Male rats exhibit higher oxidative protein damage than females of the same chronological age

Abstract

The basis of the difference in life expectancy between males and females is still unknown. Previous studies have provided compelling evidence for the presence of oxidized proteins, and lipids in advanced human atherosclerotic lesions. The gender factor responsible for such protein oxidation is unknown and controversial. Our aim was to reveal the difference between protein oxidation parameters of male and female rats of the same chronological age to understand the protein oxidation mechanisms enabling females live longer than males. In the current study, we investigated the relation between protein hydroperoxide levels (P-OOH) and other protein oxidation parameters such as protein carbonyl (PCO), total thiol (T-SH), advanced oxidation protein products (AOPP), and nitrotyrosine (NT). Our study also covered other oxidative stress parameters such as lipid hydroperoxides (L-OOH), and superoxide dismutase (SOD) activity in the plasma of male and female aged rats. Plasma P-OOH and AOPP levels of male rats were significantly higher compared with those of the female rats. T-SH levels were significantly lower in the aged male rats compared with those of the female rats. On the other hand, PCO, NT, and L-OOH levels, and SOD activity were all found to be not different. These data support the hypothesis that elevated levels of P-OOH and AOPP contribute to the extent of protein, but not lipid, oxidation in plasma of aged male rats. Furthermore, the results presented here may also rationalize studies, which have shown that protein oxidation is modulated on the basis of gender dependency.

Introduction

The basis of the difference in life expectancy between males and females is still unknown (Borrás et al., 2003). Life expectancy doubled in Europe and the United States between 1900 and 1992 (Fernandez-Ballesteros et al., 1999, Wilmoth et al., 2000). In all cases life expectancy of women was higher than that of men. In Western countries in 1992, life expectancy for men was 73.7 years and for women it was 83.8 years, i.e., 9.9% higher than for men (Fernandez-Ballesteros et al., 1999). According to the International Committee on Supercentenarians, there are currently 86 people aged 110 or older alive in the world today. Eighty of them are women (Gerontology Research Group, 2007). Scientifically, ageing is an extremely complex, multifactorial process, and numerous ageing theories have been proposed; the most important of these are probably the genomic and free radical theories. Thus, the genomic and free radical theories are closely linked. Although it is abundantly clear that our genes influence ageing and longevity, exactly how this takes place on a chemical level is only partially understood (Knight, 2000). The mitochondrion releases H₂O₂ to the cytosol, and the general level of oxidative stress in the cell will depend on the balance between H₂O₂ generation and its elimination by cellular antioxidants (Davies et al., 2001, Barja, 2002). Female rats generate half the amount of peroxides than those of males. Mitochondrial antioxidant enzyme expression and activities in females were higher than those in males of the same chronological age (Borrás et al., 2003).

For many years lipid oxidation has been in the focus of investigation, but due to their relatively high abundance it is now recognized that proteins are the main targets for oxidants. Recently, however, the number of reports supporting that protein and lipid oxidation are coupled processes has increased. On the other hand, the interaction between lipids and proteins during oxidation is not well understood. Considerable evidence indicates that the maintenance of protein redox status is of fundamental importance for cell function, therefore structural changes in proteins are considered to be among the molecular mechanisms leading to endothelial dysfunction (Kaneda et al., 2002, Woods et al., 2003, Škvařilová et al., 2005). It is well established that exposure of proteins to reactive oxygen species (ROS) can alter the physical and chemical structure of the target causing consequent oxidation of side chain groups, protein scission, backbone fragmentation, cross-linking, unfolding, and formation of new reactive groups. The latter include oxidation of hydrophobic amino acyl residues to hydroxy and hydroperoxy (P-OOH) derivatives, protein carbonylation (PCO), oxidation of thiol (T-SH) groups, nitrotyrosine (NT) formation, and many others. The conformational changes that result from this complex of reactions lead to the decrease or loss of protein biological function. These processes have been widely reviewed in (Beal, 2002, Stadtman, 2004). Recently, a new marker of protein oxidation, advanced oxidation protein products (AOPP) has begun to attract the attention of various investigators (Kaneda et al., 2002, Woods et al., 2003, Škvařilová et al., 2005, Çakatay and Kayali, 2005). AOPP are proteins damaged by oxidative stress; in plasma this is predominately albumin. (Kalousová et al., 2005). They contain dityrosines, which allow cross-linking, disulfide bridges and carbonyl groups (Capéillere-Blandin et al., 2004) and are formed mainly by chlorinated oxidants—hypochloric acid and chloramines resulting from myeloperoxidase activity (Capéillere-Blandin et al., 2004, Kalousová et al., 2005). Protein oxidation products mediated by chlorinated species generated by an enzyme myeloperoxidase were found in the extracellular matrix of human atherosclerotic plaques (Woods et al., 2003) and increased levels of AOPP were described as an independent risk factor for coronary artery disease (Kaneda et al., 2002).

In the current study, we investigated the relation between P-OOH and other protein oxidation parameters such as PCO, T-SH, AOPP, and NT. Our study also covered other oxidative stress parameters such as lipid hydroperoxides (L-OOH), and superoxide dismutase (SOD) activity in the plasma of male and female aged rats.