Protein, lipid and DNA repair systems in oxidative stress: the free-radical theory of aging revisited.

Aerobic organisms are constantly exposed to oxygen radicals and related oxidants. The antioxidant compounds and enzymes they have evolved remove most of the potentially damaging radicals/oxidants; however, damage to cellular proteins, lipids, nucleic acids and carbohydrates can be observed even under normal physiological conditions. Re-reduction of cellular components (direct repair) may be important for some biomolecules. In most cases studied to date, however, enzymatic degradation (by proteases, lipases, nucleases) appears to release damaged elements for excretion and conserve undamaged components for reutilization (indirect repair). In addition, the removal of damaged components appears to prevent or diminish the potential cytotoxicity of oxidized macromolecules. Several studies have reported an accumulation of oxidatively damaged cellular components with age (e.g., cataract formation, lipofuscin). Such reports are evidence that oxidant damage is one of several factors which contribute to the aging process, and provide at least partial support for the free-radical theory of aging. Studies of age-related changes in the activities, or levels of antioxidant enzymes and antioxidant compounds, however, have not provided complete understanding of the putative role of free radicals/oxidants in the aging process. In this review, we present the hypothesis that decreased activities or constitutive levels of oxidant repair enzymes may contribute to a progressive accumulation of oxidant damage with aging. Furthermore, the ability to mount an effective response to oxidative stress (induction of oxidant stress genes and proteins) may decline with age, thus predisposing older cells and organisms to oxidant damage.