Results revealed a decrease in the activity of hepatic antioxidative enzymes i.e. SOD, CAT, GST, GR and GPx was observed in all treated groups in dose and time dependent manner (shown in tables). In gills, an increase in the activities of SOD, CAT and GPx but decrease in the activities of GR and GST was found in T1 group after 30 days of exposure. However, a significant decrease in activities of these enzymes was observed in all treated groups in dose dependent manner after 45 days of exposure. Moreover, the activities of antioxidative enzymes were found to be decreased in time dependent manner (shown in tables). In kidneys, increase in the activities of antioxidative enzymes was observed in T1 group followed by their decrease in T2 and T3 groups in dose and time dependent manner (shown in tables). However, LPO was found to be increased in vital organs of all treated groups in dose and time dependent manner (shown in table VI).
Oxidative stress can be defined as a disturbed balance between the formation of damaging reactive species i.e. reactive oxygen species (ROS) and reactive nitrogen species (RNS) and a living organism’s ability to deal with them. Increase in the production of ROS and RNS or decrease in the antioxidant defense against ROS and RNS can cause oxidative damage of molecules (Halliwell and Gutteridge 2015). Thus, the control of oxidative stress is crucial for the normal functioning of any organism (Prokic et al. 2018). To provide protection against the attack of potentially destructive reactive species, organisms which utilize oxygen have developed a bio-chemical cellular antioxidative system. This system involves numerous enzymatic, non-enzymatic components and biochemical pathways designed to prevent cellular damage ((Halliwell and Gutteridge 2015, Prokic et al. 2019).
Exposure to xenobiotics results in the elevation in the concentrations of reactive species and/or subsequent decline in antioxidant defences against the produced reactive species. Approach of assessing the levels of oxidative stress is widely used in studies which deals with the mechanisms of environmental toxicity and ecotoxicity in living organisms exposed to contaminants (Bartoskova et al. 2013, Regoli and Giuliani 2014, Faggio et al. 2016). Oxidative stress is measured directly through the production of free radicals, indirectly via antioxidant defences against the reactive species, and by assessing the end-results of oxidative stress and oxidative damage. Most of the studies which examine the oxidative stress induced by MPs have involved two components of oxidative stress i.e. antioxidant defences and oxidative damage (Prokic et al. 2019).
The anti-oxidative system plays an important role in the process of detoxification and removal of harmful toxicants from the organism’s body. Thus, knowledge about the response of the anti-oxidative system can provide crucial information of the possible underlying expenses of xenobiotic poisoning and an organism’s ability to respond to it. A stressful condition can either stimulate or inhibit the working of anti-oxidative system, depending upon the duration of exposure and severity of the stressor (Lushchak 2011). From the set of components of anti-oxidative system, the enzymes namely superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione-S-transferase (GST) and glutathione reductase (GR) have been studied in assessing the effects of MPs (Prokic et al. 2019).
SOD, CAT and GPx are regarded as three major enzymes of first line defense mechanism that directly helps to remove harmful ROS. The enzyme SOD works by catalyzing the dismutation of toxic superoxide anion (O2-) to molecular oxygen (O2) and less toxic hydrogen peroxide (H2O2). This H2O2 is further eliminated by the combined action of CAT and GPx. These enzymes works by catalyzing the reduction of H2O2 to harmless products. GPx can also act on other peroxides (Halliwell and Gutteridge 2015). Glutathione (GSH), being non-enzymatic component of the anti-oxidative system, is directly involved in scavenging a broad variety of free radicals or indirectly through the working of GSH-dependent system which includes three enzymes i.e. GPx, GST and GR. GSH acts as a co-factor in GPx reactions. GST conjugates the GSH to the breakdown products of lipid peroxides thus playing its role in preventing oxidative damage. Glutathione is regarded as one of the principal components of the antioxidative system as it prevents oxidation of proteins and lipids caused in organisms by exposure to different environmental xenobiotics. GR helps in maintaining the normal functioning of GSH by reducing the oxidized form of glutathione (GSSG) to its reduced form (GSH) (Oost et al. 1998, Prokic et al. 2019).
The antioxidative system of organisms exposed to MPs revealed a variety of responses, ranging from induction and reduction to changes which are non-significant, depending on the size, type and concentrations of MPs and the studied tissues and organisms.
Lipid peroxidation (LPO) is a self-reliant chain reaction of molecular events that causes oxidative damage to cell membranes, lipo-proteins and other structures containing lipids (Nam 2011). Alterations in the structure and function of lipid bilayers by peroxidation of membrane lipids changes membrane permeability and encourages penetration of toxicants in the cell (Ayala et al. 2014). Organisms exposed to xenobiotics usually show increase in LPO (Ferreira et al. 2005) hence, measuring the levels of malondialdehyde (MDA) as the product of LPO is a extensively used biomarker of exposure to environmental pollutants (Prokic et al. 2019).
The liver is the main organ which plays a vital role in the detoxification of pollutants as it is the site of storage of many substances, hence, exposure and accumulation of pollutants may affect its vital functions. Gills are paired organs in fish which possess osmoregulatory, respiratory and excretory functions. Their direct contact with the surrounding aquatic environment makes them more prone to exposure of various pollutants. Kidneys are the paired organs that are supposed to be exposed to various pollutants due to its excretory role.
Decrease in the activity of anti-oxidative enzymes in the liver with the increasing concentration of MPs and exposure period may be attributed to the fact that stress induced by the overproduction of ROS in the cells exceeds the self-clearing approach of the antioxidative defence system thereby inhibiting the anti-oxidative enzymes activity. The time dependent decrease in the GPx activity in the liver of all treated groups might be due to an increased production levels of hydroperoxides (Ajima et al. 2017), supposed tobe caused by the adverse effect of LDPE MPs (Iheanacho and Odo 2020).
However, at low concentrations of MPs, the activity of antioxidative enzymes is not always inhibited and consequently the ROS levels are not significantly increased. This suggests that the antioxidative system of fish is resistant to oxidative stress induced by low concentration of MPs, but once the concentration of MPs exposure is increased, the antioxidative capacity of fish is destroyed and fish could not resist the oxidative stress induced by the MPs (Yang et al. 2020). Moreover, increase in the exposure period of MPs to fish is thought to play an important role in decrease in the antioxidative capacity of fish due to continous contact of vital organs (like gills) with MPs. These facts could possibly explain the trends of antioxidative enzymes activities observed in gills and kidneys after exposure to MPs in the present study.
In the present study, LPO levels were found to be increased in all vital organs of fingerlings of all treated groups in a dose and time dependent manner in comparison to control group. This might be due to the increased levels of MDA, produced as a result of increased ROS, which causes damage to structure of cell membranes and cells hence causing oxidative stress in fingerlings (Umamaheswari et al. 2020).
There are enough studies reporting the decrease in the activities of antioxidative enzymes in the liver of MPs exposed fish but studies involving the changes in the activities of these enzymes in gills and kidneys after exposure to MPs are lacking.
Decrease in the SOD and CAT activity in the liver of European sea bass (Dicentrarchus labrax L.) was also reported by Espinosa et al. (2019) on exposure of fish to100 or 500 mg of virgin PE-MPs/ kg diet for 3 weeks. However, GR activity was not affected by MPs exposure. Moreover, no significant effects on the activities of these anti-oxidative enzymes were observed when fish were exposed to PVC MPs diets for 3 weeks.
Marine medaka were exposed to 2, 20 and 200 µg/L of PS MPs in a semistatic system for 60 days. After exposure period, the activities of various antioxidative enzymes and MDA content in gills and liver were recorded. In liver, the activity of GPx in liver was found to be significantly decreased in all exposure groups. But, fish groups exposed to high concentrations of MPs (i.e. 20 and 200 µg/L MPs) showed significant decrease in the activities of CAT and GST in liver and increase in the activity of SOD and GST in gills. Regarding SOD activity, it showed a significant decline only in highest (200 µg/L MPs) treated group. Moreover, the MDA content was found to be significantly declined in both gills and liver of all treated groups which indicated that MPs caused excess production of ROS resulting in increased LPO in these organs, thus suggesting the MPs induced oxidative stress in these organs of fish (Wang et al. 2019).
Similar to the findings of the present study, adult male zebrafish exposed to 10 µg/L and 100 µg/L of polystyrene MPs in water for a period of 35 days resulted a dose and time dependent significant decrease in the activities of antioxidant enzymes (SOD, CAT and GPx) and significant increase in the LPO levels in the liver of fish. However, the activity of GST was found to be increased in the liver of MPs exposed groups in a dose and time dependent manner which may be due to increased efficiency of GST to detoxify the ROS produced as a result of MPs exposure (Umamaheswari et al. 2020).
Exposure of Clarias gariepinus juveniles to 0.5, 1.5 and 3.0% PVC MPs in their diet for 45 days resulted in a time dependent decrease in the activities of CAT and GPx in the liver of juveniles. However, the activity of SOD was found to be increased in all treated groups after 30 days followed by its decrease after 45 days of exposure. Moreover, the levels of LPO were found to be significantly increased progressively in all treated groups in a time dependent manner (Iheanacho and Odo 2020).
In contrary, there are several studies reporting the increase in the activities of antioxidative enzymes and consequently decline in the LPO levels in various organs of fish exposed to MPs. This could be due to the use of different species of fish and their life stages; different types, shapes, sizes of MPs; different concentrations and exposure.