Evaluation of the significance of metal-binding proteins in the gastropod Littorina littorea

Abstract

Measurements of metals in Littorina littorea from clean and contaminated sites in Great Britain confirmed that body concentrations of Ag, Cd and Hg vary according to environmental contamination, while the essential elements Cu and Zn are regulated. Investigation of the cytosolic distribution of metals in L. littorea, using Sephadex G-75 gel-permeation chromatography, revealed important distinctions in the partitioning of elements. In contrast to Ag and Hg, which were mainly associated with high molecular weight ligands, Cd was bound predominantly to an intermediate molecular weight, soluble protein (CdBP-I, apparent mol. wt=20 000). However, although the main function of CdBP-I, both in clean as well as contaminated individuals, was sequestration of Cd, significant quantities of other non-essential metals (Ag, Hg) were similarly bound. Laboratory experiments confirmed the role of CdBP-I in complexing cadmium, and also revealed the induction of a second cadmium-binding protein (CdBP-II) in response to high cadmium levels. The apparent molecular weight (10 000), absorbance characteristics and high (inducible)-SH content of CdBP-II suggest similarities with metallothionein. A dose-related increase in the-SH content of very low molecular weight (>3 000) fractions was also observed in cadmium-exposed L. littorea, although no cadmium was associated with these ligands. A detoxifying role is tentatively proposed for the metal-binding proteins CdBP-I and II in L. littorea. However, some “spillover” of cadmium to the high molecular weight protein pool was observed in individuals exposed to cadmium in the field and laboratory. The use of L. littorea in biochemical assays of environmental contamination is discussed.