Detoxification enzymes : cellular metabolism and susceptibility to various diseases

Detoxification enzymes act in the cellular metabolism of substances that are strange to the organism (xenobiotic) and endogenous compounds that could cause cellular and tissue damage, such as compounds derived from the oxidative stress. Cellular metabolism occurs in three successive phases: in phase 1, potentially toxic compounds are activated, especially through the action of P-oxidases cytochrome (CYP) enzymes.1 In phase 2, the compounds that were activated in the previous phase are conjugated to substances that will make them more water soluble and easily excreted. In this phase, glutathione S-transferase (GSTs), UDP-glucuronosyltransferases (UGT), Sulfotransferases (SULT) and Nacetyl-transferase (NAT) enzymes are the main players.2 In phase 3, conjugates compounds are eliminated extracellularly through membrane transporters, which are proteins with ion and molecule transport function through the cellular membrane.3 Figure 1 shows a scheme of the cellular metabolism by the action of detoxification enzymes. FIGURE 1 Scheme of cellular metabolism by the action of detoxification enzymes. Author: Marcelo S. M. Silva

Detoxification enzymes act in the cellular metabolism of substances that are strange to the organism (xenobiotic) and endogenous compounds that could cause cellular and tissue damage, such as compounds derived from the oxidative stress.Cellular metabolism occurs in three successive phases: in phase 1, potentially toxic compounds are activated, especially through the action of P-oxidases cytochrome (CYP) enzymes. 1 In phase 2, the compounds that were activated in the previous phase are conjugated to substances that will make them more water soluble and easily excreted.In this phase, glutathione S-transferase (GSTs), UDP-glucuronosyltransferases (UGT), Sulfotransferases (SULT) and N-acetyl-transferase (NAT) enzymes are the main players. 2 In phase 3, conjugates compounds are eliminated extracellularly through membrane transporters, which are proteins with ion and molecule transport function through the cellular membrane. 3Figure 1 shows a scheme of the cellular metabolism by the action of detoxification enzymes.

DETOXIFICATION GENES AND GENETIC POLYMORPHISMS
Detoxification enzymes are proteins produced from the expression of detoxification genes.By default, the gene receives the same acronym of the protein coded by it, however it is written in italic.Polymorphisms in detoxification genes may affect the function of enzymes coded by them, and conse-quently, the cellular metabolism.Genetic polymorphisms are alterations in the DNA sequence that can increase the organism susceptibility to develop certain diseases.Polymorphisms characterized by the complete deletion of a gene eliminate any functional activity of the enzyme coded by it.On the other hand, polymorphisms that are duplications of a whole gene may result in higher levels of enzymatic activity. 4There are also replacements of amino acids that originate from the single-nucleotide polymorphisms, better known by the acronym SNPs.If the polymorphism is located within the coding region of a gene, there may be the replacement of amino acids in the coded peptide, resulting in protein alterations that may affect the cellular metabolism.An example of that is a single-nucleotide polymorphism that occurs in the apolipoprotein E gene and that is associated with a higher risk for Alzheimer's disease. 5

POLYMORPHISMS STUDY METHODS
Genetic polymorphisms can be studied through several methods, such as PCR (Polymerase Chain Reaction) DNA amplification, followed by agarose or polyacrylamide gel electrophoresis; DNA sequencing; PCR amplification associated with the use of restriction enzymes (this technique is especially used for SNPs investigation), among other methods.In Figure 2 some methods used for the study of genetic polymorphisms are schematized.

ENVIRONMENTAL FACTORS AND DETOXIFICATION ENZYMES
Environmental factors are biological or non-biological factors found in the environment that affect living organisms.Among the biological factors are the microorganisms, parasites, food consumed, among others.With relation to the non-biological factors (also known as abiotic), there are air and water industrial pollutants, solar radiation, environment temperature, air humidity, etc.In this context, it is important to consider air pollution and its consequence to the populations' health.Both pollution produced by industrial activities and the burning of fuel by transport vehicles, among others, release a variety of toxic compounds in the air.Respiratory diseases, allergies, increased cases of several types of cancer and cardiovascular diseases are among the main health issues that may be caused by the combustion of petroleum products.Among the compounds generated are the nitrogen oxides, particulate matter and ozone. 6he influence of environmental factors in the aetiology of several diseases is well known, especially through epidemiologic studies that count on expres- The close relation between the presence of certain environmental factors and the development of malignant tumours highlights the importance of detoxification enzymes that act to minimize potential damages to the organism.In addition to malignant neoplasia, many other pathologies have been associated to the influence of environmental factors, such as inflammatory bowel disease, respiratory diseases, food poisoning, pesticide poisoning and occupational illnesses.An example of labour disease caused by environmental factor is the green tobacco sickness, which is an acute nicotine poisoning caused by the farmers skin contact with tobacco leaves.9 Figure 3 shows some environmental factors that affect human health.The association between detoxification enzymes alterations and the development of some diseases has been investigated in scientific papers.A case-control study carried out in the city of Goiânia with 100 individuals with glaucoma and 52 healthy controls found 84% of a single-nucleotide polymorphism in gene CYP1A1 (P-oxidase cytochrome A 1) in the glaucoma patients and 46% in healthy individuals.10 The absence of GSTT1 (glutathione S-transferase theta 1) expression.has been associated in scientific literature with the risk of bladder, 11 oral cavity 12 and lung cancers.13 A case-control study with 98 participants carried out in Southern Brasil showed the association between the GSTM1 (glutathione S-transferase mi 1) deletion and the occurrence of breast cancer.14 A meta-analysis of 23 studies, including 4,065 cases and 5,390 controls, suggested that the GSTT1 null genotype is associated with the risk of lung cancer in Asian population.15 Another meta-analysis including 26,666 cases and 37,210 controls of several ethnicities in different continents suggested that GSTT1 null genotype is significantly associated with an increased risk of urinary system cancer.16 An Indian case-control study with 540 participants reported that the GSTM1 deletion is a decisive risk factor for lung cancer in Northern India population.17 A meta-analysis with 16 studies, including 3,366 patients and 6,013 controls showed that GSTT1 null genotype is significantly associated (p < 0.001) with an increased susceptibility to inflammatory bowel disease.18 A case-control study carried out in South Africa with 202 participants (102 cases and 100 controls) reported association between polymorphisms in genes GSTM1 and GSTP1 and coronary artery disease in the studied population.19

IMPLICATIONS IN THE DEVELOPMENT OF CANCER AND CHEMORESISTANCE
For the organism, chemotherapy drugs are xenobiotics and, therefore, must be metabolized and eliminated from the cell.Scientific evidence has established the importance, especially of GSTs, in the patient's response with cancer to the use of chemotherapy.These enzymes are able to metabolize and inactivate anti-cancer agents. 21GSTP (glutathione S-transferase pi) class is the most abundantly expressed in human cancer and many solid tumours 22 and leukaemia; the high expression of GSTP1 has been associated with chemoresistance, treatment failure and low survival rate of patients. 21

CONCLUSION
The relevance of detoxification enzymes in processes connected to the metabolism of toxic substances has been confirmed through scientific investigation.The functioning of these enzymes can influence susceptibility to many diseases, as well as resistance to drugs.This knowledge highlights the importance of seeking better understanding, through different types of studies (case-control, meta-analysis, biochemical studies, etc.) of the interaction between environmental factors, detoxification enzymes and health condition of individuals and populations.

FIGURE 1
FIGURE 1 Scheme of cellular metabolism by the action of detoxification enzymes.Author: Marcelo S. M. Silva

FIGURE 2
FIGURE 2 Methods of studying genetic polymorphisms.Author: Marcelo S. M. Silva

FIGURE 3
FIGURE 3 Environmental factors that influence human health.Author: Marcelo S. M. Silva