Stimulatory effects of a partial respiratory Complex I inhibition on

Author(s)

Luiz Guilherme Michelini Bueno

• Document type: Master's Dissertation
• Institution: Universidade Estadual de Campinas. Faculdade de Ciências Médicas
• Defense date: 21 fev. 2014

Examining board members

Carmen Veríssima Ferreira Halder; Claudia Barbosa Ladeira de Campos

• Advisor: Roger Frigério Castilho

Abstract

Partial inhibition of mitochondrial Complex I is associated with the development of neurochemical, behavioral, and neuropathological features of Parkinson's disease in rats chronically and systemically treated with rotenone. The aims of this work were (i) to characterize the effects of partial inhibition of respiratory Complex I by rotenone on H2O2 production by rat brain mitochondria in different respiratory states and (ii) to evaluate the susceptibility of brain mitochondria from old rats (24 month-old) to rotenone-induced inhibition of oxygen consumption and increased generation of H2O2 when compared with organelles from adult rats (3-4 month-old). Flow cytometric analysis of membrane potential in isolated mitochondria indicated that rotenone leads to uniform respiratory inhibition when added to a suspension of these organelles. When mitochondria were incubated in the presence of a low concentration of rotenone (10 nM) and NADH-linked substrates, oxygen consumption was reduced from 45.9±1.0 to 26.4±2.6 nmol O2.mg-1.min-1 and from 7.8±0.3 to 6.3±0.3 nmol O2.mg-1.min-1 in respiratory states 3 (ADP-stimulated respiration) and 4 (resting respiration), respectively. Under these conditions, mitochondrial H2O2 production was stimulated from 12.2±1.1 to 21.0±1.2 pmol H2O2.mg-1.min-1 and 56.5±4.7 to 95.0±11.1 pmol H2O2.mg-1.min-1 in respiratory states 3 and 4, respectively. Similar results were observed when comparing mitochondrial preparations enriched with synaptic or nonsynaptic organelles or when 1-methyl-4-phenylpyridinium (MPP+) ion was used as a respiratory Complex I inhibitor. Rotenone-stimulated H2O2 production in respiratory states 3 and 4 was associated with a high reduction state of endogenous nicotinamide nucleotides. In succinate-supported mitochondrial respiration, where most of the mitochondrial H2O2 production relies on electron backflow from Complex II to Complex I, low rotenone concentrations inhibited H2O2 production. Rotenone had no effect on mitochondrial elimination of micromolar concentrations of H2O2. In intact synaptosomes, we observed that 10 nM rotenone stimulated H2O2 release by 20.2 ± 3.3% under basal respiratory state. When comparing isolated brain mitochondria from adult and old rats we observed that oxygen consumption under respiratory state 3 and citrate synthase activity were 21.0±3.3% and 17.0±5.4% lower in mitochondria from old rats. Experiments conducted in the presence of different rotenone concentrations (5, 10 and 100 nM) showed that brain mitochondria from adult and old rats have similar sensitive to rotenone-induced inhibition of oxygen consumption in respiratory state 3, with IC50 of 7.8±0.4 and 6.5±0.5 nM for adult and old rats, respectively. In line with these results, similar stimulations in H2O2 production were observed in mitochondria from adult and old rats treated with different concentrations of rotenone. We conclude that partial Complex I inhibition may result in mitochondrial energy crisis and oxidative stress, the former being predominant under oxidative phosphorylation and the latter under resting respiration conditions. Rotenone exerts similar effects on oxygen consumption and H2O2 production by isolated brain mitochondria from adult and old rats. (AU)