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    • It is difficult however to assure that observed alterations

    2021-10-15

    It is difficult, however, to assure that observed alterations of pro-oxidant/antioxidant levels within the studied organisms are related directly to heavy metal pollution, rather than to other accompanying environmental stressors. Environmental factors like oxygen partial pressure, temperature, salinity and the presence of organic contaminants may also alter the antioxidant-pro-oxidant status of organisms (Winston and Di Giulio, 1991, Lesser, 1997, Abele et al., 1998a, Abele et al., 1998b, Regoli, 2000, Vega and Pizarro, 2000, Rout and Shaw, 2001, Guerriero et al., 2002, Livingstone, 2003). In this study changes in temperature and oxygen levels between seasons across sites were unrelated with those observed for biochemical responses, whereas salinity levels (reported in conductivity values), enzymatic activities (CAT, GST) and lipid peroxidation levels increased substantially between upper and downstream reaches, thus suggesting that salinity may also be a causal agent of the observed effects. Reported studies conducted in plants and fish Remoxipride hydrochloride sale have shown that salinity induced antioxidant enzyme activities, depleted antioxidant compounds and caused oxidative damage (Rout and Shaw, 2001, Guerriero et al., 2002, Martinez-Alvarez et al., 2002). Additionally, there is also evidence that the presence of redox cycling chemical contaminants like organochlorinated compounds and polycyclic aromatic hydrocarbons tends to increase from headwaters to downstream reaches in the Llobregat system (Fernandes et al., 2002, López-Martín et al., 1995). The previous mentioned contaminants are known to modulate antioxidant defensive systems and to cause oxidative damage in aquatic organisms through production of superoxide anion radicals by redox cycling. Superoxide anion may then interact with other ROS to produce the highly reactive hydroxyl radical (Sjölin and Livingstone, 1997, Halliwell and Gutteridge, 1999, Livingstone et al., 1990). Thus increasing salinity, nutrient load, metallic and organic contaminant levels within the Llobregat system are likely to be causal factors of the oxidative effects observed in this study. In the present study the enzymatic activities of CAT and GST increased up to two-fold from up-to downstream reaches, whereas that of SOD and of GSTPX, apart from the rather low activities detected in site 4, were similar across the studied sites. Although one of the important features of antioxidant enzyme systems is their inducibility under conditions of oxidative stress, higher, equal or lower activities of various antioxidant enzymes have been observed in polluted compared to cleaner areas (Narbonne et al., 1999, Porte et al., 2000), suggesting that antioxidant enzyme responses are transient and variable for different species, enzymes and chemicals (Livingstone, 2001). In insects SOD, which dismutates the superoxide anion (O2−) to H2O2, CAT that further reduces H2O2 to water, and GSTPX which reduces organic peroxides, constitute a major line of defense against reactive oxygen species (Ahmad, 1992). Redox reactions with metals, oxyradicals generated in the oxidative metabolism of organochlorinated compounds and polycyclic aromatic hydrocarbons (Halliwell and Gutteridge, 1999, Livingstone, 2001) and enhanced metabolism caused by salinity stress due to activation of osmoregulatory mechanisms (Guerriero et al., 2002, Martinez-Alvarez et al., 2002) are important sources of oxyradicals. Therefore, increasing CAT activities observed in Hydropsyche larvae collected in downstream reaches are likely to be related to enhanced levels of H2O2. Studies conducted in phytophagous insects, nematodes and bacteria have shown that induction of catalase by H2O2 protects those organisms against oxidative stress (Ahmad and Pardini, 1990, Elkins et al., 1999, Kotze, 2003). For SOD and GPX, however, the presence of high basal activities in Hydropsyche larvae may have allowed detoxification of excess reactive oxygen species (i.e. O2− and hydroperoxides), hence preventing the induction of these enzyme systems. The previous argument is consistent with results observed for Lepidoptera larvae feeding pro-oxidant plant allelochemicals (Ahmad and Pardini, 1990, Aucoin et al., 1991, Ahmad, 1992). The primary role of GST multifunctional enzyme system is to facilitate conjugation of endogenous glutathione with electrophiles, thus resulting in more polar compounds to be excreted or further metabolised (Ketterer et al., 1983). GST also functions as an antioxidant enzyme by conjugating breakdown products of lipid peroxides to glutathione (Ketterer et al., 1983, Lee, 1991). Thus, elevated GST activities found in middle and downstream sites in summer and in site 2 in spring are likely to be related to increasing metabolism of xenobiotics and/or of lipid peroxides. In agreement with the previous statement several studies have reported induction of GST in aphid and insect species exposed to xenobiotics and oxidative agents (Parkes et al., 1993, Tang and Tu, 1995, Sivori et al., 1997, Francis et al., 2005).