GTRI-02

Three new metabolites (2-4), together with one known compound, GTRI-02, (1) were isolated from a fermentation broth of Streptomyces violaceoruber derived from Equus burchelli feces. The structures of the new compounds 2-4 were established using comprehensive NMR spectroscopic data analysis as well as UV, IR and MS data. The anti-inflammatory activity of compounds 1-4 was tested by examining their ability to inhibit nitric oxide (NO) production in lipopolysaccharide (LPS)-stimulated RAW 264.7 macrophage cells. Compound 2 showed a moderate inhibition of NO production with IC50 value of 51.2 μm.

A systematic study of the secondary metabolites of the wild granaticin-producing strain Streptomyces vietnamensis GIMV4.0001 led to the isolation of six known early shunt products related to actinorhodin, SEK34 (3), SEK34b (4), mutactin (5), dehydromutactin (7), EM18 (8) and GTRI-02 (9). While the other shunt products were minor or trace products, the production ratio of SEK34 (3) and SEK34b (4) to granaticins was strikingly high. Nearly 64% of the intermediate with the first ring closed went to the SEK34/SEK34b aberrant pathway. The high level of the aberrant metabolic flow toward the early shunt products SEK34 and SEK34b indicated that the second ring closure of the granaticin (1) biosynthesis is a key limiting step in the granaticin biosynthetic machinery of S. vietnamensis GIMV4.0001.

Partially reduced aromatic polyketides are bioactive secondary metabolites or intermediates in the biosynthesis of deoxygenated aromatics. For the antibiotic GTRI-02 (mensalone) in different Streptomyces spp., biosynthesis involving the reduction of a fully aromatized acetyltrihydroxynaphthalene by a naphthol reductase has been proposed and shown in vitro with a fungal enzyme. However, more recently, GTRI-02 has been identified as a product of the ActIII biosynthetic gene cluster from Streptomyces coelicolor A3(2), for which the reduction of a linear polyketide precursor by ActIII ketoreductase, prior to cyclization and aromatization, has been suggested. We have examined three different ketoreductases from bacterial producer strains of GTRI-02 for their ability to reduce mono-, bi-, and tricyclic aromatic substrates. The enzymes reduced 1- and 2-tetralone but not other aromatic substrates. This strongly suggests a reduction of a cyclized but not yet aromatic polyketide intermediate in the biosynthesis of GTRI-02. Implications of the results for the biosynthesis of other secondary polyketidic metabolites are discussed.