(1) Superoxide Dismutase and Peroxidases

As mentioned before, HO• is chemically very reactive, so the first line of defense of parasitic flatworms is to prevent its production, the controlled reduction of H2O2 to H2O being vital. The classic enzyme that carries out this reaction in aerobic organisms is catalase (CAT), which is absent in the genomes of parasitic flatworms [92,133] but is conserved in their free-living counterparts [134]. Hence, these parasites depend on enzymes glutathione peroxidase (GPx) and peroxiredoxin (Prx) in addition to the superoxide dismutase (SOD) necessary for O2 •− scavenging.

SOD is a family of metalloenzymes specialized in carrying out the dismutation of O2 •−, generating O2 and H2O2. In animals, it is possible to identify two isoforms: (a) cytoplasmic SOD dependent on one atom of copper and another of Zn (Cu/Zn SOD); (b) mitochondrial SOD, dependent on a manganese atom (Mn SOD). Experimentally, it has been possible to isolate the enzyme [71] or clone the gene [135] from some parasitic flatworms, such as *T. solium*; thus, it was not surprising to confirm that, in genomes/transcriptomes, both trematode [125] and cestode [92,136] genes are present in both isoforms. Regarding their expression, their presence has been determined in all stages of the life cycle associated mainly with the tegument of these organisms [31], although a greater expression has been observed in the adult forms with respect to the larval [137].

GPx comprises isoenzymes that carry out the reduction of H2O2 to H2O, requiring electrons from two molecules of the tripeptide glutathione (GSH), taking it to its oxidized disulfide form (GSSG). Although this family has many representatives in mammals [138], in trematodes [125] and cestodes it has only been possible to identify a single gene [92] corresponding to a selenium-dependent GPx. This GPx type turned out to be membrane integral and it is exclusive for the reduction of lipid hydroperoxides (Ph-GPx); thus, it makes sense that it has been located in the tegument [137] of the different stages of trematodes *S. mansoni* [125] and *Fasciola gigantica* [139], and of the cestode *E. granulosus* [140]. Like SOD, this enzyme has a differential expression that is a function of the pO2 to which the parasite is exposed, finding its highest expression in the aerobic life cycle stages and its lowest in the anaerobic [141]. However, the overexpression of this enzyme responds to the exposure of exogenous oxidants, as shown in *C. sinensis* under in vitro conditions [142]. Cai et al., suggesting that Ph-GPx activity could be more focused on egg production than on the maintenance of the redox status [142].

Prxs are homodimeric proteins that catalyze the reduction of H2O2 and alkyl hydroperoxides to water and alcohol, respectively. The electrons that translocate frequently come from the 12 kDa protein thioredoxin (Trx), which is why these enzymes are also called thioredoxin peroxidases (TPx), although some isoforms can also obtain electrons from GSH. Despite the abundance of Prxs, their catalytic efficiency is lower than that of CAT or GPxs by one to three orders of magnitude [143]. However, this family of enzymes seems to be the most important in the H2O2 degradation process, both in trematodes and cestodes. In fact, in the genomes of *E. granulosus*, *E. multilocularis*, *H. microstoma*, and *T. solium*, it has been possible to identify three different genes encoding Prx 1–3 [92], which is consistent with the identification of three Prxs in *S. mansoni* [85]. Wang et al. reported that GPx activity in echinococcal cysts is practically undetectable, suggesting the relevance of Prxs in this parasite [144]. Using western blot analysis, it was found that in the trematode *O. viverrini*, OvTPx-1 is expressed in all stages of development; even if its location is different,

depending on the isoenzyme [127]. For example, in adult flukes of *Schistosoma japonicum* it has been possible to locate Prx-1 in the tegument whereas Prx-2 has been found associated with the parenchyma, vitelline glands, and gastric epithelium [145]. In *H. diminuta*, a peroxidase-like activity was described in 1968 [146].
