Biochemical Basis of CVD:Part-1 Role of Free radicals & Antioxidants

What are free radicals? Any molecule containing one or more unpaired electrons

Free Radicals Body uses oxygen in metabolic reactions. Sometimes oxygen reacts with body compounds and forms unstable molecules called free radicals. Free radicals can also be formed by environmental factors: Ultraviolet radiation Air pollution Tobacco smoke. Free Radicals and Disease The body uses oxygen in numerous metabolic reactions. Sometimes oxygen reacts with body compounds and forms very unstable molecules called free radicals. Free radicals can also be formed by environmental factors such as ultraviolet radiation, air pollution, and tobacco smoke.

Free Radicals A free radical has one or more unpaired electrons. An electron without a partner is unstable and very reactive. To gain stability, a free radical attacks another stable but vulnerable compound and steals an electron. A free radical has one or more unpaired electrons. An electron without a partner is highly unstable and very reactive. To gain stability, a free radical attacks another stable but vulnerable compound and steals an electron.

Types of Free Radicals Reactive Oxygen Species (ROS) - Reactive Nitrogen Species (RNS) - NO. Reactive Metabolites or Intermediates - metabolic activation of drugs, toxins, pollutants, cigarette smokes, etc.

Reactive Oxygen Species (ROS) Superoxide (O2.-) Hydrogen Peroxide (H2O2) Hydroxyl Radical (OH.) - product of Fenton reaction catalyzed by free Fe and Cu Singlet Oxygen - oxygen at an excited state, requiring photosensitizers and photons

Sources of oxygen free radicals In mitochondria: - generation of energy - ATP - glucose, fatty acids, amino acids - O2 2H2O 4e- - leakage of O2-. (superoxide) H2O2 (hydrogen peroxide)

In Smooth Endoplasmic Reticulum (microsome) - detoxification (cytochrome P-450s) - toxins, drugs and xenobiotics - O2 + RH R-OH and H2O - leakage of O2-.

In Peroxisomes - containing oxidases for degradation of various substrates - glucose, amino acids, xanthine, etc. - requires O2 - byproduct is H2O2

In Cytoplasm - nitric oxide (NO.) production from Arginine - functions as a biological messenger - in brain, vascular endothelial cells, and macrophages - NO. + O2-. ONOO. (peroxynitrite)

nitric oxide (NO): a Biological Messenger NO is a neurotransmitter (brain- bNOS) NO regulates blood pressure (vascular endothelial cells- eNOS) NO is a cytotoxic agent (macrophages- iNOS)

Production of Singlet Oxygen - photosensitizers in the biological system (bilirubin, riboflavin, retinal, porphyrin) - requires light , O2 and photosensitizers -

Defending Against Free Radicals Together, vitamin and mineral nutrients and phytochemicals with antioxidant activity protect against certain diseases and decrease free radical damage by: Limiting free-radical formation Destroying free radicals or their precursors Stimulating antioxidant enzyme activity Repairing oxidative damage Stimulating repair enzyme activity Together, vitamin and mineral nutrients and phytochemicals with antioxidant activity protect the body against certain diseases and decrease free radical (oxidative) damage by: Limiting free-radical formation Destroying free radicals or their precursors Stimulating antioxidant enzyme activity Repairing oxidative damage Stimulating repair enzyme activity

Antioxidants Prevents the transfer of electron from O2 to organic molecules Stabilizes free radicals Terminates free radical reactions

Free Radical Defense System Antioxidant Enzymes Antioxidant Quenchers Antioxidant from Foods – nutrients/non-nutrients

Antioxidant Enzymes Superoxide Dismutase (SOD) – to get rid of superoxide produced from electron transport chain, the product is hydrogen peroxide. MnSOD (mitochondria). CuZn SOD (cytosol).

Oxygen Radical Defense Enzymes GSH Peroxidase Cu Zn SOD O2•¯ H2O2 H2O + O2 Fe2+ Mn SOD Catalase OH•

Antioxidant Enzymes - 2 Glutathione Peroxidase (GSH PX) – to get rid of hydrogen peroxide (H2O2) and some lipid peroxide. It requires reduced glutathione (GSH) as substrate and produces oxidized glutathione (GSSG) as product. A cytosolic enzyme. GSH is a substrate for two enzymes that are responsible for detoxification and antioxidation.

Antioxidant Enzymes - 3 Catalase –to get rid of hydrogen peroxide produced in peroxisomes.

Antioxidant Quenchers Cellular proteins which chelate pro-oxidant minerals (iron and copper or others) Transferrin – iron transport protein Ferritin – iron storage protein Metallothionein – minerals and heavy metals (Zn/Cu/Cd/Hg) Ceruloplasmin – copper transport and storage

Antioxidants From Food Two antioxidant vitamins the body uses are in defense of free radicals are vitamin E and vitamin C. Vitamin E protects body lipids (cell membranes and lipoproteins) by stopping the free-radical chain reaction.

Free radical hypothesis of atherosclerosis The development of atherosclerosis is a multifactorial process in which both elevated plasma cholesterol levels and proliferation of smooth muscle cells play a central role. Atherogenesis is an alteration of the artery wall that includes two major phases: Adhesion of monocytes to the endothelium and their migration into the sub endothelial space and differentiation into macrophages. These cells ingest (oxidized) low density lipoproteins(LDL) and through this process they are transformed into "foam cell". 2. Vascular smooth muscle cells migration from the media into the intima and their proliferation with the formation of atherosclerotic plaques. free radicals are involved throughout the atherogenic process, beginning from endothelial dysfunction in an otherwise intact vessel wall, up to the rupture of a lipid-rich atherosclerotic plaque, leading to acute myocardial infarction or sudden death.

Modification of LDL LDL Apo B-100 Oxidation: Derivatization: Degradation of B-100 by reactive oxygen species Derivatization: Aldehydes Glucosylation eg. diabetes Derivatized LDL Oxidized LDL

The Scavenger Receptors How macrophages deal with oxidized or modified LDL The scavenger receptor recognizes modified and/or oxidized LDL and internalizes the modified LDL. Accumulation of these modified LDL in the cell leads to the accumulation of cholesterol droplets in the macrophage and the formation of foam cells.

LDL and Atherosclerosis Elevated LDL: Increased residence time in plasma Increased modification/oxidation of LDL Monocyte Endothelial cells Cytokines oxLDL Artery wall oxLDL (stimulates cytokine secretion) Cytokines Macrophage Smooth muscle cell proliferation Macrophage foam cell

HDL Protective Role HDL Monocyte Endothelial cells oxLDL HDL Artery wall UC HDL + UC ABCA1 apoA-I PL UC oxLDL = oxidized LDL UC = unesterified cholesterol Macrophage foam cell Nascent HDL

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