Streptovaricin C

Streptovaricin C is a naphthalenic ansamycin antibiotic structurally similar to rifamycins with potential anti-MRSA bioactivities. However, the formation mechanism of the most fascinating and bioactivity-related methylenedioxy bridge (MDB) moiety in streptovaricins is unclear. Based on genetic and biochemical evidences, we herein clarify that the P450 enzyme StvP2 catalyzes the MDB formation in streptovaricins, with an atypical substrate inhibition kinetics. Furthermore, X-ray crystal structures in complex with substrate and structure-based mutagenesis reveal the intrinsic details of the enzymatic reaction. The mechanism of MDB formation is proposed to be an intramolecular nucleophilic substitution resulting from the hydroxylation by the heme core and the keto-enol tautomerization via a crucial catalytic triad (Asp89-His92-Arg72) in StvP2. In addition, in vitro reconstitution uncovers that C6-O-methylation and C4-O-acetylation of streptovaricins are necessary prerequisites for the MDB formation. This work provides insight for the MDB formation and adds evidence in support of the functional versatility of P450 enzymes.

[Carboxy-14C]-3-amino-5-hydroxybenzoic acid (AHBA) has been shown to be incorporated by Streptomyces spectabilis to the extent of greater than 0.1% (35: 1 dilution) in the ansamycin antibiotic streptovaricin C, the major component of the streptovaricin complex. When [carboxy-13C]AHBA was similarly administered, C-21 (quinone methide carbonyl at 188.3 ppm) of streptovaricin C was specifically labeled (at twenty one times natural abundance). In preparation for the 13C incorporation study the 13C NMR spectrum of streptovaricin C was investigated, making extensive use of short- and long-range HETCOR. These assignments revise some of those proposed earlier for streptovaricin C.

Streptomyces sp. KM1-30 was isolated from soil as a producer of antimutagens by screening with a modified Ames test. The chemical structure of the antimutagenic metabolite was identified as streptovaricin C, which is known to inhibit DNA dependent RNA polymerase from E. coli and RNA dependent DNA polymerase from RNA tumor viruses, by MS and 1H-, 13C-NMR analyses. Addition of streptovaricin C to the cultures of UV treated Salmonella typhimurium TA100 or Trp-P-2-treated S. typhimurium TA98 decreased the frequency of mutation without a decrease in viable cell counts. The effect of streptovaricin C to the mutation induced by UV and Trp-P-2 was not desmutagenic, but antimutagenic.