Isolation and biochemical characterization of a new topoisomerase I inhibitor from Ocotea leucoxylon

In a continuation of our search for potential tumor inhibitors from plants, we found that a crude extract from Ocotea leucoxylon showed selective acti vity typical of inhibitors of the enzyme topoisomerase I in a yeast assay f or DNA-damaging agents. Using a bioassay-directed fractionation approach, t he major bioactive compound was isolated and identified as the known aporph ine alkaloid dicentrinone (4); the inactive alkaloid dicentrine (3) was als o isolated. Compound 4 showed selective bioactivity against the rad52 repai r-deficient yeast strain RS322 (IC12 49 mu g/mL) and was inactive against t he rad52- and topo1-deficient strain RS321 (IC12 > 2000 mu g/mL) and agains t the repair-proficient strain RJ03 (IC12 > 2000 mu g/mL). Biochemical stud ies with recombinant human topoisomerase I indicated that dicentrinone (4) is an inhibitor of the human enzyme. Colony formation studies suggest that it is weakly cytotoxic, but that its mechanism of toxicity differs from tha t of camptothecin and its derivatives.