Grisorixin

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.

The effects of grisorixin, a monocarboxylic ionophore, were studied on isolated working rat hearts perfused with a suspension of washed pig erythrocytes (10% hematocrit). Grisorixin (2.5 microM) induced a transient stimulation of heart work, maximal at 5 min, expressed by an increase in heart rate (+21%) and aortic flow (+17%) and by an increase in coronary flow, maximal at 10 min (+47%). Concomitantly, myocardial Vo2 was slightly enhanced and the myocardial creatine phosphate level dropped (2 min). The lactate production increased by 82% (5 min) then dropped to the control value (10 min) and increased again till the 45th min (+211%), indicating a cardiac metabolic drift towards anaerobic glycolysis due to partial inhibition of the oxidative metabolism. Owing to its properties as an ionophore, grisorixin also induced a strong and rapid increase of potassium concentration in the perfusate and a decrease of sodium. Grisorixin was tested on hearts submitted to 20 min of hypoxic conditions. The hypoxia was rather mild and induced only very slight modifications of the ultrastructure. In the control series, heart rate and aortic flow decreased regularly while coronary flow and lactate production increased. Upon reoxygenation, the heart performances were rapidly restored. Grisorixin was administered according to four different protocols. When injected at the onset of hypoxia or 5 min later, it was able to maintain the aortic flow during the first minutes and induce a higher coronary dilation. These beneficial effects were short-lasting and no deleterious effects were found on the ultrastructure of hearts subjected to grisorixin whether after hypoxia or after reoxygenation.

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).