Cytorhodin A

Antibiotic-producing microorganism can develop strategies to deal with self-toxicity. Cytorhodins X and Y, cosmomycins A and B, and iremycin, are produced as final products from a marine-derived Streptomyces sp. SCSIO 1666. These C-7 reduced metabolites show reduced antimicrobial and comparable cytotoxic activities relative to their C-7 glycosylated counterparts. However, the biosynthetic mechanisms and relevant enzymes that drive C-7 reduction in cytorhodin biosynthesis have not yet been characterized. Here we report the discovery and characterization of a reductase, CytA, that mediates C-7 reduction of this anthracycline scaffold; CytA endows the producer Streptomyces sp. SCSIO 1666 with a means of protecting itself from the effects of its anthracycline products. Additionally, we identified cosmomycins C and D as two intermediates involved in cytorhodin biosynthesis and we also broadened the substrate specificity of CytA to clinically used anthracycline drugs.

Drug-induced myelotoxicity is usually the dose-limiting factor of treatment of malignant tumors with cytostatic drugs. Suppression of in vitro myelopoiesis (CFU-GM) by cytostatics may be a suitable model reflecting the in vivo situation. Thus the inhibitory effects of the anthracyclines doxorubicin, theprubicin, idarubicin and cytorhodin S on CFU-GM were compared. Normal human bone marrow cells were incubated with these drugs for one hour and alternatively, for the whole culture period. For each substance and each incubation time a dose-response curve was established and the D50 determined. As certain calcium antagonists can increase the toxicity of some cytostatic drugs in various tumor models, the effect of the addition of verapamil (2 microM) was also investigated. It could be shown, that the myelotoxicity on CFU-GM of the drugs mentioned above was not increased after short-term or permanent exposure to this calcium antagonist.

Anthracycline antitumor cytorhodins X and Y feature a rare 9α-glycoside and 7-dexoy-aglycone. Characterization of the cytorhodin gene cluster from Streptomyces sp. SCSIO 1666 through gene inactivations and metabolite analyses reveals three glycosyltransferases (GTs) involved in the sugar tailoring steps. The duo of CytG1 and CytL effects C-7 glycosylation with l-rhodosamine whereas the iterative GT CytG3 and CytW similarly modifies both C-9 and C-10 positions. CytG2 also acts iteratively by incorporating the second and third sugar moiety into the trisaccharide chains at the C-7 or C-10 position.