**4. Oxidative alterations and the pathophysiology of diabetes and its complications**

Many studies suggest that patients with diabetes present alterations in the levels of reactive oxygen species (ROS), a type of free radical whose electron is centered in the oxygen atoms [34]. This fact is justified by the toxic character of the persistent excess of glucose in the organism that ends up promoting glycation of proteins, hyperosmolarity and increase in the levels of sorbitol inside the cells [35].

Glucose is a vital source of energy for cells, and their serum levels are controlled by various organs such as intestine, liver, pancreas, skeletal muscle, adipose tissue and kidneys [36]. This regulation is facilitated by the action of hormones (glucagon and insulin), central and peripheral nervous system, as well as metabolic requirements of the body [37].

DM is defined as a heterogeneous group of metabolic disorders characterized by unusual hyperglycemia resulting from defects in insulin production and/or action [1]. In this situation, to revert the toxicity of excess glucose, this component undergoes auto-oxidation and, as consequence, ROS are generated (**Figure 4**) [37].

During auto-oxidation, excess glucose binds (protein glycation) [37] to lysine and valine residues in tissue and plasma proteins. This interaction results in the formation of Schiff's base, a labile or unstable compound that spontaneously transforms into ketoamine (glycated hemoglobin) through the *Amadori* rearrangement [35].

These oxidation and rearrangement processes, followed by further dehydration and fragmentation of *Amadori* product, promote the formation of advanced glycation end products (AGEs) (**Figure 4**) and generate other compounds with chemically active carbonyl groups. These compounds favor the oxidative stress that affects β cells of the pancreas, responsible for synthesizing and secreting insulin [8, 38].

Accumulated AGEs bind to membrane receptors on endothelial cells and promote the onset of tissue damage and the activation of the proinflammatory pathway that involves the NF*κ*B transcription factor responsible for regulating the expression of other inflammatory cytokines (**Figure 4**) [37].

amino group of the cysteine residue is then acetylated by the intracellular N-acetyltransferase enzymes and thereby forms the mercapturic acid which, depending on its characteristics, is rapidly led to circulation, bile, urine or metabolized until it is eliminated (**Figure 3**) [22].

**Figure 3.** Schematic representation of main glutathiones S-Transferases correlated with oxidative stress in different biological conditions. A, normal intracellular environment; B, oxidative stress in an individual without polymorphism;

Once free, glutamate and glycine are reabsorbed by the cell and used in the regeneration of GSH through the catalytic cycle. In this stage of regeneration, three groups of enzymes are important: glutathione oxidase (GO) and glutathione peroxidase (GSH-Px), which catalyze the oxidation of GSH to GSSG, and the enzyme glutathione reductase (GR) that is responsible

for the regeneration of GSH, from GSSG, in the presence of NADPH [33].

C, oxidative stress in an individual with oxidative stress.

52 Glutathione in Health and Disease

Moreover, the chronicity of this hostile environment causes the deactivation of the nitric oxide vasodilator (NO) formed by the endothelial cells [38]. This compromises the relaxation of vascular smooth muscle cells and has a degenerative effect on the vessels causing tissue death [34] and favoring the development of microvascular complications of diabetes, such as diabetic nephropathy (DN) (**Figure 4**).

decrease the uptake of myoinositol and inhibit ATPase Na<sup>+</sup>

of a diabetic individual is the picture of oxidative stress [31].

(val), which results in a lower activity of this isoform [56].

**microvascular diabetic complications**

cular complication [46].

ROS, worsening renal disease [51].

group [55, 56].

affecting nerve function (**Figure 4**). The accumulation of sorbitol associated with reduced hypoxia and blood flow in the nervous tissue favors the development of diabetic neuropathy [37]. This hyperglycemia may also alter gene and protein expression, endothelial cell permeability, and depletion of antioxidant molecules, including Glutathione S-transferases (GSTs), which play an important role in the cellular detoxification process [37, 41, 43–45].

Diabetic nephropathy (DN) is the main secondary complication of diabetes. Associated with an increased risk for cardiovascular disease and high mortality rates, DN is the leading kidney disease worldwide. Approximately 40% of diabetic patients are affected by this microvas-

The mechanisms related to the development of DN are unclear and probably involve a number of dynamic events occurring early and with the progression of diabetes. It is known that the clinical characterization of this pathology is preceded by an established morphological renal lesion that results in imbalance of normal renal homeostasis [47]. These lesions are triggered by functional and metabolic changes. A common metabolic manifestation in the body

There are several factors that are involved in generating oxidative stress during diabetes. There is strong evidence that hyperglycemia results in the activation of PKC in diabetic glomeruli and, as a consequence, mesangial expansion, glomerular basement membrane thickening, endothelial cell dysfunction leading to diabetic renal disease, inflammation, apoptosis [48–50]. Diabetic renal disease, on the other hand, intensifies the formation and activation of

Considering that, in situations of oxidative stress, GSTs play an important role in cellular detoxification, studies of polymorphisms in the genes encoding these enzymes have been gaining prominence and arousing curiosity about a possible association with the susceptibility of this complication [52–54]. In this context, the deletions of *GSTM1* and *GSTT1* together with the *GSTP1* Ile105Val polymorphism are among the most studied isoforms in the GSTs

It is described that individuals with *GSTM1* deletion polymorphisms are unable to produce the GSTM1 protein. On the other hand, the conversion of adenine to guanine at position 313 at codon 105 in the *GSTP1* gene causes the amino acid isoleucine (Ile) to be replaced with valine

In the last decade, some investigations have made DM associations and their complications with the genetic polymorphism in GSTs. Notably, the expression of the GST gene has been described in different variations among ethnic populations. Studies with Egyptian children and adolescents, for example, show that the null genotype of *GSTT1* conferred a 4.2-fold

**5. Impact of genetic polymorphism on GSTs for patients with** 

/K<sup>+</sup>

Impact of Oxidative Changes and Possible Effects of Genetics Polymorphisms of Glutathione…

from the membrane, thus

55

http://dx.doi.org/10.5772/intechopen.76222

**Figure 4.** Main complications of *Diabetes mellitus.* NADH, nicotinamide and adenine dinucleotide; ROS, oxygen-reactive species; AGE, advanced glycation end product; DN, diabetic neuropathy.

ROS, generated by hyperglycemia, also interfere with other biochemical pathways [39]. The Krebs cycle, which, due to oxidative stress, favors the increase of the number of proton donors in the mitochondria, the main source of free radicals [37, 40]. This generates an even greater accumulation of free radicals, mainly superoxide (O2 − ) and hydroxyl compounds (OH− ) [41]. This mitochondrial production is the primary cause of long-term complications of diabetes.

The cascade signaling also suffers from oxidative stress in that it affects the activation of protein kinase C (PKC) [37], a serine/threonine kinase pathway that forms part of the mitogenic protein kinase (MAPK) [42] and plays an important role in several intracellular processes such as signal transduction, response to specific hormonal, neuronal and growth factor stimuli [28, 40].

Furthermore, hyperglycemia increases the NADH/NAD<sup>+</sup> ratio and decreases the NADPH/ NADP<sup>+</sup> ratio (**Figure 4**). The substrates of this alteration are directed to the polyol pathway, which, at normal glucose concentrations, is not active [38]. In excess, in the polyol pathway, glucose is reduced to sorbitol, an osmotically active compound [37]. These disorders result in changes in redox homeostasis and in a variety of known effects for pathogenesis and progression of diabetes.

The accumulation of sorbitol in the ocular tissue, for example, contributes to the development of diabetic cataracts (**Figure 4**). In nerve tissue, high concentrations of this component decrease the uptake of myoinositol and inhibit ATPase Na<sup>+</sup> /K<sup>+</sup> from the membrane, thus affecting nerve function (**Figure 4**). The accumulation of sorbitol associated with reduced hypoxia and blood flow in the nervous tissue favors the development of diabetic neuropathy [37]. This hyperglycemia may also alter gene and protein expression, endothelial cell permeability, and depletion of antioxidant molecules, including Glutathione S-transferases (GSTs), which play an important role in the cellular detoxification process [37, 41, 43–45].
