. In this review, selected examples the wonderful chemical diversity produced by fungi in kaurane diterpenes is presented. This diversity includes mainly hydroxylation of nearly all carbon atoms of the kaurane molecule, many of them carried out stereoselectively, as well as ring rearrangements, among other chemical modifications. Sources of starting materials, general biotransformation protocols employed, fungi with most consistent regioselectivity towards kaurane skeleton, as well as biological activities associated with starting materials and products are also described.

Pimarane-type diterpenes obtained by biotransformation: antimicrobial properties against clinically isolated Gram-positive multidrug-resistant bacteria

The present study describes the antimicrobial activity of five pimarane-type diterpenes obtained by fungal biotransformation against several nosocomial multidrug-resistant bacteria. Among the investigated metabolites, ent-8(14),15-pimaradien-3β-ol was the most active compound, with very promising minimal inhibitory concentration values (between 8.0 and 25.0 µg mL(-1)). Time-kill assays using this metabolite against Staphylococcus aureus (HCRP180) revealed that this compound exerted its bactericidal effect within 24 h at all the evaluated concentrations (8.0, 16.0, and 24.0 µg mL(-1)). When this metabolite was associated with vancomycin at their minimal bactericidal concentration values, the resulting combination was able to drastically reduce the number of viable strains of S. aureus within the first 6 h, compared with these chemicals alone. The checkerboard assays conducted against this microorganism did not evidence any synergistic effects when this same combination was employed. In conclusion, our results point out that ent-8(14),15-pimaradien-3β-ol is an important metabolite in the search for new effective antimicrobial agents.

Two novel hydroperoxylated products of 20(S)-protopanaxadiol produced by Mucor racemosus and their cytotoxic activities against human prostate cancer cells

Microbial transformation of 20(S)-protopanaxadiol (1) by Mucor racemosus AS 3.205 yielded two novel hydroperoxylated metabolites and three known hydroxylated metabolites. The structures of the metabolites were identified as 26-hydroxyl-20(S)-protopanaxadiol (2), 23,24-en-25-hydroxyl-20(S)-protopanaxadiol (3), 25,26-en-24(R)-hydroperoxyl-20(S)-protopanaxadiol (4), 23,24-en-25-hydroperoxyl-20(S)-protopanaxadiol (5), and 25-hydroxyl-20(S)-protopanaxadiol (6). 4 and 5 are new compounds. Metabolites 2, 4, and 5 showed the more potent inhibitory effects against DU-145 and PC-3 cell lines than the substrate.

Anticariogenic properties of ent-pimarane diterpenes obtained by microbial transformation

In the present work, the anticariogenic activities of three pimarane-type diterpenes obtained by fungal biotransformation were investigated. Among these metabolites, ent-8(14),15-pimaradien-19-ol was the most active compound, displaying very promising MIC values (ranging from 1.5 to 4.0 μg mL(-1)) against the main microorganisms responsible for dental caries: Streptococcus salivarius, S. sobrinus, S. mutans, S. mitis, S. sanguinis, and Lactobacillus casei. Time kill assays performed with ent-8(14),15-pimaradien-19-ol against the primary causative agent S. mutans revealed that this compound only avoids growth of the inoculum in the first 12 h (bacteriostatic effect). However, its bactericidal effect is clearly noted thereafter (between 12 and 24 h). The curve profile obtained by combining ent-8(14),15-pimaradien-19-ol and chlorhexidine revealed a significant reduction in the time necessary for killing S. mutans compared with each of these two chemicals alone. However, no synergistic effect was observed using the same combination in the checkerboard assays against this microorganism. In conclusion, our results point out that ent-8(14),15-pimaradien-19-ol is an important metabolite in the search for new effective anticariogenic agents.

Hydroxylation at carbon-2 of ent-16-oxo-17-norkauran-19-oic acid by Fusarium proliferatum

A new product of biotransformation of ent-16-oxo-17-norkauran-19-oic acid (1) by Fusarium proliferatum was isolated and identified as a 2beta-hydroxy derivative (2). The structure of 2 was elucidated on the basis of spectroscopic data interpretation and single-crystal X-ray diffraction analysis. The allelopathic activity of compound 2 was evaluated on the growth of radicals and shoots of Lactuca sativa (lettuce). This is the first time that fungal hydroxylation at position C-2 has been reported on an ent-kaurane diterpene skeleton.