Abstract
Zanthoxylum rhoifolium (Rutaceae) is a plant alkaloid that grows in South America and has been used in Brazilian traditional medicine for the treatment of different health problems. The present study was designed to evaluate the antimicrobial activity of the steam bark crude methanol extract, fractions, and pure alkaloids of Z. rhoifolium. Its stem bark extracts exhibited a broad spectrum of antimicrobial activity, ranging from 12.5 to 100 µg/mL using bioautography method, and from 125 to 500 µg/mL in the microdilution bioassay. From the dichloromethane basic fraction, three furoquinoline alkaloids (1–3), and nine benzophenanthridine alkaloids (4–12) were isolated and the antimicrobial activity of the benzophenanthridine alkaloids is discussed in terms of structure-activity relationships. The alkaloid with the widest spectrum of activity was chelerythrine (10), followed by avicine (12) and dihydrochelerythrine (4). The minimal inhibitory concentrations of chelerythrine, of 1.50 µg/mL for all bacteria tested, and between 3.12 and 6.25 µg/mL for the yeast tested, show this compound to be a more powerful antimicrobial agent when compared with the other active alkaloids isolated from Z. rhoifolium. To verify the potential importance of the methylenedioxy group (ring A) of these alkaloids, chelerythrine was selected to represent the remainder of the benzophenanthridine alkaloids isolated in this work and was subjected to a demethylation reaction giving derivative 14. Compared to chelerythrine, the derivative (14) was less active against the tested bacteria and fungi. Kinetic measurements of the bacteriolytic activities of chelerythrine against the bacteria Bacillus subtilis (Gram-positive) and Escherichia coli (Gram-negative) were determined by optical density based on real time assay, suggesting that its mechanism of action is not bacteriolytic. The present study did not detect hemolytic effects of chelerythrine on erythrocytes and found a protective effect considering the decrease in TBARS and AOPP (advanced oxidized protein products) levels when compared to the control group.
Highlights
Many studies have underlined the importance and use of plants as a source of antimicrobial agents [1]
We have reported the antibacterial activity of the crude MeOH extract, aqueous extract, acid and basic fractions of Z. rhoifolium by disc diffusion method [20] and of some isolated alkaloids by the TLC bioassay [21]
Because many diseases that are widespread among underprivileged and indigenous populations, such as fever, stomach upset and respiratory diseases, among others, can be caused by fungi and bacteria, this study investigated the antimicrobial activities of the crude methanol extract, fractions obtained from this extract and purified compounds from the stem bark of Z. rhoifolium collected in Rio Grande do Sul, Brazil, were initially screened by bioautography method [23,24] to select extracts, fractions and active alkaloids
Summary
Many studies have underlined the importance and use of plants as a source of antimicrobial agents [1]. Fractions 35–38 obtained from CH2Cl2: MeOH, 80:20, and consisting of one alkaloid (TLC), were combined and concentrated in vacuum to give chelerythrine (10) after crystallization from Et2O-MeOH (120.0 mg).
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