CADMIUM-BINDING IN THE BLOOD-PLASMA OF 2 MARINE BIVALVES

Cadmium transport in the plasma of Mytilus edulis L. displays attribut es similar to those previously described for Mercenaria mercenaria (L. ). The majority of Cr, Cu, Ni, and Zn is partitioned in the plasma rat her than the hemocytes in both species, although differences in Fe and Mn are apparent. Mussels however contain twice the number of circulat ing hemocytes (2.1 +/- 0.8% of whole blood weight; n = 75 vs 1.2 +/- 0 .3%; n = 70 for the quahog) and approximately three times more plasma protein than the quahog. Titration experiments using ion-specific elec trode (ISE) measurements and equilibrium dialysis (ED) experiments ind icate that both species have a low-affinity, high-capacity system for the internal transport of Cd. At low blood Cd concentrations (<8.9 mu M) and short dialysis times (24 h), approximately 90% of the Cd in mus sel plasma is bound to plasma proteins. At higher Cd concentrations an d at longer dialysis times, the percentage of free Cd increases substa ntially. The Cd affinity was slightly lower in mussel plasma compared to the quahog (log(10)K = 2.6-3.9/M vs 3.7-4.3/M), and mussel plasma C -L values were appreciably lower (0.16-1.11 mmol/g protein) than those for the quahog (1.97-2.26 mmol/g protein). Both ISE and ED experiment s on quahog plasma yielded similar estimates of K and C-L.