Aquaculture plays an increasingly important role in global food security, contributing nearly 8% of animal protein consumed worldwide. However, its expansion is paralleled by widespread use of antibiotics, of which oxytetracycline (OTC) remains one of the most common. Despite its efficacy in disease control, OTC has the potential to induce biochemical disturbances in aquatic organisms. The present study was conducted to evaluate the enzymatic responses of juvenile common carp, Cyprinus carpio (5.3 ± 0.7 g), to graded dietary concentrations of OTC (25, 50, 75, and 100 mg/kg feed) over 45 days. Enzymatic biomarkers including aspartate aminotransferase (AST), alanine aminotransferase (ALT), and lactate dehydrogenase (LDH) were assayed to assess metabolic disruption and hepatotoxic stress in liver.
Aspartate aminotransferase (AST) is a key enzyme in amino acid metabolism . In the present study, OTC exposure elicited a dose-dependent elevation of hepatic AST activity, with the highest increase observed in fish fed with 100 mg/kg (Fig 1) . This elevation strongly suggests hepatocellular damage and leakage of transaminases into circulation, reflecting altered amino acid turnover and metabolic stress.
Alanine aminotransferase (ALT), another critical transaminase, exhibited a similar trend to AST, with a significant rise in activity corresponding to increasing OTC concentrations (Fig 2) . Since ALT is predominantly localized in the liver, its marked elevation provides more direct evidence of hepatocellular damage compared to AST. Elevated ALT activity in OTC-treated groups indicates impairment of protein metabolism and transamination processes, which are essential for nitrogen balance and energy metabolism.
Lactate dehydrogenase (LDH), an enzyme central to anaerobic glycolysis, catalyzes the conversion of pyruvate to lactate. OTC exposure induced a consistent increase in LDH activity (Fig 3), reflecting a metabolic shift from aerobic to anaerobic pathways in response to cellular stress. Elevated LDH suggests compromised oxidative metabolism, increased energy demand under toxic stress, and possible tissue hypoxia.
The dose-dependent elevations of AST, ALT, and LDH demonstrate the hepatotoxic potential of OTC in C. carpio and establishes these enzymes as sensitive biomarkers for monitoring antibiotic-induced biochemical stress in aquaculture species.