The effects of reactive oxygen and nitrogen species during yeast replicative ageing.

Free radicals are considered the most important cause of cellular ageing. We have investigated ageing process in the yeast Saccharomyces cerevisiae. We have compared the wild type strain with the mutant cells with constitutively active Ras oncogen, which generates increased amounts of free radicals. Increased generation of oxygen-derived free radicals resulted in the Ras mutant cells accumulation of lipofuscin-like pigments during ageing. Ageing wild type cells did not accumulate lipofuscin-like pigments. This is quite unique feature among known biological models. It may be caused by increased concentration of alpha tocopherol (the most prominent lipophilic antioxidant) in the wild type cells. In contrast, the Ras mutant cells contained decreased levels of alpha tocopherol even in the young cells. This observation indicates that the increased free radical generation can overwhelm the endogenous antioxidant system. We have documented the involvement of nitrogen-derived free radicals in the yeast metabolism. Protein nitrotyrosine, a marker of the reactive nitrogen species, has significantly increased in the senescent Ras mutant cells. The wild type cells contained basic level of nitrotyrosine corresponding to its concentration found in non-activated mammalian macrophages.