(+)-myristinins A and D from Knema elegans, which inhibit DNA polymerase β and cleave DNA

A survey of crude plant extracts for DNA polymerase inhibitors resulted in the identification of a methyl ethyl ketone extract prepared from Knema elegans that strongly inhibited the enzyme. Subsequent bioassay-guided fractionation of the extract, using an assay to monitor the activity of DNA polymerase beta, led to the isolation of two potent inhibitors, (+)-myristinins A (1) and D (2), which are known flavans having unusual structures. (+)-Myristinins A and D exhibited IC50 values of 12 and 4.3 mu M, respectively, as inhibitors of DNA polymerase beta in the presence of bovine serum albumin (BSA), and 2.7 and 1:2 mu M in the absence of BSA. As such, they are the most potent DNA polymerase inhibitors reported to date. Compounds 1 and 2 potentiated the cytotoxicity of bleomycin toward cultured P388D(1) cells, reducing the number of viable cells by at least 30% when employed at 9 mu M concentration for 6 h in the presence of an otherwise nontoxic concentration of bleomycin (75 nM). Principles 1 and 2 also induced strong Cu2+-dependent DNA strand scission in a DNA cleavage assay. Accordingly, 1 and 2 exhibit two activities, namely, DNA polymerase beta inhibition and DNA damage.