**2.3 The role of Reactive Oxygen Species (ROS) in glycation process**

Free radicals in biological materials were discovered less than 60 years ago. Soon thereafter, Denham Harman hypothesized that reactive oxygen radicals may be formed as by-products of enzymatic reactions *in vivo.* In 1956 he described free radicals as a Pandora's box of evils that may account for gross cellular damage, mutagenesis, cancer, and, last but not the least, the degenerative process of biological aging (Harman, 1956). Presently, the list of cell and tissue disorders caused by free radicals is very long and the diseases, such as diabetes and/or impairments in mitochondria functions, also belong to the "victims" of ROS attack.

Glycation and oxidative stress are closely linked, and both phenomena coincide in a vicious process referred to as ''glycoxidation''. In all steps of glycoxidation there is a massive generation of oxygen-free radicals, some of them being common with lipidic peroxidation pathways. Besides, glycated proteins, and especially their advanced adducts, activate membrane receptors, such as RAGE, and induce an intracellular oxidative stress and a proinflammatory status. Glycated proteins may modulate functions of cells involved in oxidative metabolism and induce inappropriate responses. Finally, some oxidative products (reactive aldehydes such as methylglyoxal) or lipid peroxidation products (malondialdehyde) may bind to proteins and amplify glycoxidation generated lesions (Hunt et al., 1998).

Recently, oxygen free radicals, antioxidant defences and the cellular redox status have been considered as central players in pathogenesis of diabetes. The role of glycaemic control on the pro-oxidant/antioxidant balance deserves special attention. Metabolic disturbances and oxidative stress seem to be closely related, improved glycaemic control being associated with a lowered pro-oxidant status (Wierusz-Wysocka et al., 1995).

It was also evidenced that there is a relationship between oxidative stress and insulin resistance observed in diabetes. Hyperinsulinaemia increases the concentrations of ROS, which, in turn, may be responsible for the impaired intracellular insulin actions. Amongst ROS, hydrogen peroxide has been shown to contribute to insulin receptor signaling, and may play a key role in the modulation of the signalling transduction pathways regulated by insulin through coupled receptors.

Consequently, the inactivation of hydrogen peroxide by catalase could represent a critical step for the removal of intracellular ROS in insulin-producing cells. On the other side, the inhibition of catalase under conditions of insulin resistance could also represent an adaptive response to maintain the homeostasis of intracellular hydrogen peroxide as an intermediate of the insulin-activated physiological processes. Overall, relationships between ROS and diabetes seem extremely complex (Bonnefont-Rousselot, 2002).

A second source of ROS formation is an excessive production of AGEs, especially due to a hyperglycaemia-induced overproduction of methylglyoxal. AGEs are also able to produce oxygenated free radicals via complex biochemical mechanisms. AGEs have been shown to interact with their specific receptors (RAGE) and thus they induce oxidative stress, enhance vascular cell adhesion molecule type 1 (VCAM-1) expression, and increase endothelial adhesiveness for monocytes. This overproduction of AGEs appears to play a key role in the pathogenesis of diabetic complications. In particular, the accumulation of two AGEs biomarkers, namely carboxymethyllysine and pentosidine, has been related to the severity of diabetic nephropathy and the so-called 'carbonyl stress'. The toxic effects of AGEs result from structural and functional alterations in proteins, especially the cross-linking of proteins, and from their interactions with RAGEs leading to the enhanced formation of oxygen free radicals (Miyata et al., 2001; Singh et al., 2001).

### **2.4 Diabetes – a frequent disease or an epidemic?**

In 1993 the World Health Organisation (WHO) Ad Hoc Diabetes Reporting Group published standardized global estimates for the prevalence of diabetes and impaired glucose tolerance in adults, based on data from 75 communities in 32 countries. These estimates provided, for the first time, comparable information on the prevalence of abnormal glucose tolerance from many populations worldwide. However, they did not meet the needs of those who frequently refer to the WHO diabetes program for information on the number of people with diabetes in a particular country/community, nor did they take account of future trends in the burden of diabetes (King & Rewers, 1993). Therefore, a further study has been undertaken that links data from the global database collected by WHO with demographic estimates and projections issued by the United Nations to estimate the number of people with diabetes in all countries of the world at three points in time, i.e., the years 1995, 2000, and 2025. The results of this study suggest that for the world as a whole, between the years 1995 and 2025, the adult population will increase by 72%, prevalence of diabetes in adults will increase by 35%, and the number of people with diabetes will increase by 122% (Fig. 2). For the developed countries, there will be an 11% increase in the adult population, a 27% increase in the prevalence of adult diabetes, and a 42% increase in the number of people with diabetes. For the developing countries, there will be an 82% increase in the adult population, a 48% increase in the prevalence of adult diabetes, and a 170% increase in the number of people with diabetes (King et al., 1998).

Fig. 2. Global projections for the diabetes epidemic in years 2003-2025.

biomarkers, namely carboxymethyllysine and pentosidine, has been related to the severity of diabetic nephropathy and the so-called 'carbonyl stress'. The toxic effects of AGEs result from structural and functional alterations in proteins, especially the cross-linking of proteins, and from their interactions with RAGEs leading to the enhanced formation of

In 1993 the World Health Organisation (WHO) Ad Hoc Diabetes Reporting Group published standardized global estimates for the prevalence of diabetes and impaired glucose tolerance in adults, based on data from 75 communities in 32 countries. These estimates provided, for the first time, comparable information on the prevalence of abnormal glucose tolerance from many populations worldwide. However, they did not meet the needs of those who frequently refer to the WHO diabetes program for information on the number of people with diabetes in a particular country/community, nor did they take account of future trends in the burden of diabetes (King & Rewers, 1993). Therefore, a further study has been undertaken that links data from the global database collected by WHO with demographic estimates and projections issued by the United Nations to estimate the number of people with diabetes in all countries of the world at three points in time, i.e., the years 1995, 2000, and 2025. The results of this study suggest that for the world as a whole, between the years 1995 and 2025, the adult population will increase by 72%, prevalence of diabetes in adults will increase by 35%, and the number of people with diabetes will increase by 122% (Fig. 2). For the developed countries, there will be an 11% increase in the adult population, a 27% increase in the prevalence of adult diabetes, and a 42% increase in the number of people with diabetes. For the developing countries, there will be an 82% increase in the adult population, a 48% increase in the prevalence of adult diabetes, and a 170% increase in the number of people with diabetes (King et al., 1998).

oxygen free radicals (Miyata et al., 2001; Singh et al., 2001).

Fig. 2. Global projections for the diabetes epidemic in years 2003-2025.

**2.4 Diabetes – a frequent disease or an epidemic?** 

It is well known that diabetes is one of the most costly and burdensome chronic diseases of our time and is a condition that is increasing in epidemic proportions throughout the world. The prevalence of abnormal glucose tolerance in any population is of public health concern, since diabetes may increase disability burden and health care utilisation.

The relationship between blood glucose concentration in diabetes and the incidence of disease complications was demonstrated in large epidemiological studies. Accurate metabolic control in diabetes is not always feasible, and therefore the issue of molecular mechanisms underlying the damaging effects of hyperglycaemia on body cells and tissues, as well as the possibilities of their pharmacological inhibition, are of the utmost importance in a diabetological practice and anti-diabetic treatment.

The complications resulting from the disease are a significant cause of morbidity and mortality and are associated with the damage or failure of various organs such as eyes, kidneys, and nerves. Although the treatment of diabetes has become increasingly sophisticated, with over a dozen pharmacological agents available to lower blood glucose, a multitude of ancillary supplies and equipment available, and a clear recognition by health care professionals and patients that diabetes is a serious disease, the normalisation of blood glucose for any appreciable period of time is seldom achieved. In addition, in wellcontrolled so called "intensively" treated patients, serious complications still occur, and the economic and personal burden of diabetes remains (Turner et al., 1999).

Nowadays, diabetes is treated not only as a disease, but as an epidemic. However, as a discipline, diabetes epidemiology is relatively young. The first significant gathering of researches interested in diabetes epidemiology took place just in 1978. Then, in the relatively short span of 2 decades, epidemiology studies have had a profound impact on diabetes research, care and prevention. This explosion of interest and activity in the epidemiology of diabetes should contribute to an effective reduction in the number of patients with this disease.
