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Shifts in tissue nitrogen isotope composition may be a more sensitive general indicator of stress than measurement of high-turnover defensive biomolecules such as metal-lothionein and glutathione. As a physical resource transmitted along the trophic web, perturbations in protein nitrogen metabolism may also help resolve issues concerning the effects of contaminants on organisms and their consequential hierarchical linkages in ecotoxicology. Snowy egret nestlings (Egretta thula) fed mercury-contaminated diets of constant nitrogen isotope composition exhibited increased relative δ15N values in whole liver (p = 0.0011) and the acid-soluble fraction (ASF) of the liver (p = 0.0005) when compared to nestlings fed a reference diet When nitrogen isotope data were adjusted for the source term of the diet, liver mercury concentrations corresponded with both whole liver relative 15N enrichment (r² = 0.79, slope 0.009, p < 0.0001) and relative 15N enrichment in the acid-soluble fraction of the liver (r² = 0.85, slope 0.026, p < 0.0001). Meanwhile, significant differences were not observed in hepatic levels of the metal-binding peptides metallothionein and glutathione despite a nearly 3-fold difference in liver mercury content Because increases in tissue δ15N values result from increased rates of protein breakdown relative to synthesis, we propose that the increased relative liver δ15N values reflect a shift in protein metabolism. The relationship between ASF and mercury was significantly stronger (p < 0.0001) than that for whole liver, suggesting that the relationship is driven by an increase in bodily derived amino acids in the acid-soluble, free amino acid pool. [ABSTRACT FROM AUTHOR]

A mercury (Hg) saturation assay for measuring metallothionein (MT) in fish liver was modified by optimizing binding conditions to minimize the mercury and tissue consumed. The revised method uses stable Hg at low concentrations instead of 203Hg. At the reduced Hg concentrations used, MT concentrations in livers homogenized in saline appeared to increase systematically with dilution in both bluegill sunfish (Lepomis macrochirus) and largemouth bass (Micropterus salmoides). This error suggested a binding limitation due to sulfhydryl oxidation or competition for and removal of mercury by non-MT proteins. Homogenizing tissues in trichloroacetic acid (TCA) eliminated the interference. To further evaluate the method, the protocol was tested in the laboratory and field. Metallothionein in bluegill injected with 0.6 mg/kg zinc chloride increased at a rate of 0.03 nmole MT/g liver/h (r² = 0.53, p = 0.001). Linearity improved when data were corrected for protein content (r² = 0.74, p < 0.0001). Metallothionein levels in bluegill from a coal ash--contaminated environment were significantly increased over that of hatchery-reared sunfish (F = 20.17, p = 0.0003). The microscaled procedure minimizes concerns related to radioisotope use and waste generation while retaining the high sensitivity of the 203Hg assay. [ABSTRACT FROM AUTHOR]