Metallothionein mRNA and protein response to arsenite in juvenile winter flounder, {\it Pleuronectes americanus\/}

Date of Completion

January 1996

Keywords

Biology, Molecular|Health Sciences, Toxicology|Biology, Oceanography|Environmental Sciences

Degree

Ph.D.

Abstract

The metal-inducible protein, metallothionein (MT), plays an important role in metal homeostasis and detoxification in fish and is a good biomarker candidate of metal impact. Long Island Sound (LIS) winter flounder (Pleuronectes americanus) contain elevated levels of potentially toxic metals. A new PCR-based technique was developed which identified differential MT mRNA responses between adult and juvenile winter flounder exposed to water-borne CdCl$\sb2$. Subcutaneous exposure of juveniles to NaAs$\sp{3+}$ ranging from sublethal doses to the LD40 significantly elevated MT regardless of baseline variation in liver MT and metal content. The reduced variance associated with MT measurements makes it a more sensitive indicator of sublethal NaAs$\sp{3+}$ exposure than liver As accumulation.^ Saturation of the MT response was linked to mortality by correlating the MT dose response maximum to total liver As content. The upper limit of this response ($\sim$200 ug/g liver) provides a framework for evaluating metal-induced stress among juvenile winter flounder. Fish with high levels of pre-existing MT ($>$100 ug/g) exhibited a narrower MT response window such that MT reached a maximum faster than fish with lower levels of pre-existing MT ($<$75 ug/g). Pre-existing MT correlated with liver Zn content while metal-induced MT correlated with the exposed metal (Cd or As). This suggests that stress effects may be separated from metal-induced MT in the field by measuring both liver MT and Zn.^ The potency of MT induction reflects metal toxicity and this was demonstrated by the stronger MT protein response to CdCl$\sb 2$, than NaAs$\sp{3+}$. The continued rise in MT mRNA after the MT protein reached a maximum suggests this parameter is a better short-term indicator of high NaAs$\sp{3+}$ exposure. Exposure to sublethal doses of NaAs$\sp{3+}$ did not significantly elevate liver As content and even very high levels of exposure caused only a transient liver As accumulation. More work is needed to determine whether MT protein remains elevated after liver As returns to baseline. The environmental relevance of MT induction by NaAs$\sp{3+}$ and its associations with liver As content can not be determined until liver As is speciated in both baseline and metal-exposed LIS winter flounder. ^

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