The identification of metallothioneins in the bivalve <i>Macoma balthica</i> from the Western Scheldt Estuary

Macoma balthica have been sampled every two months for two years at two different locations, one brackish and one marine, from the Western Scheldt estuary and sumitted to in vitro short term exposure to Cd, Cu and Zn. Total and heat stable cytosolic metal concentrations have been measured before and after exposure to study the effects of the sampling season on the metal uptake by the bivalve. Along with this, concentrations of metal-binding proteins, exhibiting the characteristic properties of metallothioneins (MTs) have been measured in the entire cytosol and the chromatographically separated cytosol to see if they increase after the metal exposure of the clams and to determine their molecular weight. Much higher uptakes of Cd and, to a lesser extent, Cu occurred in winter while Zn appears to be constantly regulated. These phenomena are amplified in the cytosol, especially for Cd. The respective contents of the various elements, either at the total or at the cytosolic level, are not constant, showing that the body weight seasonal fluctuations of the clams are not the only parameter responsible for the metal uptake variability. As well as this, the proportion of cytosolic metal was found to vary up to maxima of 50% depending on the absolute cytosolic concentration. MT or MLP concentrations have been shown to increase on metal exposure and are found in the cytosolic chromatographic fractions in the molecular weight range of 6000-12000 Da. Metal maxima, after the exposure were found in these same fractions suggesting that the cytosolic phase, with its heat stable proteins, plays an important role in trace metal detoxification mechanisms of clams. The use of MT seasonal "norm" charts has been investigated for their suitability to be used as indicators of trace metal pollution