The effects of temperature and metal exposures on the profiles of metallothionein-like proteins in Oreochromis niloticus

Freshwater fish Oreochromis niloticus were exposed individually to 1 mg/L concentrations of cadmium and zinc for 14 days at 20, 25 and 30° C. Heat-treated (80° C) liver tissue homogenates were run on a column packed with Sephadex G-75 to obtain eluats for determination of metallothionein-like proteins, sulfhydryl groups and metal concentrations. Elution profiles of the liver showed that there were three heat-resistant protein peaks detected at 254 and 280 nm in all samples. Approximate molecular weights of proteins obtained from the first, second and third peaks were 70,000, 43,000 and 6500 Da, respectively. Metallothionein-like proteins (6500) of control fish contained sulfhydryl groups and zinc, but not cadmium. Concentrations of zinc and cadmium in MT-like proteins increased in metal exposed fish and these increases were temperature dependent. However, zinc concentration in control fish decreased in relation to temperature increase. Data indicated that temperature plays significant role on metals bound on metallothioneins. This suggests that season of sampling should be taken into account when natural monitoring studies are carried out with metallothioneins.