Microorganisms are a plentiful source of structurally diverse bioactive substances. In particular, over the last 25 years myxobacteria have proved to be a consistent source of interesting and novel bioactive secondary metabolites. One of the most prominent examples is epothilones, novel nontaxane antimicrotubule agents obtained from the fermentation of the cellulose degrading myxobacteria, Sorangium cellulosum. The potential antitumor effects of epothilones had been proved much better than taxol.
The genus Sorangium belongs to the myxobacterial suborder Sorangineae. Strains in the genus are classified as one species, Sorangium cellulosum, by ignoring divergent morphological characteristics.
Myxobacteria are usually obtained from herbivore excrement, decaying plant material and bark of living and dead trees. However, research on this group of bacteria has not been extensive. Conventional isolation methods do not apply, because the fruiting bodies form mostly on natural substrates. It is difficult to cultivate and produce fruiting bodies on ordinary media used in the laboratory. Therefore, this studies established in isolation and purification methods of Sorangium cellulosum from soil of different regions in Korea. As a result of, Sorangium cellulosum in soil were isolated by 96 strains. Morphological characteristics and 16S rDNA analysis indicated that the strains belongs to the genus Sorangium.
In continuation of our screening program for novel bioactive compounds, Sorangium cellulosum strain KM0141 was selected for its pronounced antibiotic activity and epothilone-producing.
The products were isolated and purified by solvent extraction, silica gel column chromatography, RP18 column chromatography and semi-preparative HPLC. Therefore, two novel highly cytotoxic compounds isolated from culture extracts of strain KM0141, propose the named isospirodienal and spirodienal B.
Their structures were elucidated by spectroscopic studies including various NMR experiments. The previous unknown molecular framework common to isospirodienal and spirodienal B includes a highly functionalized spiroketal core structure, a side chain bearing a diene chromophore and a terminal aldehyde group, and a total of twelve stereocenters. The relative stereochemistry of the spiroketal structure was proposed on the basis of vicinal protone couplings and ROESY data. The molecular formula of spirodienal B differs from that of isospirodienal only by an additional CH2 group, and their close relationship was evident from comparison of their 1H and 13C NMR spectroscopic data. Both compounds demonstrated potent cytotoxicity against human tumor cell lines but showed no growth inhibition against various microorganisms including bacteria and yeast. Against human tumor cells such as HCT-15, the activity of spirodienal B(IC50=0.02 nM) was more than 4000 times stronger than that of doxorubicin in terms of IC50.