From Allium stipitatum, three pyridine-N-oxide alkaloids (1-3) possessing disulfide functional groups were isolated. The structures of these natural products were elucidated by spectroscopic means as 2-(methyldithio)pyridine-N-oxide (1), 2-[(methylthiomethyl)dithio]pyridine-N-oxide (2), and 2,2′-dithio-bis-pyridine-N-oxide (3). The proposed structure of 1 was confirmed by synthetic S-methylthiolation of commercial 2-thiopyridine-N-oxide. Compounds 1 and 2 are new natural products, and 3 is reported for the first time from an Allium species. All compounds were evaluated for activity against fast-growing species of Mycobacterium, methicillin-resistant Staphylococcus aureus, and a multidrug-resistant (MDR) variants of S. aureus. Compounds 1 and 2 exhibited minimum inhibitory concentrations (MICs) of 0.5-8 μg/mL against these strains. A small series of analogues of 1 were synthesized in an attempt to optimize antibacterial activity, although the natural product had the most potent in vitro activity. In a whole-cell assay at 30 μg/mL, 1 was shown to give complete inhibition of the incorporation of 14C-labeled acetate into soluble fatty acids, indicating that it is potentially an inhibitor of fatty acid biosynthesis. In a human cancer cell line antiproliferative assay, 1 and 2 displayed IC50 values ranging from 0.3 to 1.8 μM with a selectivity index of 2.3 when compared to a human somatic cell line. Compound 1 was evaluated in a microarray analysis that indicated a similar mode of action to menadione and 8-quinolinol by interfering with the thioredoxin system and up-regulating the production of various heat shock proteins. This compound was also assessed in a mouse model for in vivo toxicity.

The emergence of multidrug-resistant strains of many human pathogens has led to an urgent need for the discovery and development of new antimicrobial agents. As part of an ongoing investigation into the antibacterial properties of the Alliaceae, the isolation of 1 (canthin-6-one), 2 (8-hydroxy-canthin-6-one) and 3 (5(zeta)-hydroxy-octadeca-6(E)-8(Z)-dienioc acid)) from A. neapolitanum, a perennial bulbous herb found in open pastures of the Mediterranean is reported. Compounds 1 and 2 were isolated by Sephadex LH-20 from fractions exhibiting a positive reaction with Dragendorff's reagent on TLC, compound 3 was isolated after HPLC purification of Sephadex fractions. Structures were elucidated by extensive 1D and 2D NMR experiments and are in accordance with published data, however, the 13C NMR data for compound 2 and the 1H and 13C NMR data for compound 3 are reported here for the first time. Canthin-6-one alkaloids are well-known constituents of the Simaroubaceae and Rutaceae, and display a wide range of biological activities. These metabolites are reported as constituents of the Alliaceae here for the first time, and displayed minimum inhibitory concentrations (MICs) in the range 8-32 microg/mL against a panel of fast-growing Mycobacterium species and 8-64 microg/mL against multidrug-resistant (MDR) and methicillin-resistant (MRSA) strains of Staphylococcus aureus. Compound 3 displayed antimycobacterial activity in the range of 16-32 microg/mL.

Bioassay-guided fractionation of a hexane extract of strobile hops (Humulus lupulus) was undertaken to isolate and characterize the antimycobacterial constituents using the fast-growing mycobacterial species Mycobacterium fortuitum. Activity was associated with a low polarity fraction and 1H NMR spectra indicated the presence of a fatty acid mixture with unsaturated components. GC-MS of the derivatives indicated the presence of palmitic, stearic and oleic acids with small quantities of lignoceric, arachidic, behenic and linoleic acids. These compounds were assessed against M. fortuitum and all saturated fatty acids were inactive at concentrations greater than 256 microg/ml, whereas the unsaturated fats oleic and linoleic acids displayed minimum inhibitory concentrations of between 4 and 16 microg/ml against the fast-growing species tested. The widespread occurrence of these components could render screening for antimycobacterials from natural sources highly problematic without adequate dereplication. We propose that GC-MS of derivatised components of lipophilic extracts be a first step before any antimycobacterial bioassay-guided study, as this technique is the method of choice for dereplication of fatty acids.

No abstract