INTRODUCTION

Diabetes:

Diabetes is a disease in which the body is unable to regulate blood sugar on its own. And

it does not produce or properly use insulin, which is a hormone that is needed to convert

sugar, starches and other food into energy needed for daily life. Although both genetics

and environmental factors such as obesity and lack of exercise appear to play a role, the

actual cause of diabetes still remains unknown. There are two major types of diabetes,

called type 1 and type 2.

Type 1 diabetes: A chronic disease in which high levels of sugar

(glucose) are found in blood. Type 1 diabetes can occur at any age

but it is most often observed in children, adolescents and young adults.

The insulin hormone which is responsible for producing specialized

cells called beta cells produce little or no insulin and this results in the

formation of type1 diabetes. Without enough insulin, glucose builds up

in the bloodstream instead of entering into the cells. The body is now

unable to use this glucose as a source of energy, and this leads to the

formation of symptoms of type1 diabetes. The exact cause for type1

diabetes is not known but it is considered to be an autoimmune

disorder. In type 1 diabetes, the pancreas undergoes an autoimmune

attack by the body itself, and is rendered incapable of making insulin.

Figure: 1- Schematic representation of onset of type 1 diabetes

Type 2 diabetes: A chronic disease in which high levels of sugar

(glucose) is found in blood. It is considered to be the most common

form of diabetes. During the condition of type 2 diabetes the

components like fat, liver and muscle cells do not respond properly to

the insulin. This phenomenon is called “insulin resistance”, due to

which the blood sugar cannot enter into the cells. When sugar cannot

enter the cells they start building up in the blood at high amounts. This

phenomenon of building up of sugar in high amounts in the blood

stream is called “hyperglycemia”.

Type 2 diabetes usually occurs slowly over time. Most people with the

disease are overweight when they are diagnosed. Type2 diabetes is

most seen in elderly people. Family history and genes play a large role

in type 2 diabetes. Low activity level, poor diet, and excess body

weight around the waist increase your risk.

Figure: 2- Schematic representation of onset of type 1 diabetes

Causes of diabetes:

Insufficient production of insulin (either absolutely or relative to the

body's needs), production of defective insulin (which is uncommon), or

the inability of cells to use insulin properly and efficiently leads to

hyperglycemia and diabetes. This latter condition affects mostly the

cells of muscle and fat tissues, and results in a condition known as

"insulin resistance." This is the primary problem in type 2 diabetes. The

absolute lack of insulin, usually secondary to a destructive process

affecting the insulin producing beta cells in the pancreas, is the main

disorder in type 1 diabetes. For essentially, if someone is resistant to

insulin, the body can, to some degree, increase production of insulin

and overcome the level of resistance. After time, if production

decreases and insulin cannot be released as vigorously, hyperglycemia

develops.

Insulin is a hormone that is produced by specialized cells (beta cells) of

the pancreas. (The pancreas is a deep-seated organ in the abdomen

located behind the stomach.) In addition to helping glucose enter the

cells, insulin is also important in tightly regulating the level of glucose

in the blood. After a meal, the blood glucose level rises. In response to

the increased glucose level, the pancreas normally releases more

insulin into the bloodstream to help glucose enter the cells and lower

blood glucose levels after a meal. When the blood glucose levels are

lowered, the insulin release from the pancreas is turned down. It is

important to note that even in the fasting state there is a low steady

release of insulin than fluctuates a bit and helps to maintain a steady

blood sugar level during fasting. In normal individuals, such a

regulatory system helps to keep blood glucose levels in a tightly

controlled range.

Risk Factors for Type 2 Diabetes:

Type 2 diabetes occurs when the body can't use the insulin that's

produced, a condition called insulin resistance. Though it typically

starts in adulthood, type 2 diabetes can begin anytime in life. Because

of the current epidemic of obesity among U.S. children, type 2 diabetes

is increasingly found in teenagers. Here are the risk factors for

developing type 2 diabetes.

Obesity or being overweight. Diabetes has long been linked to

obesity and being overweight. Research at the Harvard School of

Public Health showed that the single best predictor of type 2 diabetes

is being obese or overweight.

Impaired glucose tolerance or impaired fasting

glucose. Prediabetes is a milder form of diabetes that's sometimes

called impaired glucose tolerance. It can be diagnosed with a simple

blood test. Prediabetes is a major risk factor for developing type 2

diabetes.

Insulin resistance. Type 2 diabetes often starts with cells that are

resistant to insulin. That means they are unable to take in insulin as it

moves glucose from the blood into cells. With insulin resistance, the

pancreas has to work overly hard to produce enough insulin so cells

can get the energy they need. This involves a complex process that

eventually leads to type 2 diabetes.

Ethnic background. Diabetes occurs more often in Hispanic/Latino

Americans, African-Americans, Native Americans, Asian-Americans,

Pacific Islanders, and Alaska natives.

High blood pressure . Hypertension, or high blood pressure, is a

major risk factor for diabetes. High blood pressure is generally defined

as 140/90 mm Hg or higher. Low levels of HDL "good" cholesterol and

high triglyceride levels also put you at risk.

History of gestational diabetes. If you developed diabetes while

you werepregnant, you've had what is called gestational diabetes.

Having had gestational diabetes puts you at higher risk of developing

type 2 diabetes later in life.

Sedentary lifestyle. Being inactive -- exercising fewer than three

times a week -- makes you more likely to develop diabetes.

Family history. Having a family history of diabetes -- a parent or

sibling who's been diagnosed with this condition -- increases your risk

of developing type 2 diabetes.

Polycystic ovary syndrome. Women with polycystic ovary syndrome

(PCOS) are at higher risk of type 2 diabetes.

Oxidative stress in type 2 diabetes

Hyperglycemia underlies the development of diabetic complications

possibly due to an increase in oxidative stress. Oxidative stress is

defined as the imbalance of oxidants and antioxidants in the favor of

oxidants. This imbalance reflects either a loss of the protective

antioxidant network or the increased production of free radicals.

Oxidative stress has been strongly associated with tissue damage in

diabetic individuals. Mechanisms by which hyperglycemia can induce

oxidative stress include enhanced glycoxidation, increased

carbohydrate flux through the polyol pathway, formation of AGEs,

increased glucose flux through the hexosamine pathway, activation of

DAG-activated protein kinase C and inflammation. The unifying event

in these mechanisms is the production of free radicals, more

specifically ROS and reactive nitrogen species (Sarah Akbar et al.,

2011).

Reactive Oxygen Species (ROS)

ROS include a number of chemically reactive molecules derived from oxygen [26–29].

Some of those molecules are extremely reactive, such as the hydroxyl radical, while some

are less reactive (superoxide and hydrogen peroxide). Intracellular free radicals, i.e., free,

low molecular weight molecules with an unpaired electron, are often ROS and vice versa

and the two terms are therefore commonly used as equivalents. Free radicals and ROS

can readily react with most biomolecules, starting a chain reaction of free radical

formation. In order to stop this chain reaction, a newly formed radical must either react

with another free radical, eliminating the unpaired electrons, or react with a free radical

scavenger (a chain-breaking or primary antioxidant). In Table 1, the most common

intracellular forms of ROS are listed together with their main cellular sources of

production and the relevant enzymatic antioxidant systems scavenging these ROS

molecules. The step-wise reduction of molecular oxygen via 1-electron transfers,

producing and also connecting the ROS molecules listed in Table 1, can be summarized

as follows:

Table: Reactive oxygen species sources and their products

ROS molecule Main sources Enzymatic defense Product(s)

systems

Superoxide (O2•¯)

Leakage of electrons from

the electron transport chain

Activated phagocytesXanthine oxidase

Flavoenzymes

Superoxide dismutase

(SOD)

Superoxide reductase

(in some bacteria)

H2O2 + O2

H2O2

Hydrogen

peroxide

(H2O2)

From O2•¯ via superoxide dismutase (SOD)NADPH-oxidase

(neutrophils)

Glucose oxidaseXanthine oxidase

Glutathione

peroxidase

Catalases

Peroxiredoxins (Prx)

H2O +

GSSG

H2O + O2

H2O

Hydroxyl

radical (•OH)

From O2•¯and H2O2 via transition metals (Fe or

Cu)Nitric oxide

(NO)Nitric oxide

synthases

Glutathione/TrxRGSNO

Antioxidants and antioxidant-related enzymes

Defense mechanisms against free radical-induced oxidative damage include the

following: (I) catalytic removal of free radicals and reactive species by factors such as

catalase (CAT), superoxide dismutase (SOD), peroxidase, and thiol-specific antioxidants;

(II) binding of proteins (e.g., transferrin, metallothionein, haptoglobins, caeroplasmin) to

pro-oxidant metal ions, such as iron and copper; (III) protection against macromolecular

damage by proteins such as stress or heat shock proteins; and (iv) reduction of free

radicals by electron donors, such as GSH, vitamin E (tocopherol), vitamin C (ascorbic

acid), bilirubin, and uric acid (Fig. 1).

Animal catalase is heme-containing enzymes that convert hydrogen peroxide (H2O2) to

water and O2, and they are largely localized in subcellular organelles such as

peroxisomes. Mitochondria and the endoplasmic reticulum contain little CAT. Thus,

intracellular H2O2 cannot be eliminated unless it diffuses to the peroxisomes [2].

Glutathione peroxidases (GSH-Px) remove H2O2 by coupling its reduction with the

oxidation of GSH. GSH-Px can also reduce other peroxides, such as fatty acid

hydroperoxides. These enzymes are present in the cytoplasm at millimolar concentrations

and also present in the mitochondrial matrix. Most animal tissues contain both CAT and

GSH-Px activity. SODs are metal-containing proteins that catalyze the removal of

superoxide, generating water peroxide as a final product of the dismutation. Three

isoforms have been identified, and they all are present in all eukaryotic cells. The copper-

zinc SOD isoform is present in the cytoplasm, nucleus, and plasma. On the other hand,

the manganese SOD isoform is primarily located in mitochondria. Dietary micronutrients

also contribute to the antioxidant defense system. These include ɣ -carotene, vitamin C,

and vitamin E (the vitamin E family comprises both tocopherols and tocotrienols, with _-

tocopherol being the predominant and most active form). Water-soluble molecules, such

as vitamin C, are potent radical scavenging agents in the aqueous phase of the cytoplasm,

whereas lipid soluble forms, such as vitamin E and ɣ -carotene, act as antioxidants within

lipid environments. Selenium, copper, zinc, and manganese are also important elements,

since they act as cofactors for antioxidant enzymes. Selenium is considered particularly

important in protecting the lipid environment against oxidative injury, as it serves as a

cofactor for GSH-Px.

The most abundant cellular antioxidant is the tripeptide, GSH(L- ɣ -

glutamyl -L-cysteinyl glycine). GSH is synthesized in two steps. First, _-

glutamylcysteine synthetase (ɣ-GCS) forms a ɣ-peptide bond between

glutamic acid and cysteine, and then GSH synthetase adds glycine.

GSH prevents the oxidation of protein thiol groups, either directly by

reacting with reactive species or indirectly through glutathione

transferases.

Catalase

Catalase was first noticed in 1811 when Louis Jacques Thénard, who discovered H2O2

(hydrogen peroxide), suggested its breakdown is caused by an unknown substance. In

1900, Oscar Loew was the first to give it the name catalase, and found it in many plants

and animals. Catalase gene located on the short (p) arm of chromosome 11 at position 13.

More precisely, the CAT gene is located from base pair 34,460,471 to base pair

34,493,606 on chromosome 11.

Figure:------Schematic Representation of Catalase gene located on chromosome 11: base pairs 34,460,471 to 34,493,606.

It is a ubiquitously occurring enzyme that catalyses the decomposition

of H2O2 to water and oxygen. The enzyme has one of the highest

turnover rates, converting millions of H2O2 molecules per single

catalase molecule each second. The enzyme is a tetramer with

polypeptide chains that are more than 500 amino acids long. Catalase

is usually determined in the serum.

 

2H2O2  2H2O + O2

Structure of Catalase Enzyme Catalase was first noticed in 1811 when Louis Jacques Thénard, who discovered H2O2

(hydrogen peroxide), suggested its breakdown is caused by an unknown substance. In

1900, Oscar Loew was the first to give it the name catalase, and found it in many plants

and animals (Loew, 1900). In 1937 catalase from beef liver was crystallised by James B.

Sumner and Alexander Dounce (Sumner & Dounceand, 1937) the molecular weight was

worked out in 1938 ((Sumner & Dounceand, 1938) . In 1969, the amino acid sequence of

bovine catalase was worked out (Schroeder et al., 1969) then in 1981, the three-

dimensional structure of the protein was revealed.

Figure:------Schematic Representation of 3D Catalase Structure

 Oxidative stress in diabetes mellitus

Many studies have shown that increased lipid peroxides or oxidative stress are present in

diabetic subjects. Oxidative stress can be increased before clinical signs of diabetic

complications. However, the role of oxidative stress in the initiation and progression of

diabetes remains uncertain. It is debatable whether oxidative stress precedes the

appearance of diabetic complications or whether it merely reflects the presence of

complications or consequence of complications. In diabetes, oxidative stress seems

caused by both increased Production of ROS, sharp reduction in antioxidant defenses and

altered cellular redox status. Hyperglycemia may lead to an increased generation of free

radicals via multiple mechanisms. Patients with diabetes may be especially prone to acute

and chronic oxidative stress which enhances the development of late diabetic

complications. Although the source of this oxidative stress remains unclear, it has been

suggested that the chronic hyperglycemia in diabetes enhances the production of ROS

from glucose autoxidation, proteinglycation and glycoxidation, which leads to tissue

damage (34).

Figure:------Schematic Representation Oxidative stress leads to tissue damage

Catalase and Diabetes:

The metabolic effects of oxidants, which are believed to contribute to many diseases,

may influence the development of some forms of diabetes. As we discuss earlier the

oxidant hydrogen peroxide (H2O2) is a by-product of normal cellular respiration and is

also formed from superoxide anion by the action of superoxide dismutase. H2O2 has been

reported to damage pancreatic β-cells (Murata et al., 1998) and inhibit insulin signaling

(Hausen et al., 1999)).

The enzyme catalase has a predominant role in controlling the concentration of H 2O2, and

consequently, catalase protects pancreatic β-cells from damage by H2O2 (Tiedge et al.,

1998). Low catalase activities, which have been reported in patients with schizophrenia

and atherosclerosis (Góth et al., 1996), are consistent with the hypothesis that long-term

oxidative stress may contribute to the development of a variety of late-onset disorders,

such as type 2 diabetes (Góth et al., 2000).

Loew O (May 1900). "A New Enzyme of General Occurrence in Organisms". Science 11 (279): 701–702.

Sumner JB, Dounce AL (April 1937). "Crystalline catalase". Science 85 (2206): 366–367.

Sumner JB, Gralén N (March 1938). "The molecular weight of crystalline catalase". Science 87 (2256): 284–284.

Schroeder WA, Shelton JR, Shelton JB, Robberson B, Apell G (May 1969). "The amino acid sequence of bovine liver catalase: a preliminary report". Arch. Biochem. Biophys. 131 (2): 653–655.

Murata M, Imada M, Inoue S, Kawanishi S: Metal mediated DNA damage by diabetogenic alloxan in the presence of NADH. Free Radic Biol Med 25:586–595, 1998

Hausen LL, Ikeda Y, Olsen GS, Busch AK, Mosthaf L: Insulin signaling is inhibited by micromolar concentrations of H2O2: evidence for a role of H2O2 in tumor necrosis factor alpha-mediated insulin resistance. J Biol Chem 274:25078–25804, 1999

Tiedge M, Lortz S, Munday R, Lenzen S: Complementary action of anti-oxidant enzymes in the protection of bioengineered insulin-producing RINm5F cells against toxicity of reactive oxygen species. Diabetes 47:1578–1585, 1998

Góth L, Vitai M: Hypocatalasemia in hospital patients. Clin Chem 42:341–342, 1996