Introduction

Bisphenol A (BPA) is a raw material mostly used in the plastic industry and an endocrine disruptor. The wide range of use increases the likelihood of human exposure to this chemical (Kang et al. 2006). BPA synthesized in 1891 firstly is one of the most widely used industrial chemicals (Dodds and Lawson 1938). The half-life of BPA in water is between 3 and 5 days and it is sufficient to affect the aquatic organisms (Fürhacker et al. 2000). BPA may cause toxic effects to aquatic organisms even with low concentrations (Tatar et al. 2018).

There is a significant balance between the formation of reactive oxygen species (ROS) and their removal by the antioxidant defense system. ROS are removed by enzymatic and nonenzymatic antioxidants (Serdar 2019). ROS formation in organisms exposed to BPA has also been shown in previous studies (Kabuto et al. 2003; Wu et al. 2011; Jemec et al. 2012). When cells are under oxidative stress, they often increase levels of antioxidant enzymes such as superoxide dismutase (SOD) and catalase (CAT) (Valavanidis and Vlachogianni, 2010; Serdar et al. 2018). SOD is a very important enzyme that catalyzes the conversion of superoxides into O2 and H2O (Ding et al. 2008). The main function of CAT is to remove hydrogen peroxide, synthesized in some stages of metabolism in the presence of molecular oxygen (Keha and Küfrevioğlu 2001). When oxidative stress increases in the organism, the formation of free radicals increases. GSH is responsible for protecting cells against the destructive effects of these free radicals (Meister and Anderson 1983). Lipid peroxidation occurs when the levels of free radicals increase and exceed the antioxidant capacity of the cells. Lipid peroxidation is defined as conversion of lipid hydroperoxide to aldehydes and other carbonyl compounds. One of these compounds is malondialdehyde (MDA) and can be used to determine the level of lipid peroxides (Khoschsorur et al. 2000). MDA reacts with thiobarbituric acid and forms TBARS. Lipid peroxidation and antioxidant defense mechanisms were used as biomarkers of oxidative stress in order to determine pollutant effects in water (Livingstone 2001; Valavanidis and Vlachogianni, 2010).

Gammarus sp. is an appropriate organism for the determination of the ecotoxicological effects of pollutants (Kunz et al. 2010; Yildirim and Yaman 2019).

This study was designed to determine some enzymatic (SOD and CAT) and nonenzymatic (GSH and MDA) antioxidants that are biomarkers of oxidative stress in G. pulex individuals exposed to BPA.

Materials and methods

Experimental setup

The model organisms were collected by hand nets from The Munzur River in Tunceli, Turkey, and were quickly brought to a laboratory at 18 °C and 12:12 light/dark period. It was placed in 20-L aerated aquariums filled with water and sediment taken from the natural environment. The organisms were fed with willow leaves during 2 weeks before starting the experimental studies (De Lange et al. 2006).

The lethal concentration of BPA to G. pulex was obtained by probit analysis and the value was calculated as 249.97 ± 1.4 μg/l BPA. Synthetic solutions containing three sublethal doses of BPA were applied to G. pulex (n: 10 per group) for 24 and 96 h. Four application groups were designed as A (group containing 25 μg BPA); B (group containing 12.5 μg BPA); C (group containing 6.25 μg BPA), and control group. G. pulex individuals were not fed during the experimental applications and were controlled per 24 h. Inactivity was accepted as the death criterion.

Determination of biochemical response

G. pulex organisms were weighed. Phosphate buffered saline (PBS buffer) was added at 1/5 w/v and homogenized with ice by using a homogenizer. These homogenized samples were centrifuged for 15 min at 17000 rpm in a refrigerated centrifuge and the obtained supernatants were stocked in the − 86 °C freezer until measurement was complete.

Biochemical biomarkers were determined by using the ELISA kits (catalog numbers of SOD, CAT, GSH, and TBARS: 706002; 707002; 703002 and 10009055, respectively) purchased from The CAYMAN Chemical Company.

Statistical analyses

SPSS 18 software was used for statistical analysis. One-way ANOVA and Duncan’s multiple range tests were employed to evaluate the statistical differences in three application groups (A, B, C) in the same exposure time (abcp < 0.05). Two-tailed independent T test was used to compare the differences between exposure times in the same application group (*p < 0.05).

Results

SOD activities increased in all groups compared with the control group after 24 h (p < 0.05). However, no statistically significant changes were found in all groups compared with the control group after 96 h (p > 0.05). When the exposure times were compared, a significant difference was not found in all application groups (p > 0.05; Fig. 1a).

Fig. 1
figure 1

a SOD activities (U/ml), b CAT activities (nmol/min/ml), c GSH levels (μM) and d TBARS levels (μM) of G. pulex exposed to BPA. The asterisk shows statistical differences according to the two-tailed independent T test between different exposure times in the same group; *p < 0.05. The letters (a, b, c) show statistical differences of Duncan’s multiple range test between all application groups in the same exposure time; abcp < 0.05. Values represent mean ± SE; n = 10

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CAT activities decreased in all groups compared with that in the control group after 24 and 96 h (p < 0.05). This decrease was not statistically significant only in group A after 24 h (p > 0.05; Fig. 1b).

GSH levels increased in all groups compared with the control group after 24 and 96 h (p < 0.05). When the exposure times were compared, significant differences were found in A and B groups (p > 0.05; Fig. 1c).

TBARS levels increased in all groups compared with the control group after 24 and 96 h (p < 0.05). When the exposure times were compared, significant differences were found in A and C groups (p > 0.05; Fig. 1d).

Discussion

BPA, which is an endocrine disrupter, is a raw material in the plastics industry. BPA has become a critical environmental problem due to its widespread use in the sector. According to the latest estimates, worldwide BPA production is annually 6.8 million tons (Jandegian et al. 2015). Estrogenic activity of BPA has been confirmed in many toxicological studies using animals. Physiological functions in the reproductive organs and endocrine system can be impaired (Crain et al. 2007; Zhang et al. 2016). In this study, some enzymatic and nonenzymatic antioxidant parameters were investigated in G. pulex exposed to various concentrations of BPA (Watts et al. 2001; Schirling et al. 2006). In our study, the enzymatic antioxidants SOD and CAT and nonenzymatic antioxidants MDA and GSH levels were used as biochemical biomarkers to determine the effects of BPA application.

SOD and CAT enzyme activities are the first defense systems against oxygen toxicity (Zagal and Mazmancı 2011). SOD converts reactive and toxic superoxide radicals to hydrogen peroxide (H2O2) (Duman and Kar 2015). CAT enzyme reduces H2O2 to nontoxic H2O and O2, which are innocuous to cells (Li et al. 2012). These enzyme activities can be used as early indicators against xenobiotics that is the source of oxidative stress. In our study, the SOD activity increased after 24 h in all groups compared with the control group and the highest activity was found in group A. After 96 h, an increase was observed only in group A and no change was observed in groups B and C compared with the control group. It is thought that the increase in the SOD activity may lead to H2O2 accumulation and this will cause the inhibition of the CAT activity (Li et al. 2012). Previous studies have reported that ROS may inhibit the CAT activity (Kono and Fridovich 1982; Escobar et al. 1996). The effect of BPA on the antioxidant defense system in Arabidopsis thialiana was investigated and an increase in SOD activity was detected (Imran et al. 2017). In our study, CAT activity decreased after 24 and 96 h of application compared with the control group. Oxidative stress caused by BPA may have led to depletion of SOD and CAT activities, which may be indicative of weakening of the antioxidant system (Li et al. 2012). Özaydın et al. (2018) investigated the effect of BPA on antioxidant defense system in rats and they found that SOD and CAT activities were decreased significantly at the end of the application. Faheem and Parvez Lone (2017), in their study on Ctenopharyngodon idella, reported that BPA reduces CAT activity. In another study that applied BPA to chicken embryos, SOD activity was found to be increased (Sravani et al. 2015).

It is suggested that increased GSH levels have been reported to be a response to adaptation to increasing stress conditions (Moreno et al. 2010; Serbes 2011). The use of GSH in elimination of detoxified peroxide products formed by lipid peroxidation caused to decrease the GSH levels (El-Gendy et al. 2010). In our study, the duration of exposure was found to affect GSH levels in groups A and B. The highest GSH level was found in group B after 96 h, and GSH levels in groups A and C increased after 24 and 96 h compared with the control group. Therefore, the increases observed in G. pulex may be due to adaptation mechanisms developed against stress. Özaydın et al. (2018) found that GSH levels decreased significantly at the end of BPA exposure in their study on rats. In a study with Ctenopharyngodon idella, BPA was found to significantly reduce GSH levels (Faheem and Parvez Lone 2017).

The increase in MDA levels, which is an indicator of lipid peroxidation, suggests that BPA causes oxidative stress in G. pulex. Xenobiotics can increase the amount of MDA by increasing free radical formation or by reducing the cell’s defend capacity against peroxidation reactions (Lackner 1998). In our study, the duration of exposure was found to affect MDA levels in groups A and C. The highest levels of MDA were found in group A after 96 h of administration and MDA levels in groups B and C increased after 24 and 96 h compared with the control group. MDA levels were found to be increased in a study in which BPA was applied to chicken embryos (Sravani et al. 2015). In a study conducted on freshwater fish, MDA levels were found to be increased at the end of the application similar to our study (Faheem and Parvez Lone 2017).

Conclusion

According to the data obtained in the present study, it is believed that BPA, which is mostly used in the plastics industry, has a toxic effect on G. pulex and pollution caused by BPA may pose a great risk to the environment. MDA levels increased in G. pulex exposed to BPA compared with the control group. This is an indicator of lipid peroxidation. Changes in lipid peroxidation and increased GSH levels indicate antioxidant defense. Changes in the level of GSH suggest that there is a tendency to adapt to stress. Partial reductions in the level of GSH are due to the use of GSH in removal of ROS.

Furthermore, the differences in exposure time and application dose according to the obtained data changed the biochemical responses of G. pulex. For changes in the antioxidant enzymes and GSH, MDA levels showed that stresses caused by BPA metabolism are modifiable. It has been indicated that the toxicity of metabolites induced the changes in the antioxidant defense system.

As a compensatory response to low levels of oxidative stress, the antioxidant enzyme system can be induced. When severe oxidative stress occurs, the antioxidant enzyme activities can inhibit and cause oxidative damage. It was concluded that the biomarkers we used to investigate the toxic effects of BPA on G. pulex were useful biomarkers.