CONICET | Buscador de Institutos y Recursos Humanos

The effect of NO between cytochromes b and c of the mitochondrial respiratory chain were studied using submitochondrial particles (SMP) from bovine heart and GSNO and SPER-NO as NO sources. Succinate-cytochrome c reductase (complex II-III) activity (222 ± 4 nmol/min . mg protein) was inhibited by 51% in the presence of 500 uM GSNO and by 48% in the presence of 30 uM SPER-NO, in both cases at ~1.25 uM NO. Neither GSNO nor SPER-NO were able to inhibit succinate-Q reductase activity (complex II; 220 ± 9 nmol/min . mg protein), showing that NO affects complex III. Complex II-III activity was decreased (36%) when SMP were incubated with L-arginine and mtNOS cofactors, indicating that this effect is also produced by endogenous NO. GSNO (500 uM) reduced cytochrome b562 by 71%, in an [O2] independent manner. Hyperbolic increases in O2?- (up to 1.3 ± 0.1 nmol/min . mg protein) and H2O2 (up to 0.64 ± 0.05 nmol/min . mg protein) productions were observed with a maximal effect at 500 uM GSNO. The O2?-/H2O2 ratio was 1.98 in accordance to the stoichiometry of the O2?- disproportionation. Moreover, H2O2 production was increased by 72-74% when heart coupled mitochondria were exposed to 500 uM GSNO or 30 uM SPER-NO. SMP incubated in the presence of succinate showed an EPR signal (g = 1.99) compatible with a stable semiquinone. This EPR signal was increased not only by antimycin but also by GSNO and SPER-NO. These signals were not modified under N2 atmosphere, indicating that they are not a consequence to the effect of NOx species on complex III area. These results show that NO interacts with ubiquinone-cytochrome b area producing antimycin-like effects. This behaviour comprises the inhibition of electron transfer, the interruption of the oxidation of cytochromes b, and the enhancement of [UQH?]ss which, in turn, leads to an increase in O2?- and H2O2 mitochondrial production rates.