The influence of dietary and waterborne zinc on heat-stable metal ligands in rainbow trout, Salmo gairdneri Richardson: quantification by 109Cd radioassay and evaluation of the assay

The '"Cd binding assay of Eaton & Toal was critically evaluated and then used to assess the induction of cytosolic metal-binding ligands in rainbow trout exposed to Zn in the diet and/or in the water for 16 weeks. With purified rabbit Cd-Zn metallothionein(MT), W d binding and total Cd recovery in the assay were linear up to 5 pg of protein; gel chromatography revealed a single peak. With heat-denatured extracts of gill, liver and intestine from control and Cdand Zninjected trout, ""Td binding was generally linear with sample size. Gel chromatography demonstrated that " T d was bound by a protein with the same apparent weight as MT ( 1 I 000 daltons), but significant binding occurred also at three other regions [molecular weight (rnol wt) > 70 000. 30 000 and < 30001. In the dietary/waterborne Zn exposure. induced 'Td-binding activity occurred not in the MT peak but in the low mol wt peak ( < 3000). Activity in the gill rose in response to both dietary and waterborne Zn, but the liver did not respond. The maximum fivefold elevation in the gill was primarily a waterborne effect. In the intestine, the maximum rise was 25-fold due to both factors. The thresholds for induction were > 39 pg Zn I ' in water, and > 90 mg kg ' in the diet, but only when waterborne Zn was also high. There was no correlation between ""Td binding and acid soluble thiol levels, which tended to decline at higher Zn exposures.