Production of reactive oxygen species and loss of viability in yeast mitochondrial mutants: protective effect of Bcl-xL

The capacity of yeast cells to produce reactive oxygen species (ROS), both as a response to manipulation of mitochondrial functions and to growth conditions, was estimated and compared with the viability of the cells. The chronological ageing of yeast cells (growth to late-stationary phase) was accompanied by increased ROS accumulation and a significantly higher loss of viability in the mutants with impaired mitochondrial functions than in the parental strain. Under these conditions, the ectopic expression of mammalian Bcl-x_L, which is an anti-apoptotic protein, allowed cells to survive longer in stationary phase. The protective effect of Bcl-x_L was more prominent in respiratory-competent cells that contained defects in mitochondrial ADP/ATP translocation, suggesting a model for Bcl-x_L regulation of chronological ageing at the mitochondria. Yeast can also be triggered into apoptosis-like cell death, at conditions leading to the depletion of the intramitochondrial ATP pool, as a consequence of the parallel inhibition of mitochondrial respiration and ADP/ATP translocation. If respiratory-deficient (ρ⁰) cells were used, no correlation between the numbers of ROS-producing cells and the viability loss in the population was observed, indicating that ROS production may be an accompanying event. The protective effect of Bcl-x_L against death of these cells suggests a mitochondrial mechanism which is different from the antioxidant activity of Bcl-x_L.