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To evaluate the effect of formalin disinfection on the oxidative stress biomarkers in the cardiac and hepatic tissue, rainbow trout (Oncorhynchus mykiss Walbaum) were assigned into test and control groups. The test group was disinfected by formalin in final concentration 200 mL per m3. Fish were bathed for 20 min, three times, every 3 days. Two days after the last bathing fish were sampled.
2-Thiobarbituric acid reactive substances (TBARS) and carbonyl derivatives of protein oxidative destruction, as well as antioxidant defense biomarkers (superoxide dismutase, catalase, glutathione reductase, glutathione peroxidase activity, total antioxidant capacity) were determined. The formaldehyde-exposed animals showed decrease of aldehydic and ketonic derivatives of oxidatively modiﬁed proteins and increased superoxide dismutase activity in the hepatic tissue compared to untreated group. In cardiac tissue, TBARS level, aldehydic and ketonic derivatives of oxidatively modified proteins were increased in formalin-exposed trout and down-regulated antioxidant status versus control group. It could be concluded that the disinfection of rainbow trout by formalin not contributed to the hepatic injury and may not be toxic. Toxic effect to cardiac tissue was more considerable.