**7. Genetic variability of metabolic enzymes**

hydrogen peroxide (H2O2), hydroxyl radical (OH˙) and nitric oxide (NO) [46, 48, 56, 57]. Asbestos may stimulate the production of ROS in two different ways. The first mechanism involves redox-active iron (Fe2+, Fe3+) in asbestos that catalyses the formation of OH˙, whereas the second mechanism involves the production of ROS by alveolar macrophages during the phagocytosis of asbestos fibres [58–60]. Reactive oxygen species in lungs may lead to the

It has also been suggested that asbestos fibres may upregulate the activity of inducible nitric oxide synthase (iNOS), thus inducing the production of NO by alveolar macrophages and pulmonary epithelial cells [51, 61–64]. Because NO is a free radical, it reacts readily with other

−

most importantly peroxynitrite [65–69]. Nitric oxide may play a role in the initiation and progression of asbestosis [51, 64, 70, 71]. However, the data presented by Dörger et al. [72] indicate that iNOS-derived NO plays a dual role in acute asbestos-induced lung injury and that although iNOS deficiency resulted in an exacerbated inflammatory response, it improved

Reactive oxygen species and RNS can damage all types of biomolecule, including lipids, proteins and deoxyribonucleic acid (DNA). Complex defence mechanisms, including enzymes, proteins and antioxidants, are involved in the prevention of cell damage [73, 74].

**6. Enzymes involved in the detoxification of reactive oxygen and nitric**

−

Zn in the catalytic centre and is located in the extracellular space [74, 82, 83].

Human tissues contain specific enzyme systems to detoxify ROS and RNS. Superoxide dismutases (SODs) and catalase (CAT) together with glutathione peroxidases represent an important line of the primary antioxidant enzyme defence system against ROS. Superoxide

quently catalyses the conversion of H2O2 to water (H2O) and O2 [48, 75–82]. Three distinct SOD isoenzymes have been identified in mammals: a cytosolic copper-zinc SOD (CuZnSOD or SOD1) localised in cytoplasmic compartment with cooper (Cu) and zinc (Zn) in the catalytic centre, manganese SOD (MnSOD or SOD2) that is localised in mitochondria and uses manganese (Mn) as a cofactor and extracellular SOD (ECSOD or SOD3) that also contains Cu and

Another important family of enzymes involved in the detoxification of xenobiotics and electrophiles produced by ROS and RNS is glutathione S-transferases (GSTs) [84–87]. They catalyse the conjugation of reduced glutathione to different electrophiles [88]. These conjugation reactions mostly result in less reactive products [89]; however, in some cases, the products are more reactive and consequently more harmful than the parent compound [90, 91]. Seven classes of cytosolic GST isoenzymes have been recognised in mammals (Alpha, Mu, Pi, Sigma, Theta, Omega, Zeta) [84–86, 91, 92]. The major GST enzyme in the human lung is GSTP1, which belongs to the Pi class [90, 91, 93], while GSTM1 (Mu class) and GSTT1 (Theta class) were most

), leading to the formation of toxic metabolites,

to H2O2 and oxygen (O2), whereas CAT subse-

production of cytotoxic and potentially genotoxic electrophilic compounds [46].

reactive oxygen metabolites (as, for instance, O2

oxidant-promoted lung tissue damage.

dismutases catalyse the dismutation of O2

frequently investigated [90, 91].

**species**

162 Occupational Health

Genetic polymorphisms are the most common cause for genetic variability of detoxification and antioxidative enzymes [15–17, 80, 91, 94–99].

The most common functional single nucleotide polymorphism (SNP) of the *MnSOD* gene is C to T substitution (c.201C>T, rs4880), which results in alanine (Ala) to valine (Val) amino acid change at position −9 of the mitochondrial targeting sequence (MnSOD p.Ala-9Val) [96, 97, 100]. It has been suggested that this SNP alters the secondary structure of the protein and hence may affect the efficiency of transport of the MnSOD into the mitochondria, where it would be biologically available [96, 97].

ECSOD is secreted into extracellular space where it binds lung matrix components and inhibits their fragmentation in response to oxidative stress [101, 102]. In the *ECSOD* gene, a C to G substitution (c.896C>G, rs1799895) leads to amino acid change from arginine (Arg) to glycine (Gly) at position 213 (p.Arg213Gly) [89, 100, 103–105]. This polymorphism causes an 8- to 15 fold increase in the concentration of plasma ECSOD levels due to impaired binding to the extracellular matrix [103, 104].

The most common functional SNP of the catalase gene (*CAT*) consists of a C to T substitution at position −262 in the promoter region (*CAT* c.−262C>T) and has a substantial impact on the basal expression as well as the CAT levels in red blood cell [80]. The findings of later studies indicated lower CAT activity in subjects with the −262TT genotype than those with the CT and CC genotypes [106–111].

Regarding GSTs, the most common polymorphism of the *GSTM1* and *GSTT1* genes in most of the populations is null polymorphism due to homozygous deletion (null genotype) of these genes, which result in the absence of the GSTM1 and GSTT1 enzyme activity [17, 91]. *GSTM1*-null genotype has been associated with an increased risk of asbestosis in some studies [16, 86], while this association has not been proved in the others [15, 17]. No association has been found between *GSTT1* deletion polymorphism and asbestosis in the studies published so far [17, 86]. As for the *GSTP1* gene, two common single nucleotide polymorphisms in the coding sequence were reported to result in amino acid substitution that may lead to reduced conjugating activity of the enzyme [91, 98, 112, 113]. The first polymorphism is characterised by adenine (A) to guanine (G) transition of nucleotide 313 in exon 5 (c.A313G), which causes an isoleucine (Ile) to valine (Val) substitution at position 105 of the GSTP1 enzyme (p.Ile105Val), resulting in three possible genotypes: 105 Ile/Ile, 105 Ile/Val or 105 Val/Val. The second polymorphism involves the cytosine (C) to thymine (T) transition at nucleotide 341 in exon 6 (c.C341T), which results in alanine (Ala) to Val substitution at position 114 of the GSTP1 enzyme (p.Ala114Val). Regarding codon 114, three genotypes are also possible: 114 Ala/Ala, 114 Ala/Val or 114 Val/Val [91, 98]. Based on the presence of the polymorphisms in both codons 105 and 114, *GSTP1* genotypes can be combined into groups with a presumed high, intermediate or low conjugation capacity of the enzyme.

The human *iNOS* gene is also known to be polymorphic. Several types of polymorphisms have been identified in the promoter region of the *iNOS* gene [99, 114]. The CCTTT pentanucleotide tandem repeat polymorphisms have been associated with the transcriptional promoter activity, which has been shown to increase with the CCTTT repeat number. Based on that, alleles with 11 or fewer CCTTT repeats are usually defined as short alleles (S) and the ones with 12 or more repeats as long alleles (L). Accordingly, the subjects can have SS, SL or LL genotype [115].
