Enzymes and dietary antioxidants
What are antioxidants?
Antioxidants protect biological systems from oxidative damage produced by oxygen-containing free radicals and redox transition metal ions such as iron, copper and cadmium1.During the oxidative metabolism of glucose in mitochondria, superoxide anion (O2-) is produced as a by-product of the reduction of coenzyme Q complex III. Superoxide dismutase (product no.S128537) converts superoxide anions into hydrogen peroxide (H2O2), which in turn can be converted into peroxyl radicals (RO2-), hydroxyl radicals (OH-) or hypochlorite (ClO-) ions. The superoxide anion can also react with nitric oxide (NO) to form the highly reactive peroxynitrite anion (ONOO-)2.Under normal conditions, these cellular oxidants are reduced or scavenged by intracellular antioxidants and antioxidant enzymes, the most important of which are glutathione (product no. L274260), thioredoxin, superoxide dismutase, catalase（product no.C163049） and peroxidase（product no.P105528）. Dietary antioxidants such as ascorbic acid (vitamin C), vitamin E, beta-carotene and other carotenoids, as well as selenium, have been identified as important components of the total antioxidant capacity of cells and blood plasma.Carotenoids, lutein and zeaxanthin are important antioxidants for the eye and retina3. Vitamin E is a mixture of tocopherols and tocotrienols, of which alpha-tocopherol (product number:T105540) is the main antioxidant4 and is the main lipid-soluble antioxidant in cells, playing an important role in the protection against membrane lipid peroxidation.Low-density lipoprotein (LDL) carries vitamin E into cells and subsequently prevents peroxidation of LDL by supplying hydrogen to lipid peroxyl radicals5. Polyphenolic compounds, particularly flavonoids, have recently been shown to be powerful antioxidants in cultured cells.Human studies on flavonoids have also shown that their effects can be partly attributed to their antioxidant effects6. Antioxidants can act directly as reducing agents, providing protonic hydrogen to unpaired oxygen electrons, or by stabilising or transferring free radical electrons7. In this process, the reducing agent is oxidised; for example, the cysteine sulfhydryl groups of two glutathione molecules are oxidised to form the intermolecular cysteine of oxidised glutathione (Figure 1).
Figure 1. Structure of oxidized glutathione
Research on dietary antioxidants
Lipoic acid is an endogenous antioxidant that has recently gained interest as a dietary supplement because not only does it scavenge free radicals, but its dihydrolipoate form is also a very effective reducing agent.Lipoic acid reduces the oxidized forms of other antioxidants and ultimately maintains the concentration of reduced glutathione in tissues8.Some antioxidants capture or scavenge free radicals and become free radicals themselves in the process. When the carotenoids astaxanthin (Figure 2), luteolin and zeaxanthin scavenge oxygen radicals, the charge of unpaired electrons becomes dispersed throughout the polyene chain of the molecul9.The flavonol quercetin is oxidised to quinone, which can react with thiols10.
Figure 2. Structure of astaxanthin
Flavonoids and carotenoids are the main dietary antioxidants and are ubiquitous in fruits and vegetables.Epidemiological studies have shown that people with diets high in these phytochemicals also have a lower incidence of chronic diseases associated with oxidative stress, such as atherosclerosis, diabetes, neurodegenerative diseases and cancer8. Polyphenols are treated as xenobiotics by the body and are rapidly metabolised and conjugated in the intestinal mucosa and liver. With the exception of gallic acid conjugates of catechins (product no. C114051), such as epigallocatechin gallate (product no. E107404)13, most flavonoids in blood are glucuronides, sulphate esters or O-methylated conjugates4,14.
Latest Research Developments
Moskaug et al. hypothesized that flavonoids enhance the antioxidant defence system of cells by inducing chronic low levels of oxidative stress in cells (the hormone principle)
15. In contrast, Halliwell et al. concluded that unabsorbed micromolar concentrations of flavonoids and other phenolic compounds remain in the small intestine and colon and that their antioxidant, metal chelating and other effects are protective against the development of colorectal cancer. Recent studies have confirmed that polyphenols have many intracellular effects in addition to scavenging free radicals and chelating metal ions.Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are also potential signalling molecules. They have been studied through a class of redox-sensitive transcription factors that regulate gene expression, including Nrf2, which induces the expression of antioxidant/detoxification enzymes, and NFkB and AP-1, which induce the production of inflammatory cytokines, cell adhesion molecules, acute phase proteins and anti-apoptosis1,2,16. These transcription factors are activated in response to oxidative stress; persistently elevated levels of reactive oxygen species (ROS) activate NFkB by inducing phosphorylation and segregation of its inhibitory subunit IkB.Silymarin (product no. S408123) and silymarin (product no. S109809)17, catechins and procyanidins (product no. P413229),1,11 and other flavonoids16 have been reported to block the activation of NFkB.Forman et al. cite evidence that hydrogen peroxide and superoxide anions exhibit second messenger properties by mediating redox signalling reactions. They hypothesised that peroxides reversibly interact with key cysteine sulfates present at the active site of signalling proteins 18. Protein tyrosine phosphatases and thioredoxins are known to have active site cysteines in the form of sulfates, while transcription factors AP-1 and NFκB and some cysteaspartic enzymes have redox-sensitive cysteines that may be in the form of sulfates.Zinc-bound cysteines in the regulatory site of some protein kinase c (product no. P343699) isoforms may also be oxidised by hydrogen peroxide. Peroxisome production by receptor stimulation also leads to the activation of all mitogen-activated protein kinase pathways (ERK, JNK and p38 MAPK). Other signalling proteins and enzymes are targets for nitric oxide, peroxides or both oxidants (Table 1). Thus, antioxidant polyphenols that scavenge reactive oxygen intermediates may have profound effects on the intracellular signaling pathways that mediate the cellular response to oxidative stress.Signaling Protein | Modulator Oxidant |
PTP1B | NO, H₂O₂ |
SHP-2 | H₂O₂ |
LMW-PTP | NO, H₂O₂ |
PTEN | H₂O₂ |
Trx | NO, H₂O₂ |
Src | H₂O₂ |
Ras | NO, H₂O₂ |
GSTp/JNK | H₂O₂ |
Gi/Go | H₂O₂ |
NMDA | NO |
Table1 . Additional targets for redox signallin
References
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