Pieter C. Zietsman

Labdane and Clerodane Diterpenoids from Colophospermum mopane

Five labdane (1-5), an isolabdane (6), and five clerodane diterpenoids (7-11), were isolated from seeds, husks, and leaves of Colophospermum mopane. Compounds 1-3 and 6-9 are new, and their structures were elucidated by means of physical data analysis (1D and 2D NMR, HRESIMS). The absolute configurations of 1, 7, and 10 were determined by single-crystal X-ray diffraction with Cu Kα radiation. For compounds 2 and 6, the absolute configurations were established by the modified Moshers method and corroborated by comparison of experimental and calculated electronic circular dichroism spectra of their 3-p-bromobenzoate derivatives. The crude extracts and compounds were evaluated for antimicrobial activity. The leaf extract was the most active against Staphylococcus aureus (125 μg/mL). Compound 11 showed the best inhibitory activity, with minimum inhibitory values of 15.6 μg/mL against Escherichia coli and Staphylococcus aureus and 31.3 μg/mL against Enterococcus faecalis.

hERG Channel Inhibitory Daphnane Diterpenoid Orthoesters and Polycephalones A and B with Unprecedented Skeletons from Gnidia polycephala

The hERG channel is an important antitarget in safety pharmacology. Several drugs have been withdrawn from the market or received severe usage restrictions because of hERG-related cardiotoxicity. In a screening of medicinal plants for hERG channel inhibition using a two-microelectrode voltage clamp assay with Xenopus laevis oocytes, a dichloromethane extract of the roots of Gnidia polycephala reduced the peak tail hERG current by 58.8 ± 13.4% (n = 3) at a concentration of 100 μg/mL. By means of HPLC-based activity profiling daphnane-type diterpenoid orthoesters (DDOs) 1, 4, and 5 were identified as the active compounds [55.4 ± 7.0% (n = 4), 42.5 ± 16.0% (n = 3), and 51.3 ± 9.4% (n = 4), respectively, at 100 μM]. In a detailed phytochemical profiling of the active extract, 16 compounds were isolated and characterized, including two 2-phenylpyranones (15 and 16) with an unprecedented tetrahydro-4H-5,8-epoxypyrano[2,3-d]oxepin-4-one skeleton, two new DDOs (3 and 4), two new guaiane sesquiterpenoids (11 and 12), and 10 known compounds (1, 2, 5-10, 13, and 14). Structure elucidation was achieved by extensive spectroscopic analysis (1D and 2D NMR, HRMS, and electronic circular dichroism), computational methods, and X-ray crystallography.

HPLC-Based Activity Profiling for hERG Channel Inhibitors in the South African Medicinal Plant Galenia africana

The human ether-a-go-go-related gene channel is a voltage-activated K(+) channel involved in cardiac action potential. Its inhibition can lead to QT prolongation, and eventually to potentially fatal arrhythmia. Therefore, it is considered a primary antitarget in safety pharmacology. To assess the risk of human ether-a-go-go-related gene channel inhibition by medicinal plants, 700 extracts from different parts of 142 medicinal plants collected in Southern Africa were screened on Xenopus laevis oocytes. A CH2Cl2 extract from the stems and leaves of Galenia africana (Aizoaceae) reduced the peak tail human ether-a-go-go-related gene current by 50.4 ± 5.5 % (n = 3) at a concentration of 100 µg/mL. By means of high-performance liquid chromatography-based activity profiling, nine flavonoids were identified in the active time windows. However, the human ether-a-go-go-related gene channel inhibition of isolated compounds was less pronounced than that of extract and active microfractions (human ether-a-go-go-related gene inhibition between 10.1 ± 5 and 14.1 ± 1.6 at 100 µM). The two major constituents, 7,8-methylenedioxyflavone (1) and 7,8-dimethoxyflavone (13), were quantified (4.3 % and 9.4 %, respectively, in the extract). Further human ether-a-go-go-related gene inhibition tests for compounds 1 and 13 at 300 µM showed a concentration-dependent inhibitory activity (33.2 ± 12.4 and 30.0 ± 7.4, respectively). In a detailed phytochemical profiling of the active extract, a total of 20 phenolic compounds, including six new natural products, were isolated and identified.

HPLC-Based Activity Profiling for GABAA Receptor Modulators in Searsia pyroides Using a Larval Zebrafish Locomotor Assay

A dichloromethane extract from leaves of Searsia pyroides potentiated gamma aminobutyric acid-induced chloride currents by 171.8 ± 54% when tested at 100 µg/mL in Xenopus oocytes transiently expressing gamma aminobutyric acid type A receptors composed of α1β2γ2s subunits. In zebrafish larvae, the extract significantly lowered pentylenetetrazol-provoked locomotion when tested at 4 µg/mL. Active compounds of the extract were tracked with the aid of HPLC-based activity profiling utilizing a previously validated zebrafish larval locomotor activity assay. From two active HPLC fractions, compounds 1 - 3 were isolated. Structurally related compounds 4 - 6 were purified from a later eluting inactive HPLC fraction. With the aid of 1H and 13C NMR and high-resolution mass spectrometry, compounds 1 - 6 were identified as analogues of anacardic acid. Compounds 1 - 3 led to a concentration-dependent decrease of pentylenetetrazol-provoked locomotion in the zebrafish larvae model, while 4 - 6 were inactive. Compounds 1 - 3 enhanced gamma aminobutyric acid-induced chloride currents in Xenopus oocytes in a concentration-dependent manner, while 4 - 6 only showed marginal enhancements of gamma aminobutyric acid-induced chloride currents. Compounds 2, 3, and 5 have not been reported previously.