Stress responses and changes in protein metabolism in carp Cyprinus carpio during cadmium exposure

Stress responses and changes in protein metabolism were studied in common carp Cyprinus carpio exposed to 0, 0.8, 4, and 20 microM cadmium (Cd) over a 29-day period. Blood and other tissue samples were taken after 4 and 29 days of exposure. The highest Cd concentration proved to be lethal to the fish, resulting in 100% mortality after 21 days of exposure. Cd accumulated in the tissues in the following order: kidney>liver>gills. Blood hematocrit, blood hemoglobin, plasma glucose, plasma lactate, and tissue total protein contents were not significantly altered. The concentrations of Cd and zinc (Zn) binding metallothioneins ((Cd, Zn)-MTs) were in the following order=liver>kidney>gills. An increase in (Cd, Zn)-MTs was observed at all exposure concentrations at days 4 and 29 in kidney and at Day 29 in gills. No significant changes in (Cd, Zn)-MT contents were found in liver. The concentrations of free amino acids and the activities of proteases were increased at Day 4 in gills, liver, and kidney of carp exposed to 4 and 20 microM Cd, and in gills and kidney at Day 29 in carp exposed to 4 microM Cd. The observed increases in the activities of aspartate aminotransferase and alanine aminotransferase suggest that the observed proteolysis is intended to increase the role of proteins in the energy production during Cd stress. However, this increased activity of both aminotransferases was not found in gills during exposure to the lethal Cd concentration, indicating that Cd may also cause an inhibitory effect on the activity of these enzymes above a certain level.