The tanshinone natural products possess a variety of pharmacological properties 
including anti-bacterial, anti-inflammatory, anti-oxidant, and anti-neoplastic 
activity. The molecular basis of these effects, however, remains largely 
unknown. In the present study, we explored the direct effect of tanshinones on 
the enzyme telomerase. Telomerase is up-regulated in the majority of cancer 
cells and is essential for their survival, making it a potential anti-cancer 
drug target. We found that the ortho-quinone tanshinone II-A inhibits telomerase 
in a time- and DTT-dependent fashion, and the hydrogen peroxide scavenger 
catalase protected telomerase from inactivation. These findings demonstrate that 
ortho-quinone containing tanshinones can inhibit telomerase owing to their 
ability to generate reactive oxygen species. The results also provide evidence 
that telomerase is directly and negatively regulated by reactive oxygen species.