Cadmium-binding to transferrin in the plasma of the common carp Cyprinus carpio

In this study, it was investigated by autoradiography with radioactive cadmium after Western blotting of two-dimensional electrophoresis gels, to which proteins cadmium is mainly bound in plasma of common carp Cyprinus carpio. The obtained results demonstrate that in carp plasma, cadmium is primarily bound to two high molecular weight proteins. Relative small amounts are bound to a protein with M-r similar to 60 000. The other metal-binding protein, with M-r similar to 70 000 and pI similar to 6.7 was identified as transferrin. The conditional equilibrium constants for the binding of cadmium ions to the two metal-binding sites of this protein were calculated as log K-1 = 5.40 +/- 0.12 and log K-2 = 4.66 +/- 0.21, which are comparable to those of human transferrin under the same experimental conditions. Transport of cadmium in plasma of carp was found to be different from that of brown trout Salmo trutta and man, where cadmium is mainly bound to albumin and transferrin. The prominent binding of cadmium to transferrin can be explained by the absence or at least the very low concentrations in which albumin is present in carp plasma. (C) 2001 Elsevier Science Inc. All rights reserved.