Epigrisorixin

The present work describes the metabolites produced by a strain identified as Streptomyces youssoufiensis, whose secondary metabolites profile has not been studied so far. The crude ethyl acetate extract was analyzed by high performance liquid chromatography-electrospray ionization mass spectrometry, leading to the detection of the ionophoric polyethers nigericin, epinigericin, abierixin and the newly isolated grisorixin methyl ester. The presence of epimeric forms of nigericin/epinigericin and grisorixin/epigrisorixin has spurred density functional theory computational calculations. This analysis was able to provide the relative stability of the most favored epimers, setting the basis for general structural considerations applicable to several other polyethers. Both nigericin sodium salt and grisorixin methyl ester showed to affect glioblastoma stem cells proliferation in a dose-dependent manner, with a higher activity for the more lipophilic grisorixin methyl ester (GI50 values of 3.85 and 3.05 μM for VIPI and COMI human glioblastoma stem cells, respectively).

With addition of methyl oleate, the increased yield of antibiotic production by nigericin-producing Streptomyces hygroscopicus NRRL B-1865 also resulted in the isolation of three additional polyether antibiotics. Two of these are abierixin and epinigericin, as new antibiotics. The third antibiotic is grisorixin. The production of both abierixin (opened ring A and 30-CH2OH) and grisorixin (ring A and 30-CH3) poses the problem of the identity of the last pathway precursor of the major metabolite, nigericin (ring A and 30-CH2OH). Transformation experiments of abierixin by S. hygroscopicus gave negative results. Hydroxylation of grisorixin to nigericin by S. hygroscopicus represents the final step in nigericin biosynthesis.

Thin-layer chromatography coupled with flame ionization detection was used to develop a method to separate and to determine simultaneously three polyether carboxylic ionophore antibiotics (abierixin, nigericin and grisorixin) produced by Streptomyces hygroscopicus NRRL B 1865. Various proportions of chloroform, methanol and formic acid (or acetic acid as a substitute for formic acid) were used in the developing solvent to determine changes in RF values of the antibiotics and to allow conditions for maximum resolution to be obtained. Development on Chromarods SII with chloroform-methanol-formic acid (97:4:0.6, v/v/v) gave satisfactory and reliable separations of the three polyether antibiotics. Under these conditions, the internal standard methyl desoxycholate was found to be suitable for their simultaneous determination in the lipid extracts of Streptomyces hygroscopicus NRRL B 1865.