Antioxidants & Pro-oxidants
Substances that inhibit oxidative changes in molecules. Many oxidative changes are destructive and this applies as much to the human body as to non-biological chemistry. But oxidation is also a need to get energy. Without energy, there is no living. This implies we do need controlled oxidation and when it becomes a threat to wellbeing it should be constrained. This is the role of anti-oxidant: constrain the negative effects of oxidation.
Radicals in Biology
In 1956, Denham Harman published his seminal Free Radical Theory of Aging. In 1972, Harman identified the mitochondria as the primary source of cellular ROS generation. Much research focused on mitochondrial ROS (mtROS mitochondrial Reactive Oxidative Species), in particular, the possibility of mtROS “leakage” to other cellular compartments, and the effects of oxidative damage to sensitive mitochondrial DNA (mtDNA). Subsequent research has shown that mtROS “leakage” is much less than originally thought. “The physiological level of ROS emission from mitochondria is minimal. Experiments have shown that oxidase overexpression lowering mtOS (mitochondrial Oxidative Stress) and mtDNA mutations, does not increase lifespan. Furthermore, increased mutations in mtDNA (500 fold) produce no signs of accelerated aging.
Given the recognized destructive potential of ROS and the ability of antioxidants to neutralize them, why has antioxidant supplementation failed to produce consistent, positive outcomes, in many cases, causing harm, even increasing oxidative stress?
ROS, RNS, and RSS
The oxygen molecule (O2) is relatively stable. Many oxygen-containing compounds on the other hand, such as peroxides and superoxide’s, are highly reactive free radicals, collectively called “reactive oxygen species” or ROS. ROS are often byproducts of cellular energy production. Many, like superoxide, are produced by the body using enzymes for goal related purposes. Free radicals containing nitrogen are referred to as “reactive nitrogen species” or RNS. RNS result from the reaction of nitric oxide and superoxide to produce peroxynitrite, and related compounds. Both ROS and RNS are highly reactive and can damage proteins, lipids, and DNA. RNS-induced damage is sometimes referred to as “nitrosative stress, to distinguish it from “oxidative stress.”
Due to their destructive potential, superoxide and RNS are produced by the body as a defense to foreign pathogens. Superoxide production is controlled by a regulated network of enzymes. Sulfur-containing radicals are referred to as “RSS” (reactive Sulphur species). They result from the reaction of thiols with ROS. Both RNS and RSS result from reactions involving ROS.
Reactive oxygen species (ROS) some returning questions answered.
(1) Are all free-radicals are positively charged?
positively charged free radicals in biological systems are not common. The fact is that most biological free radicals are negative or neutral. They follow the octet rule.
(2) Are all antioxidants negatively charged?
There are a few, but most of them are neutral. Generally, antioxidants have conjugated pi systems, which allow electron delocalization and different resonance structures. This allows the antioxidant to donate an electron to a radical without becoming overly reactive itself.
Prooxidant refers to any endobiotic or xenobiotic that induces oxidative stress either by the generation of ROS or by inhibiting antioxidant systems. It can include all reactive, free radical containing molecules in cells or tissues. Prooxidants may be classified into several categories.
Some antioxidant flavonoids have acted as prooxidant when a transition metal is available. The antioxidant activities and the copper-initiated prooxidant activities of these flavonoids depend on their structures. The OH substitution is necessary for the antioxidant activity of a flavonoid. Flavone and flavanone, which have no OH substitutions and which provide the basic chemical structures for the flavonoids, show neither antioxidant activities nor copper-initiated prooxidant activities. The copper initiated prooxidant activity of a flavonoid also depends on the number of free OH substitutions on its structure. The more the OH substitutions, the stronger the prooxidant activity. O-Methylation and probably also other O-modifications of the flavonoid OH substitutions inactivate both the antioxidant and the prooxidant activities of the flavonoids.
Sample: The antioxidant activity of quercetin (a yellow crystalline pigment C15H10O7 occurring usually in the form of glycosides in various plants ) has been found to be better than its monoglucosides in a test system wherein lipid peroxidation was facilitated by aqueous oxygen radicals. Luteolin (a yellow coloring substance, C15H10O6, obtained from the weed Reseda luteola: used in dyeing silk and, formerly, in medicine. ) has proved to be a significantly stronger antioxidant than its two glycosides.
Flavonoids generally occur in foods as O-glycosides with sugars bound at the C3 position. Methylation or glycosidic modification of the OH substitutions leads to inactivation of transition metal-initiated prooxidant activity of a flavonoid.
The protection provided by fruits and vegetables against diseases, including cancer and cardiovascular diseases, has been attributed to the various antioxidants, including flavonoids, contained in these foods. Flavonoids, such as quercetin and kaempferol, induce nuclear DNA damage and lipid peroxidation in the presence of transition metals.
To counteract the harmful effects taking place in the cell, the physical system has evolved itself with some strategies like prevention of damage, repair mechanism to alleviate the oxidative damages, protection mechanism against damage and the antioxidant defense mechanisms.
Based on the oxidative stress-related free radical theory (Denham Harman), the antioxidants are the first line of action against the effects of the stress. Endogenous antioxidant defenses include a network of antioxidant enzymic and nonenzymic molecules that are distributed within the cytoplasm and various cell organelles. In eukaryotic organisms, several ubiquitous primary antioxidant enzymes, such as SOD, catalase, and several peroxidases catalyze a complex cascade of reactions to convert ROS to more stable molecules, such as water and O2. Besides the primary antioxidant enzymes, a large number of secondary enzymes act in close association with small molecular-weight antioxidants to form redox cycles that provide necessary cofactors for primary antioxidant enzyme functions.
Small molecular-weight non-enzymic antioxidants (e.g., GSH, NADPH, thioredoxin, vitamins E and C, and trace metals, such as selenium) function as direct scavengers of ROS. These enzymatic and non-enzymatic antioxidant systems are necessary for sustaining life by maintaining a delicate intracellular redox balance and minimizing undesirable cellular damage caused by ROS. Endogenous and exogenous antioxidants include some high molecular weight (SOD, GPx, Catalase, albumin, transferring, metallothionein) and some low molecular weight substances (uric acid, ascorbic acid, lipoic acid, glutathione, ubiquinol, tocopherol/vitamin E, flavonoids).
Natural food-derived components have received attention in the last two decades, and several biological activities showing promising anti-inflammatory, antioxidant, and anti-apoptotic-modulatory potential have been identified. Flavonoids comprise a large heterogeneous group of benzopyran derivatives present in fruits, vegetables, and herbs. They are secondary plant metabolites and more than 4000 molecular species have been described. Flavonoids exert a positive health effect, owing to their free radical-scavenging activities. One flavonoid present in a large number of fruits and vegetables is quercetin (3,5,7,3′,4′, pentahydroxyflavone) which prevents oxidative injury and cell death by scavenging free radicals, donating hydrogen compound, quenching singlet oxygen, and preventing lipid peroxidation or chelating metal ions. Red wines have a high content of phenolic substances including catechin and resveratrol, which are responsible for the antioxidant action, anti-inflammatory, antiatherogenic property, oestrogenic growth-promoting effect, and immunomodulation.
SCIENCE OF ANTIOXIDANTS