Phase II Detoxification: A Scientific Review
We are constantly exposed to external, chemical stressors. These potential toxins, including pollutants, radiation, chemotherapy and smoke inhalation, along with our own internal chemical mediators of prolonged inflammatory response, damage cellular DNA. This is the necessary precursor for the development of Cancer and is also implicated in chronic illness including Cardiovascular disease, Neurodegeneration, Diabetes, Obesity and Ageing.
Detoxification is crucial in order to remove these potential toxins from the body and optimise long-term health. Our cells have evolved a powerful defence mechanism whereby the enhanced expression of their genes increases the removal of these toxins. Mounting evidence from studies on population groups and the incidence of disease, so called epidemiology, shows associations between consumption of brassicas such as watercress, with a reduced risk of certain cancers (References 95, 96) including lung, prostate and colorectal cancer (CRC) (References 97, 98)
This review examines current research to understand the process by which Glucosinolate and other antioxidants in brassicas, enhance removal of toxins, specifically through their role in Phase II of this detoxification and therefore have potential to influence the risk of disease.
Detoxification – Phase I and II
The process by which toxins are removed by the body occurs mostly in the liver, but also in the cells lining the digestive tract, kidney and lung and is comprised of two phases of biotransformation:
Phase I involves cytochrome enzymes (P450) which begin the process of chemically transforming fat soluble toxins by oxidisation. This forms an intermediate, Reactive Oxidative Species (ROS), potentially more harmful than the original toxin, which must be further transformed rapidly, by antioxidant action in Phase II.
Phase II enzymes catalyse the addition of compounds onto the reactive intermediate from Phase I, making them more water soluble and therefore more rapidly excreted in bile or urine. Glucosinolate antioxidants in brassicas, have been found to enhance this enzyme activity by inducing gene transcription of phase II enzymes (Reference 99) and therefore reducing the levels of carcinogens and other toxins, available to damage and mutate DNA.
The Role of Glucosinolate antioxidants
Isothiocyanates (ITCs) are a type of glucosinolate antioxidant in Brassicas. Specifically, studies have shown Phenethylisothiocyanate (PEITC), of which watercress contains the highest levels, to be pivotal in Phase II detoxification. (Reference 99) Furthermore, in individuals who smoke, and who lack one specific genotype for the Phase II enzyme Glutathione S-Transferase, (GST), research has shown an enhanced effect of PEITC in altering carcinogen metabolism, suggesting the importance of diet in cancer outcomes. (Reference 100)
PEITCs - Mechanism of Action in Phase II Detoxification
PEITCs promote Phase II detoxification by enhancing our ability to produce phase II enzymes. (Reference 101) They do this by altering the expression of gene transcription. This is known as Epigenetics – heritable changes in gene function, without altering a cell’s DNA sequence, caused by external factors. (Alterations occur either by increased methylation of the DNA or decreased acetylation of associated histones).
A gene is a small section of DNA within a chromosome located in the nucleus of cells. Genes have ‘promotor regions’ which specific proteins (transcription factors) can bind to, thereby controlling the information which is transcribed from the DNA to its messenger RNA, resulting in the cellular manufacture of proteins required to perform specific functions.
In phase II detoxification, the binding of transcription factor Nrf2 onto a specific gene promotor region called ARE (Antioxidant Response Element), enables the production of numerous phase II enzymes, including GSTs, by gene transcription. PEITC has been shown to have a cytoprotective role by activating Nrf2 (Reference 102). Additionally, in-vivo studies have demonstrated an increase in toxicity of environmental pollutants, medicines, allergens and carcinogens in Nrf2 deficient animals and an associated poorer response to plant chemicals such as PEITC (Reference 99)
It is imperative to note however, that over-activation of Nrf2 activity in existing cancerous cells can lead to resistance to radio- and chemotherapies presumably due to increased drug detoxification, and therefore is associated with a poorer prognosis. This paradox must be considered when promoting the use of nutrients in health promotion. (Reference 103) Activation of Nrf2 in other chronic diseases related to DNA damage, such as neurodegenerative diseases, diabetes, and cardiovascular disease is however, purely protective. (Reference 104)
Future roles of Watercress in health, related to Phase II Detoxification
Watercress, being a rich source of PEITC, clearly has an important potential role in reducing cancer risk and health outcomes of other chronic disease states, due to its ‘naturopathic’ influence on phase II detoxification. In determining future clinical recommendations which benefit from the effects of food-derived antioxidants such as PEITCs, several factors should be considered:
· Plant Nutrient Content: Glucosinolates in brassicas vary according to plant health, soil quality and plant microbial content.
· Influence of the microbiome: Although hydrolysis and therefore activation of Glucosinolates to ITCs by the enzyme myrosinase (in different cells within the same plant) occurs on chewing, giving the characteristic bitter taste, variation exists between individuals on this initial conversion of glucosinolates due to differences in the microbiome, required in addition to myrosinase, for activation. This highlights differences between individuals in their ability to mount effective Phase II detoxification, regardless of the PEITC plant source (Reference 105)
· Bioavailability of PEITC: 50% of PEITC available in watercress is absorbed. (Reference 106) ITCs are more ‘available for absorption by the body’ from the raw vegetable form – smoothies for example, than from cooked versions, such as soup. Additionally, watercress extract has been shown to have more powerful anti-carcinogenic activity than PEITC alone (Reference 107)
· Bifunctional Mediators: In-vitro studies, i.e. using cells in a laboratory situation, have shown the function of PEITC to be dependent on its concentration; higher levels being associated with the killing of tumour cells (apoptosis) in prostate adenocarcinomas and CRC (Reference 108). These same higher levels are also cytotoxic to healthy cells. Caution will therefore be required in determining recommended intake levels for optimal health benefits. (Reference 102); a variety of smaller amounts of beneficial brassicas possibly being more beneficial by maintaining the balance of nutrients required to optimise phase II detoxification over cytotoxicity. This has been demonstrated in-vivo, with an enhanced effect from an intake combination of watercress, garden cress and broccoli (Reference 109)
· A role for Vitamin C in Sport Performance: In 2012, Fogarty et al demonstrated an association between watercress intake and reduced post-exercise cell damage (Reference 110). Further research (Reference 111) has demonstrated reduced biomarkers of cell damage in teenage boys undergoing strenuous activity, after Vitamin C supplementation, presumably associated with increased Phase II detoxification. However, Powers et al 2011 (Reference 112), reported that excessive intake of antioxidant supplements may impede the body’s own production of antioxidants (which tends to be enhanced in sports performance) A more natural form of antioxidant supplementation, such as Watercress, being rich in Vitamin C and PEITC may therefore be advantageous.
· Generational Influence: Epigenetic modifications are heritable. The mechanisms by which the detoxification role of PEITC can be preserved across generations are unclear. However, the knowledge that changes we make to our diet can ultimately reduce cell damage and that this is a heritable change, is an exciting focus for future research.
· Caution with research so far: It’s important to acknowledge that the majority of the evidence presented is from in-vitro studies using laboratory cell lines or in-vivo using animals. Therefore, translating these findings into the clinical setting currently requires caution.
In summary, we have an efficient detoxification process by our liver, kidneys, gut and lungs in ensuring the impacts of potential toxins that we come into contact with on a regular basis, are kept to a minimum. Mounting evidence is aiding our understanding of the mechansims by which antioxidants in our diet, are influential in this process. Watercress is a rich source of PEITCs and therefore a valuable brassica in optimising our ability to resist DNA damage and related chronic disease. However, the importance of including watercress in the diet as part of a mixed intake of beneficial brassicas cannot be stressed enough, in order to ensure an optimal intake of a variety of antioxidants and therefore a balance between beneficial detoxification and undesirable cytotoxic effects and therapeutic resistance.