**3. Role of oxidative stress in diabetic complications**

protein kinase C (PKC) and subsequent formation of reactive oxygen radicals [11, 12]. Hyperglycemia, not only generates more reactive oxygen species (ROS), but also attenuates

The injurious effects of hyperglycemia are separated into microvascular (involving small vessels such as capillaries) and macrovascular complications (involving large vessels, such as arteries and veins). Microvascular complications include diabetic nephropathy, neuropathy and retinopathy while macrovascular complications include coronary artery disease, periph‐

Diabetic nephropathy is a major cause of end-stage renal disease worldwide. It is a progressive decline in the glomerular filtration rate, characterized by glomerular hyperfiltration, glomer‐ ular and tubular epithelial hypertrophy, increased urinary albumin excretion, increased basement membrane thickness and mesangial expansion with the accumulation of extracel‐ lular matrix proteins (ECM) [14]. Alteration of the permeability characteristics of the glomer‐ ular capillary wall manifests clinically as abnormal albuminuria [15]. Microalbuminuria progresses to end-stage renal disease through a number of stages including normoalbuminu‐

Diabetic retinopathy results from the damage of the small vasculature of the retina, multi cellular and the light sensitive tissue at the back of the eye. It is a major cause of visual impairment worldwide [17, 18]. The retina capillaries are lined with endothelial cells respon‐ sible for maintaining the blood retinal barrier, and are surrounded by smooth muscle cells, pericytes, which provide tone to the vessels [18]. The vascular lesions that are identified at the early stage of diabetic retinopathy include pericytes disappearance from capillaries resulting in pericyte ghosts, obliteration of capillaries and small arterioles, gradual thickening of vascular basement membrane, increased permeability of endothelial cells, and formation of microaneurysms (i.e. weakening of vessel walls that results in the projection of a balloonlike

Neuropathies are characterized by a progressive loss of nerve fiber function. A widely accepted definition of diabetic neuropathy is "the presence of symptoms and/or signs of peripheral nerve dysfunction in people with mellitus after exclusion of other causes" [21]. In the periph‐ eral nervous system, diabetes causes a progressive deterioration of sensory nerves and damage to motor nerves [22]. Diabetic neuropathy is ultimately the leading cause of lower extremity amputation [23]. Peripheral neuropathy is thought to develop because of cellular damage to endothelial cells, affecting nerve blood flow and also damage to the neurons affecting con‐ ductivity of impulses [23]. Signs and symptoms of diabetic neuropathy include decrease or no sweating, numbness, or tingling, and some sort of burning sensation, weakness and loss of

antioxidative mechanisms by scavenging enzymes and substances [13].

**2. The complications of diabetes mellitus (DM)**

eral arterial disease and stroke [5].

26 Antioxidant-Antidiabetic Agents and Human Health

ria, microalbuminuria and macroalbuminuria [16].

sac), vessel leakage, exudate, and hemorrhage [19, 20].

reflexes [24].

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are the terms collectively describing free radicals and other non-radical reactive derivatives also called oxidants. Biological free radicals are highly unstable molecules which are products of normal cellular metabolism. They have electrons available to react with various organic substrates such as lipids, proteins and deoxyribonucleic acid (DNA). Free radicals are well recognized for playing a dual role as both deleterious and beneficial species, since they can be either harmful or beneficial to living systems [34]. At low or moderate levels free radicals (ROS and RNS) exerts beneficial effects such as defence against infectious agents, induction of a mitogenic response and the maturation process of cellular structures [35-37]. ROS include superoxide anion (O2 .-), hydroxyl (. OH), hydrogen peroxide (H2O2) and hypochlorous acid (HOCl) while RNS include nitric oxide (. NO), nitrogen dioxide (NO2 .-) and peroxynitrite (OONO<sup>−</sup> ) [38, 39]. High concentrations of free radicals on the other hand result in deleterious processes that can damage cell structures due to oxidative stress [40, 41].

Free radicals produced under physiological conditions are maintained at steady state levels by endogenous or exogenous antioxidants (externally supplied through foods or supple‐ ments) which act as free radical scavengers. However, oxidative stress occurs when the production of free radicals overwhelms the detoxification capacity of cellular antioxidant system causing biological damage [42-44]. The endogenous antioxidants (Table 1) com‐ prise of the enzymatic antioxidants such as superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione reductase (GR), catalase (CAT), and non-enzymatic antioxi‐ dants including glutathione (GSH), α lipoic acid, vitamins C and E [39, 45, 46]. On the other hand, the exogenous antioxidants include micronutrients and other exogenously adminis‐ tered compounds such as vitamin E, vitamin C, trace metals (selenium, manganese, zinc), carotenoids and flavonoids [39, 44, 47].


**Antioxidants and**

Non-enzymatic antioxidant

Non enzymatic

Protein

**associated complications**

activation [84].

[87].

Lipid **<sup>↑</sup>** F2-Isoprostanes

**Table 2.** Experimental evidence supporting the involvement of oxidative stress

**4.1. Aldose reductase pathway and ROS generation**

**↑** MDA

**↑** Nitrotyrosine

**4. Pathways of free radical generation in diabetes mellitus and its**

In diabetes, ROS is thought to be generated through increased polyol pathway [82], increased formation of advanced-glycation end products (AGEs) [83] and protein kinase C (PKC)

Aldose reductase is the rate limiting enzyme of the polyol pathway. The nicotinamide ade‐ nine dinucleotide phosphate (NAD(P)H)-requiring aldose reductase, catalyses the reduction of glucose to sorbitol followed by the oxidation of sorbitol to fructose by NAD+ dependent sorbitol dehydrogenase. At normal blood glucose concentration (5.5 mM), aldose reductase catalyzed reaction represents less than 3% of total glucose utilization [85]. However, hypergly‐ cemia results in saturation of hexokinase and more than 30% of glucose is directed into the polyol pathway [86]. In a diabetic state, polyol pathway increases in tissues that do not require insulin for cellular glucose uptake, such as retina, kidney, peripheral nerves and blood vessels

**Animals**

**Humans**

**macromolecules Evidence of oxidative stress Target tissue/organ References**

Lipids **↑** TBARS, lipid peroxides, MDA Kidney [69-70] DNA **↑** 8-OHdG, 8-OHG Plasma, Liver, Kidney [71-72]

Reactive oxygen species **↑** ROS Hippocampus [66]

Enzymatic antioxidants **↑** SOD,CAT, GPX Erythrocyte [74]

antioxidants **<sup>↓</sup>** GSH Erythrocyte [75]

DNA **↑** 8-OHdG Urine [78-79]

**↓** Vit E and C Liver, kidney [64]

Oxidative Stress and Diabetic Complications: The Role of Antioxidant Vitamins and Flavonoids

kidney [62-64]

[65] [66] [67] [68]

http://dx.doi.org/10.5772/57282

29

[73] [67]

[76] [77]

Kidney, hippocampus.

Retina, Heart

Retina

Urine Erythrocyte

Protein carbonyl Plasma [80, 81]

Enzymatic antioxidants **<sup>↓</sup>** SOD,CAT,GR, GPX Liver, Pancreas, Liver,

**↓** GSH /GSSG,GSH

Protein **<sup>↑</sup>** Nitrotyrosine kidney

**Table 1.** Role of antioxidants in the protection against free radical damage

Numerous experimental evidences have highlighted a direct link between oxidative stress and diabetes through the measurement of oxidative stress biomarkers in both diabetic patient and rodents. As shown in Table 2, a hyperglycemic state can lead to an increase in the levels of oxidative DNA damage markers such as 8-hydroxy-2'-deoxyguanosine (8-OHdG) and 8 oxo-7, 8-dihydro-2'-deoxyguanosine (8-oxodG); lipid-peroxidation products measured as thiobarbituric acid-reactive substances (TBARS); protein oxidation products such as nitrotyr‐ osine and carbonyl levels and also lower the activity of antioxidant enzymes. Cell culture studies using pancreatic beta cells, aortic smooth muscle cells and endothelial cells have also provided evidence for an increase in ROS production in diabetes [55, 56].

Due to their ability to directly oxidize and damage DNA, proteins, and lipids, free radicals are believed to play a key role in the onset and progression of late-diabetic complications [57]. In the absence of an appropriate condensation by antioxidant defense network, increased oxidative stress leads to activation of stress-sensitive intracellular signaling pathways and the formation of gene products that cause cellular damage and contribute to late diabetic compli‐ cations [58-61].


**Table 2.** Experimental evidence supporting the involvement of oxidative stress

**Antioxidants Cellular location Role Reference**

oxygen

generation of GSSG

lysosomes, mitochondria Conversion of superoxide radical to H2O2 [51]

their active forms

peroxidation

Numerous experimental evidences have highlighted a direct link between oxidative stress and diabetes through the measurement of oxidative stress biomarkers in both diabetic patient and rodents. As shown in Table 2, a hyperglycemic state can lead to an increase in the levels of oxidative DNA damage markers such as 8-hydroxy-2'-deoxyguanosine (8-OHdG) and 8 oxo-7, 8-dihydro-2'-deoxyguanosine (8-oxodG); lipid-peroxidation products measured as thiobarbituric acid-reactive substances (TBARS); protein oxidation products such as nitrotyr‐ osine and carbonyl levels and also lower the activity of antioxidant enzymes. Cell culture studies using pancreatic beta cells, aortic smooth muscle cells and endothelial cells have also

Due to their ability to directly oxidize and damage DNA, proteins, and lipids, free radicals are believed to play a key role in the onset and progression of late-diabetic complications [57]. In the absence of an appropriate condensation by antioxidant defense network, increased oxidative stress leads to activation of stress-sensitive intracellular signaling pathways and the formation of gene products that cause cellular damage and contribute to late diabetic compli‐

Detoxifies H2O2 and lipid peroxides with simultaneous oxidation of GSH and

Recycles Glutathione disulfide back to

Acts as a cofactor for antioxidant enzymes (GPx, GST), regenerates other antioxidants such as Vitamins C and E to

Directly scavenge singlet oxygen, peroxyl and superoxide radicals , protects against peroxidation of membrane lipids

Acts synergistically with vitamin E to terminate radical inducedlipid

Increases glutathione and vitamin C

levels [54]

glutathione using the cofactor NADPH [50]

[48]

[49]

[52]

[34]

[34, 53]

(A) Catalase Peroxisomes Decomposition of H2O2to water and

and nucleus

and nucleus

and nucleus

cytoplasm

provided evidence for an increase in ROS production in diabetes [55, 56].

**Table 1.** Role of antioxidants in the protection against free radical damage

**Enzymatic Antioxidants**

**Non enzymatic antioxidants**

(B) Glutathione peroxidase Cytoplasm, mitochondria,

28 Antioxidant-Antidiabetic Agents and Human Health

(C) Glutathione reductase Cytoplasm, mitochondria,

(A) GSH Cytoplasm, mitochondria

(B) Vitamin-E Membrane

(C) Vitamin-C Cytosol

cations [58-61].

(D) α-Lipoic acid Cell membrane and

(D) Superoxide dismutase Cytoplasm, nucleus
