Marine Environmental Research 14 (1984) 451-452 Studies on the Binding Site of the Cadmium-Binding Protein (CdBP) from the American Oyster (Crassostrea virginica) Bruce Fowler, Carol Czop, David Elliott, Prakash Mistry & Colin Chignell National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, North Carolina 27709, USA Previous studies (Ridlington, J. W. 8z. Fowler, B. A., Chem.-Biol. Interact. 25, 127-38, 1979) have shown that the American oyster (Crassostrea virginica) elaborates a low molecular weight cadmium- binding protein (CdBP) which, although similar to metallothionein (MT), differs in that it contains only 1-2 g atoms Cd/mole protein. The present studies were conducted to determine the affinity of the CdBP for Cd and to further characterize the nature of the binding site by means of 5,5'- dithiobis-(2-nitrobenzoic acid) (DTNB) titration and circular dichroism. Electrophoretically homogeneous samples of CdBP were purified from American oysters exposed to 0.2 ppm Cd in a flowing seawater system for 2 weeks. Computer-assisted Scatchard analysis of l°9Cd binding to CdB P revealed the presence of a single class of site(s) with a dissociation constant of 10- ~ M for Cd. The addition of DTNB to CdBP followed by separation of bound and free Cd on small Sephadex G-25 columns clearly showed that this sulfhydryl (SH)-specific reagent displaced Cd from CdBP. This finding confirms previously reported circular dichroism studies (Squibb, K.S. et al., Fed Proc. 41, 644, 1982) which have demonstrated that CdBP exhibits a positive band at 259nm which is characteristic of the Cd-S bond. Quantitation of the DTNB reaction at completion showed that the CdBP-binding site possessed an SH/Cd ratio of 2:1 which is different from the 4:1 ratio observed for MT. Circular dichroism studies of CdBP incubated with a 2-fold excess of Cd showed marked reduction of the 259 nm peak but no change in other portions of 451

Studies on the binding site of the cadmium-binding protein (CdBP) from the american oyster (Crassostrea virginica)

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Marine Environmental Research 14 (1984) 451-452

Studies on the Binding Site of the Cadmium-Binding Protein (CdBP) from the American Oyster (Crassostrea

virginica)

Bruce Fowler, Carol Czop, David Elliott, Prakash Mistry & Colin Chignell

National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, North Carolina 27709, USA

Previous studies (Ridlington, J. W. 8z. Fowler, B. A., Chem.-Biol. Interact. 25, 127-38, 1979) have shown that the American oyster (Crassostrea virginica) elaborates a low molecular weight cadmium- binding protein (CdBP) which, although similar to metallothionein (MT), differs in that it contains only 1-2 g atoms Cd/mole protein. The present studies were conducted to determine the affinity of the CdBP for Cd and to further characterize the nature of the binding site by means of 5,5'- dithiobis-(2-nitrobenzoic acid) (DTNB) titration and circular dichroism. Electrophoretically homogeneous samples of CdBP were purified from American oysters exposed to 0.2 ppm Cd in a flowing seawater system for 2 weeks. Computer-assisted Scatchard analysis of l°9Cd binding to CdB P revealed the presence of a single class of site(s) with a dissociation constant of 10- ~ M for Cd. The addition of DTNB to CdBP followed by separation of bound and free Cd on small Sephadex G-25 columns clearly showed that this sulfhydryl (SH)-specific reagent displaced Cd from CdBP. This finding confirms previously reported circular dichroism studies (Squibb, K.S. et al., Fed Proc. 41, 644, 1982) which have demonstrated that CdBP exhibits a positive band at 259nm which is characteristic of the Cd-S bond. Quantitation of the DTNB reaction at completion showed that the CdBP-binding site possessed an SH/Cd ratio of 2:1 which is different from the 4:1 ratio observed for MT. Circular dichroism studies of CdBP incubated with a 2-fold excess of Cd showed marked reduction of the 259 nm peak but no change in other portions of

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452 Bruce Fowler et al.

the spectrum. These studies suggest that the lower binding capacity and affinity of CdB P for Cd may stem from the presence of only 2 SH groups at the binding sites instead of the 4 found in MT. The reduction in the 259 nm band of CdBP upon the addition of excess Cd may be due to the formation of an optically inactive complex in which there is only 1 Cd per SH group.