Antioxidant mechanism of mitochondria-targeted plastoquinone SkQ1 is suppressed in aglycemic HepG2 cells dependent on oxidative phosphorylation.

Previously suggested antioxidant mechanisms for mitochondria-targeted plastoquinone SkQ1 included: i) ion-pairing of cationic SkQ1 with free fatty acid anions resulting in uncoupling; ii) SkQ1H ability to interact with lipoperoxyl radical; iii) interference with electron flow at the inner ubiquinone (Q) binding site of Complex III (Q), involving the reduction of SkQ1 to SkQ1H by ubiquinol. We elucidated SkQ1 antioxidant properties by confocal fluorescence semi-quantification of mitochondrial superoxide (J) and cytosolic HO (J) release rates in HepG2 cells. Only in glycolytic cells, SkQ1 prevented the rotenone-induced enhancement of J and J but not basal releases without rotenone. The effect ceased in glutaminolytic aglycemic cells, in which the redox parameter NAD(P)H/FAD increased after rotenone in contrast to its decrease in glycolytic cells. Autofluorescence decay indicated decreased NADPH/NADH ratios with rotenone in both metabolic modes. SkQ1 did not increase cell respiration and diminished J established high by antimycin or myxothiazol but not by stigmatellin. The revealed SkQ1 antioxidant modes reflect its reduction to SkQ1H at Complex I I or Complex III Q site. Both reductions diminish electron diversions to oxygen thus attenuating superoxide formation. Resulting SkQ1H oxidizes back to SkQ1at the second (flavin) Complex I site, previously indicated for MitoQ. Regeneration proceeds only at lower NAD(P)H/FAD in glycolytic cells. In contrast, cyclic SkQ1 reduction/SkQ1H oxidation does not substantiate antioxidant activity in intact cells in the absence of oxidative stress (neither pro-oxidant activity, representing a great advantage). A targeted delivery to oxidative-stressed tissues is suggested for the effective antioxidant therapy based on SkQ1.