Gilvocarcin V

Analysis of gilvocarcin V production by Streptomyces arenae in complex and chemically defined media revealed strong nitrogen repression of antibiotic biosynthesis. Nitrogen regulation was first suggested by the observation of a 10-fold increase in gilvocarcin V production when the ammonium ion trapping agent Mg3(PO4)2.8H2O was added to complex medium. In a chemically defined medium, cell mass increased as the initial ammonium sulfate concentrations approached 7.5 mM; however, antibiotic production was strongly repressed at ammonium sulfate concentrations exceeding 1.5 mM. Repression of gilvocarcin V production at 7.5 mM ammonium sulfate was maximally reversed by adding Mg3(PO4)2.8H2O to the medium at 25 mM; specific antibiotic production attained a level 2.5-fold higher than at the nonrepressive ammonium salt concentration of 1.5 mM. Evaluation of the effects of soluble inorganic phosphate concentrations upon gilvocarcin V titers suggested that the relatively insoluble Mg3(PO4)2.8H2O must in fact serve as an ammonium ion-trapping agent, as previously reported in other fermentation systems, not as a supplementary source of phosphate for growth and antibiotic production. These studies also revealed a minor repression of antibiotic synthesis at elevated levels of soluble phosphate. Comparisons of several amino acids as nitrogen sources in a Mg3(PO4)2.8H2O-containing medium indicated that L-aspartic acid and glycine promoted the highest yields of gilvocarcin V. Metabolism of these two amino acids into precursors of the polyketide pathway for gilvocarcin V biosynthesis is postulated.

Polycarcin V, a polyketide natural product of Streptomyces polyformus, was chosen to study structure-activity relationships of the gilvocarcin group of antitumor antibiotics due to a similar chemical structure and comparable bioactivity with gilvocarcin V, the principle compound of this group, and the feasibility of enzymatic modifications of its sugar moiety by auxiliary O-methyltransferases. Such enzymes were used to modify the interaction of the drug with histone H3, the biological target that interacts with the sugar moiety. Cytotoxicity assays revealed that a free 2'-OH group of the sugar moiety is essential to maintain the bioactivity of polycarcin V, apparently an important hydrogen bond donor for the interaction with histone H3, and converting 3'-OH into an OCH3 group improved the bioactivity. Bis-methylated polycarcin derivatives revealed weaker activity than the parent compound, indicating that at least two hydrogen bond donors in the sugar are necessary for optimal binding.

Four new analogues of the gilvocarcin-type aryl-C-glycoside antitumor compounds, namely 4'-hydroxy gilvocarcin V (4'-OH-GV), 4'-hydroxy gilvocarcin M, 4'-hydroxy gilvocarcin E and 12-demethyl-defucogilvocarcin V, were produced through inactivation of the gilU gene. The 4'-OH-analogues showed improved activity against lung cancer cell lines as compared to their parent compounds without 4'-OH group (gilvocarcins V and E). The structures of the sugar-containing new mutant products indicate that the enzyme GilU acts as an unusual ketoreductase involved in the biosynthesis of the C-glycosidically linked deoxysugar moiety of the gilvocarcins. The structures of the new gilvocarcins indicate substrate flexibility of the post-polyketide synthase modifying enzymes, particularly the C-glycosyltransferase and the enzyme responsible for the sugar ring contraction. The results also shed light into biosynthetic sequence of events in the late steps of biosynthetic pathway of gilvocarcin V.