Xanthocidin

Xanthocidin and six new derivatives were isolated from the endophytic Streptomyces sp. AcE210. Their planar structures were elucidated by 1D and 2D NMR spectroscopy as well as by HRMS. The absolute configuration of one compound was determined by using vibrational circular dichroism spectroscopy (VCD). The structural similarities of xanthocidin and some of the isolated xanthocidin congeners to the methylenomycins A, B, and C suggested that the biosynthesis of these compounds might follow a similar route. Feeding studies with isotopically labelled [13 C5 ]-l-valine showed that instead of utilizing acetyl-CoA as starter unit, which has been proposed for the methylenomycin biosynthesis, Streptomyces sp. AcE210 employs an isobutyryl-CoA starter unit, resulting in a branched side chain in xanthocidin. Further evidence for a comparable biosynthesis was given by the analysis of the genome sequence of Streptomyces sp. AcE210 that revealed a cluster of homologues to the mmy genes involved in methylenomycin biosynthesis.

Few studies have examined xanthocidin, a biotic isolated from Streptomyces xanthocidicus in 1966, because its supply is limited. Based on its chemical structure, xanthocidin has the potential to become a lead compound in the production of agrochemicals and anti-cancer drugs; however, it is unstable under both basic and acidic conditions. We recently established the total synthesis of xanthocidin using the FeCl3-mediated Nazarov reaction, and obtained two stable derivatives (#1 and #2). The results of the present study demonstrated that these derivatives exhibited the inhibitory activity of topoisomerase IIα, known as a molecular target for cancer chemotherapy, and this was attributed to the respective exo-methylene ketone group without DNA intercalation. The results obtained also suggest that these derivatives may have value as lead compounds in the synthesis of topoisomerase IIα inhibitors.

Streptomyces sp. strain AcE210 exhibited antibacterial activity toward Gram-positive microorganisms and turned out to be a rare producer of the specialized metabolite xanthocidin. The 10.6-Mb draft genome sequence gives insight into the complete specialized metabolite production capacity and builds the basis to find and locate the biosynthetic gene cluster of xanthocidin.