Chronic exposure of coho salmon to sublethal concentrations of copper--III. Kinetics of metabolism of metallothionein

Abstract

Juvenile coho salmon (Oncorhynchus kisutch) 3-8 g body weight were exposed to sublethal concentrations of copper acetate in water of hardness 280 mg/l (as CaCO3) using flow-through conditions. Concentrations of copper were approximately 1, 50, 100 or 150 micrograms Cu/l. Uptake of copper into blood, gills, kidney and liver was measured using 67Cu and into hepatic metallothionein. Chromatography on Sephadex G-75 was used to separate the protein from the cytosol. Copper was determined using atomic absorption spectrophotometry. The ratio of rates of uptake of copper and synthesis of metallothionein was consistent with published values for the copper content of copper-thionein, i.e. 7 to 1. After 4 weeks of continuous exposure the concentrations of copper and hepatic metallothionein levelled off at concentrations that were a function of the copper concentration in the water. After the plateau or steady-state had been reached, the rate of loss of metallothionein was measured by determining the change in specific-activity of isotope with time after incorporation of 35S-L-cysteine or 3H-L-lysine into the protein. The half-life for loss of metallothionein was a function of the exposure to copper, being about 13 days for control fish and 30 days for fish exposed to the greatest concentration of copper. Salmon that had been exposed to copper for 8 weeks neither gained hepatic metallothionein in a further 4 week exposure to copper, nor lost it when transferred for 4 weeks to clean water. Tolerance to copper exhibited by fish that continued to be exposed to it was partially lost after 4 weeks depuration.(ABSTRACT TRUNCATED AT 250 WORDS)