Behavior of metallothionein-bound metals in a natural population of an estuarine mollusc

Abstract

A population of oysters ( Crassostrea virginica) in the estuarine reaches of the Patuxent River (Maryland, USA) possesses elevated concentrations of certain metals and is known to exhibit a gradient in metal concentrations, increasing upstream from the mouth of the river. The accumulation of total Cd, Zn and Cu in the gill tissues of these oysters was analyzed and compared to the concentrations in the cytosol and bound to metallothioneins (MTs). The concentrations of the metals in the gill tissues exhibited the expected gradient, with concentrations increasing upstream. The acetylated form of oyster MT, CvNAcMT, was the main form of MT detected in these oysters. This is the expected form of this protein in oysters based on basic biochemical criteria and data obtained for oysters exposed to low concentrations of Cd in this study. MTs were quantitatively most significant in binding Cd, sequestering about 20% of the tissue Cd, and much less so in binding Zn and Cu, accounting for about 0·3 and 1·0%, respectively, of these metals. Complex relationships were observed when the cytosolic and MT-bound fractions of these metals were compared with the total metal concentrations. Of greatest interest was the observation that concentrations of the MT-bound fractions declined in oysters collected from the site with the highest tissue metal concentrations. Zinc bound to MTs did not exhibit any correlation with either the total or cytosolic metal concentrations. With Cd and Cu, however, strong relationships were observed between MT-bound and cytosolic fractions, irrespective of the total tissue metal concentrations. With these metals, the relative distribution on MT-bound fractions could be predicted from the cytosolic concentrations, but not from the total tissue metal concentration. The amounts of Cd bound to MTs was inversely related to that of Zn. Cu was usually bound at low concentrations, except in a few individuals whose MTs were Zn-rich and Cu-rich. The relatively complex nature of the dynamics of MT-bound metals indicated that increased emphasis should be placed on determining the sources of variability in the utilization of MTs in binding metals in these molluscs.