The scientific evidence accumulated during the last two decades indicates that, beyond the initial promises of delaying aging, antioxidants when consumed in the form of food have an important potential to reduce the development of those diseases that currently affect the most world population (cardiovascular, tumor and neurodegenerative diseases). As a result of this recognition, antioxidants have increasingly been considered by the population as " Molecules whose consumption is synonymous with health ".
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In this section, fundamental aspects related to the definition, classification, mechanisms of action and main biological actions promoted by antioxidants are addressed. In addition, concepts related to the role of free radicals and oxidative stress in health and human pathology are reviewed. These last aspects are treated additionally, in extenso , in the section " Antioxidants and health: scientific evidence ".
What is an antioxidant?
An antioxidant can be defined, in the broadest sense of the word, as any molecule capable of preventing or slowing the oxidation (loss of one or more electrons) of other molecules, usually biological substrates such as lipids, proteins or nucleic acids. The oxidation of such substrates can be initiated by two types of reactive species: free radicals, and those species which, without being free radicals, are sufficiently reactive to induce the oxidation of substrates such as those mentioned.
But what is a free radical?
From a chemical point of view, a free radical is any species (atom, molecule or ion) that contains at least one unpaired electron in its outermost orbital, and that is in turn capable of existing independently (hence the free term).
The atoms order their electrons (ê) in regions called "atomic orbitals", in the form of pairs of electrons. The latter gives the atom stability, or low chemical reactivity towards its environment.
However, under certain circumstances, said orbital may lose its parity, either by yielding or capturing an electron (ê). When this occurs, the resulting orbital exhibits an unpaired ê, converting the atom in a free radical.
The presence of an "unpaired" electron in the outermost orbital of an atom gives the latter an increased ability to react with other atoms and / or molecules present in its environment, normally, lipids, proteins and nucleic acids. The interaction between free radicals and said substrates gives rise to alterations in the structural, and eventually functional, properties of the latter.
The reactive species derived from oxygen ( ROS ) is a collective term, widely used, which includes all those reactive species that, whether or not free radicals, focus their reactivity on an oxygen atom. However, often under the name ROS, other chemical species whose reactivity is centered or derived in atoms other than oxygen are included. Strictly speaking, however, species whose reactivity derives or focuses on atoms such as nitrogen or chlorine should be referred to as RNS (Reactive Nitrogen Species) and RCS (Reactive Chlorine species), respectively.
See table that describes the main free radicals and reactive species derived from oxygen and nitrogen.
Main free radicals and reactive species derived from oxygen and nitrogen normally generated in our body.
Is the generation of reactive oxygen species in the body a normal process? The endogenous generation of ROS (free radicals and other reactive pro-oxidant species) is a normal part of the metabolism of all aerobic living beings. Indeed, under physiological conditions, most of the tissues of the human body generate significant amounts of ROS. Among the most generated ROS, the free radical superoxide anion ( O2 • - ) stands out. The generation of said radical takes place, at cellular level, mainly through the electron transport chain in the mitochondria (specifically, during the interaction between oxygen molecules and complexes I and III).
Although the chain of electron transport constitutes a series of biochemical reactions designed to generate, between the matrix and in the inter-membrane space of the mitochondria, a gradient (of protons) necessary for the re-synthesis of ATP from ADP , during the course of its operation, between 1% and up to 3% of the oxygen that regularly enters the mitochondria is converted into superoxide radicals (that is, oxygen gains an electron). Fortunately, the high presence of SOD in the mitochondria allows the dismutation of the superoxide radicals, oxygen and hydrogen peroxide. Since the accumulation of peroxide, either in the mitochondria or outside it, would be toxic to the cell, a greater part of the peroxide formed is rapidly reduced to water inside the mitochondria, by the action of the enzyme glutathione peroxidase. The hydrogen peroxide that does not reach to be reduced, leaves the mitochondria to be subsequently reduced by other peroxidases in the cytoplasm, and inside the peroxisomes by the action of the catalase.
The physiological formation of O2 • - is not limited to its mitochondrial production. Indeed, this species can also be generated in the cytosol of many cells through the action of enzymes such as xanthine oxidase (XO), glucose oxidase and amino-oxidases; at the level of endoplasmic reticulum O2 radicals - are also generated through the action of certain cytochromes P-450, and at the level of the plasma membrane by the action of the enzyme NADPH-oxidase (NOX). Although this last enzyme is present in abundance in neutrophils, such cells need to be activated as a condition to initiate the massive production of O2 • - .
Besides superoxide, what other reactive species is normally generated in the organism?
Another reactive species normally generated by the organism is nitric oxide ( NO • ). This free radical, which results from the action of the cytosolic enzyme nitric oxide synthase (NOS), is generated continuously, though not exclusively, by vascular-endothelial cells (those that "cover the inside of a blood vessel). Together with O2 • - , NO • constitutes an example of reactive species whose generation and action is not only physiological, but absolutely fundamental for the proper functioning of the organism. As discussed in the section "Antioxidants and health: scientific evidence", the controlled production of O2 • - and NO • is not only physiological, but is also fundamental to ensure the health of the human organism.
How can antioxidants be classified? The protection of the biological substrates promoted by most of the antioxidants involves their direct interaction with reactive species such as those referred to in Table I. However, it is also possible to distinguish other mechanisms through which antioxidants actively contribute to preventing or preventing retard the oxidation of a biological substrate. In order to review these mechanisms, it is necessary to previously classify those antioxidants that are normally present in the human body.
While there are different ways to classify antioxidants, from a perspective of their origin and presence in the body, it is possible to distinguish between those antioxidants that are normally bio-synthesized by the body, and those that enter it through the diet . Among the first are:
- i) enzymatic antioxidants, such as superoxide dismutase, catalase, glutathione peroxidase, glutathione S-transferases, thioredoxin-reductases and sulfoxy-methionine reductases, and
- ii) non-enzymatic antioxidants, such as glutathione, uric acid, dihydro-lipoic acid, metallothionein, ubiquinol (or Co-enzyme Q) and melatonin. Although i) and ii) are primarily bio-synthesized by the human body, the diet can also contain said antioxidants. However, it should be clarified that the contribution that could be made to the organism ingestion of (foods with) said antioxidants is not very significant because they undergo significant degradation / biotransformation throughout the gastro-intestinal tract.
Regarding the antioxidants that enter the body only through the diet, these are classified, essentially, in:
- i) vitamins-antioxidants, such as ascorbic acid, alpha-tocopherol and beta-carotene (or pro-vitamin A),
- ii) carotenoids (such as lutein, zeaxanthin and lycopene),
- iii) polyphenols, in their categories of flavonoids and non-flavonoids, and
- iv) compounds that do not fall into the three previous categories, such as some glucosinolates (eg isothiocyanates) and certain organo-sulfur compounds (eg, diallyl-disulfide).
What are the main mechanisms of action of antioxidants?
Antioxidants can prevent or slow the oxidation of a biological substrate, and in some cases reverse the oxidative damage of the affected molecules.
Direct interaction with reactive species : The most known mechanism, although not necessarily the most relevant to action, refers to the ability of many antioxidants to act as "stabilizers or quenchers of various reactive species." This last supposes the known activity "scavenger" of free radicals that have many antioxidant molecules. In the case of free radicals, such action implies their stabilization through the transfer of an electron to said reactive species. Such a mechanism, defined as "SET" (single electron transfer), allows the free radical to lose its condition by "matching" its unpaired electron. One consequence for the antioxidant is that, as a result of yielding an electron, it becomes a free radical and ends up oxidizing under a form that is low or zero reactivity towards its surroundings (Figure II). Along with the SET mechanism, many antioxidants can stabilize free radicals through a mechanism that involves the direct transfer of a hydrogen atom (this is an electron with its proton). Such mechanism is defined as "HAT" (hydrogen atom transfer). In the latter case, the free radical is also electronically stabilized.
Antioxidants whose action is promoted through SET and / or HAT mechanisms are mostly non-enzymatic antioxidants, whether these are normally bio-synthesized by the human organism or enter the body through the diet. Most of the antioxidants that act through these mechanisms present in their chemical structure, as functional groups, phenolic hydroxyl (for example, all polyphenols and tocopherols). However, other antioxidants, non-phenolic, such as glutathione, melatonin, and ascorbic, dihydro-lipoic and uric acids, are also examples of molecules whose action is promoted by SET and / or HAT mechanisms.
Together with the SET and HAT mechanisms, certain antioxidants can also act by stabilizing reactive species through a mechanism that implies "the direct addition of the radical to its structure". An example of this type of antioxidant action is that promoted by carotenes such as beta-carotene. As a result of such reaction, the free radical (eg peroxyl) loses its condition, and the carotene is covalently modified, becoming a free radical that through successive reactions is oxidized and converted into epoxide and carbonyls derivatives of markedly lower reactivity.
As expected, the direct interaction between an antioxidant and a reactive species will prevent either the initiation and / or the propagation of oxidative processes that affect the biological substrates.
Prevention of enzymatic formation of reactive species : Some antioxidants can act by preventing the formation of ROS and RNS. They do this by inhibiting the expression, synthesis or activity of pro-oxidant enzymes involved in the generation of reactive species, such as NADPH-oxidase (NOX), xanthine oxidase (XO), myeloperoxidase (MPO) and Nitric oxide synthase (NOS). This type of antioxidant action does not demand that an antioxidant exhibit in its structure characteristics that are typically associated with the mechanisms of action ET or HAT. Examples of inhibitors of the activity of these enzymes are, for compounds coming from the diet, certain polyphenols capable of inhibiting NOX, MPO and XO, and some agents used in the therapy of gout, such as allopurinol, and febuxostat that inhibit the xanthine oxidase.
Prevention of the formation of reactive species dependent on metals : A second mechanism that also involves the inhibition of the formation of reactive species is related to counterpose the capacity of certain transition metals, such as iron and copper (both in their reduced state), to catalyze (redox activity) the formation of superoxide radicals from the reduction of oxygen and hydroxyl radicals, from hydrogen peroxide (Fenton reaction). Those molecules that have the ability to bind such metals, forming complexes or chelates, manage to inhibit the redox activity of these, preventing the formation of the aforementioned reactive species. Included in this group of antioxidants are: i) certain peptides and proteins normally bio-synthesized by the body and whose physiological function involves transporting, storing and / or excreting iron (such as ferritin) or copper (such as metallothionein and ceruloplasmin), ii) certain polyphenols that access the organism through the diet and whose distinctive feature is to present in its flavonoid structure a catechol group in the B ring, and iii) some agents that are used in metal removal therapy such as desferroxamina that traps iron, and penicillamine or tetrathiomolybdate that trap copper.
Activation or induction of the activity of antioxidant enzymes : As part of the antioxidant defense, the human organism bio-synthesizes certain enzymes whose function is to remove reactive species, mainly ROS. Among these are the following: superoxide dismutase (SOD, in its Cu / Zn and Mn-dependent isoforms) that reduces superoxide radicals to hydrogen peroxide, catalase (CAT, iron-dependent) that reduces hydrogen peroxide to water, glutathione peroxidase (GSpx; dependent) that reduces lipo-hydroperoxides to their corresponding alcohols, glutathione-S-transferase (GST) in its peroxidase type that acts by reducing organic peroxides, glutathione reductase that reduces oxidized glutathione (GSSG) to reduced (GSH), and sulfoxy-methionine- reductase that regenerates methionine from its sulfoxy-oxidized metabolite (Table II).
The antioxidant action of all these enzymes results in a decrease in the cellular redox state. Among the enzymes mentioned, two cases merit an additional comment. The first, the SOD is distinguished because although its action removes a free radical (superoxide), as a product of its action forms a species that is also reactive, hydrogen peroxide. The latter highlights the importance of other enzymes capable of removing hydrogen peroxide, such as CAT and GSpx. The second enzyme that warrants comment is glutathione reductase because its antioxidant action is double because it catalyzes not only the removal of a ROS but also, as a result, results in the formation of GSH, an important cellular antioxidant.
There is evidence that certain compounds present in the human diet could induce the expression of genes that code for the synthesis of some of the antioxidant enzymes such as those described in Table II. Examples of such compounds are some polyphenols present in fruits and vegetables, various isothiocyanates (such as sulforaphane) present in cruciferous (broccoli, cauliflower) and some curcuminoids (such as curcumin) of turmeric. These compounds are, more often, known as inducers of bio-transforming enzymes of the phase II type, ie those enzymes that conjugate electrophilic xenobiotics.
Can food be a good source of antioxidant enzymes? Although food does not constitute an effective contribution of antioxidant enzymes, since after ingestion these are degraded during the digestion process, some if they can contribute to their optimal functioning by providing those microminerals that are required for the biosynthesis of such enzymes. It is necessary to clarify, however, that a greater dietary contribution of microminerals such as Cu, Zn, Mn, Fe, or Se, could suppose an increase in the activity of antioxidant enzymes when the organism exhibits a deficit condition in such microminerals. In the absence of such a deficiency or lack, it is not expected that their higher intake (or supplementation) will be translated per se in an increase in their activity.
Oxidative stress : Under certain conditions, the speed with which reactive species (ROS and RNS) are generated in the organism exceeds the speed with which these species are removed by the antioxidant defense mechanisms (that is, those that are proper to them, plus those that they are contributed by the diet). To the imbalance or redox imbalance that takes place we call it oxidative stress. The latter may arise as a result of; i) an exacerbated production of reactive species, even in the presence of a balanced dietary supply of antioxidants, ii) a decreased intake of foods rich in antioxidants, even in the absence of an increased production of reactive species, iii) a reduced biosynthesis of some of the endogenous antioxidant mechanisms (whether enzymatic or non-enzymatic), even in the presence of a balanced dietary supply of antioxidants and in the absence of an increased production of reactive species.
What is the consequence of oxidative stress for the organism?
When oxidative stress affects biological substrates, the redox imbalance that characterizes such stress translates into a oxidative damage to various macromolecules. When oxidative damage is intense, sustained over time, and can not be reversed or repaired, it will lead to the appearance of those pathologies that are currently associated with oxidative stress. Figure III shows some of the main pathologies in which oxidative stress is involved, either as a determining factor or as an aggravating condition of the damage and loss of functions that characterize such diseases.