Characterization studies on the cadmium-binding proteins from two species of New Zealand oysters

Two different types of New Zealand oysters--Ostrea lutaria (OL) and Crassostrea glomerata (CG)--contained different concentrations of zinc, copper, and cadmium. OL oysters had 5.3 micrograms Cd/g, 3.4 micrograms Cu/g, 100 micrograms Zn/g; CG oysters had 1.4 micrograms Cd/g and 936 micrograms Zn/g. Both kinds of oysters were shown by gel filtration (G-75) to contain cadmium and zinc in fractions corresponding to a high molecular weight protein (corresponding to the size of albumin or larger) which was heat labile. OL oysters contained cadmium in fractions corresponding to a molecular weight of approximately 6500. The cadmium-binding protein in these fractions was heat-stable. This protein contained no detectable amounts of zinc and was not present in the CG oysters. Further purification by gel filtration (G-50) was performed to obtain a purer protein fraction. Isoelectric focusing of the protein obtained by G-50 filtration showed one main fraction of protein with a pI approximately 5.9 at approximately 13 degrees C. CG oysters contained cadmium and zinc in a polypeptide with low molecular weight (MW 1000). The cadmium-binding oyster proteins are minimally reactive in a competitive binding radioimmunoassay in comparison to the reactivity of a typical vertebrate metallothionein; the proteins may be metallothioneins, but, if so, they do not exhibit the principal determinants characteristic of vertebrate metallothioneins.