Topopyrone C

Non-reducing iterative type I polyketide synthase genes, pnk1 and pnk2, were cloned from the fungus Phoma sp. BAUA2861, which produces the topoisomerase I inhibitors, topopyrones A to D. Heterologous expression of these polyketide synthase genes under the α-amylase promoter in Aspergillus oryzae was carried out to identify their functions. The pnk2 transformant produced topopyrones C, D, and haematommone. Therefore, the pnk2 gene was found to encode for the topopyrone nonaketide synthase.

The structures of novel topoisomerase I inhibitors, topopyrones A, B, C and D were elucidated by spectral analysis of the chemical derivatives. These compounds are an anthraquinone type containing a fused 1,4-pyrone moiety. Topopyrones A and B contain a chlorine atom, however C and D do not. It was suggested that topopyrones B and D are converted from topopyrones A and C, respectively by Wessely-Moser type rearrangement.

A series of structurally simple analogues of natural topopyrone C were synthesized and tested for cytotoxic and topoisomerase I inhibitory activities. The removal of the hydroxyl groups at the 5 and 9 positions resulted in an increased cytotoxic potency and ability to stabilize topoisomerase-mediated cleavage. In addition, the results suggest that some structural features, such as the pyrone ring and a polar group in position 11, are fundamental for topoisomerase I inhibitory effect. These structural requirements are also consistent with the cytotoxic activity.