Alterations of Antioxidant Enzymes and Oxidative Damage to Macromolecules in Different Organs of Rats During Aging

Abstract

Oxygen free radicals have been hypothesized to play an important role in the aging process. To investigate the correlation between the oxidative stress and aging, we have determined the levels of oxidative protein damage and lipid peroxidation in the brain and liver, and activities of antioxidant enzymes in the brain, liver, heart, kidney, and serum from the Fisher 344 rats at ages of 1, 6, 12, 18, and 24 months. The results showed that the level of oxidative protein damage (measured as carbonyl content) in the brain and liver was significantly higher in older animals than in young animals. No statistical difference was observed in the lipid peroxidation of the liver and brain between young and old animals. The activities of antioxidant enzymes in most tissues displayed an age-dependent decline. Superoxide dismutases in the heart, kidney, and serum, glutathione peroxidase activities in the serum and kidney, and catalase activities in the brain, liver, and kidney, significantly decreased during aging. Cytochrome c oxidase, an enzyme involved in electron transport in mitochondria, initially increased, but subsequently decreased in the aged brain, whereas no significant alteration was observed in the liver mitochondrial antioxidant enzymes. The present studies suggest that the accumulation of oxidized proteins during aging is most likely to be linked with an age-related decline of antioxidant enzyme activities, whereas lipid peroxidation is less sensitive to predict the aging process.

Introduction

Aging is the progressive deterioration in physiological functions and metabolic processes. In recent years, the reactive oxygen species (ROS) have become an active field in aging research because of their potential involvement in many degenerative processes.[1]It is well known that utilization of oxygen represents an efficient mechanism for aerobic organisms to generate energy, but ROS, as the by-products during this process and other unfavorable events, are also produced within the biological system.[2]These ROS are highly reactive and capable of damaging many biological macromolecules such as DNA, RNA, protein, and lipids.1, 2, 3A particular consequence of this ROS-mediated attack is the accumulation of oxidatively damaged macromolecules, which may lead to genetic mutation and cellular senescence, if not timely removed in vivo.[4]

To protect cells against oxidative damage by oxidants produced during the oxygen metabolism, an antioxidant system has presumably evolved in aerobic organisms.[5]Antioxidant enzymes constitute an important defense system to clear up the detrimental ROS in vivo. Superoxide dismutases (SOD) including MnSOD in mitochondria and CuZnSOD in cytosol rapidly convert superoxide anion (O₂·⁻) to hydrogen peroxide (H₂O₂). The later can be converted to more harmful hydroxy radicals (HO·) in the presence of transition metals such as iron and copper. Catalase (CAT) and glutathione peroxidase (GPx) can decompose H₂O₂ to water. Any factors that undermine the activities of antioxidant enzymes may lead to accumulation of ROS and subsequently oxidative damage to biological macromolecules.[5]

Mitochondrial electron transport chain is widely viewed as the main locus in the cell for the generation of O₂·⁻.[6]Cytochrome c oxidase plays an impotent role in the mitochondrial respiratory chain that converts molecular oxygen into water.[7]Cytochrome c oxidase activity has been found to decline with aging.8, 9Correlated with the decline of this enzyme is the concomitant increase in the flux of mitochondrial O₂·⁻ and H₂O₂ generation in both insects and mammals.10, 11To confirm the correlation between oxidative stress and the aging process, we have determined oxidatively modified lipids and proteins and the activities of antioxidant enzymes, namely, SOD, GPx, and CAT, as well as cytochrome c oxidase activities in different organs of rats during the aging processes.