An in vitro evaluation of human DNA topoisomerase I inhibition by Peganum harmala L. seeds extract and its beta-carboline alkaloids.

Purpose: Peganum harmala L. (Zygophyllaceae) seeds extract is one of the main components of an ethnobotanical preparation used in the treatment of neoplasms in Iran. Cytotoxic effects of P. harmala extract on cancerous cell-lines have been reported before. beta-carbolines like harmaline and harmine are the major alkaloids present in the seeds of the P. harmala. Considering reports concerning DNA topoisomerase inhibition by other beta-carbolines like harmane, we have used DNA relaxation assays to investigate topoisomerase I inhibitory activity of P. harmala seeds extract and its beta-carboline alkaloids to further inspecting the mechanism of its cytotoxic activity.

Methods: Harmine and harmaline contents of the extract were determined using an HPTLC method. DNA topoisomerase I enzyme needed for investigating inhibitory effect of the compounds using DNA relaxation assay, was partially purified from the human placenta. DNA relaxation assay is based on the conversion of a supercoiled plasmid substrate to its relaxed form by the catalytic activity of the enzyme. The supercoiled substrate and its relaxed product can be easily distinguished using agarose gel electrophoresis, since the relaxed topological isomers of DNA migrate more slowly than supercoiled species.

Results: Using HPTLC method, it was found that each gram of dried extract contained 55.5 and 79.0 mg of harmine and harmaline respectively. In the DNA relaxation assay, order of potency was harmine > harmane > harmaline > extract. The most active compound was harmine with IC50 value of 13.5 +/- 1.7 microg/ml.

Conclusions: Our in vitro findings demonstrate that P. harmala seeds extract do inhibit human DNA topoisomerase I and based on the results of HPTLC analysis, it appears that the biological activity of the extract can be explained by its beta-carboline content.