Cadmium binding to a histidine-rich glycoprotein from marine mussel blood plasma: Potentiometric titration and equilibrium speciation modeling

Cadmium-binding parameters (conditional stability constants and carrying ca pacities) of Mytilus edulis blood plasma histidine-rich glycoprotein (HRG)w ere investigated by potentiometric titrations using a Cd ion-specific elect rode. Titration data were applied to a single-component complexation model and expressed as Scatchard plots that were analyzed using the graphical cur ve peeling method and the algebraic statistical mechanical method. These se ts of binding parameters, derived for the purified HRG. were subsequently e ntered into the geochemical speciation model MINTEQA2 and then used to simu late the experimental titration, thereby determining which set of log K and C-L values best represented the titration data. The Cd binding to HRG was best described by a two-class model with log K values of 7.65 +/- 0.10 and 5.31 (+/-) 0.06 M-1 and carrying capacities of 6.0 +/- 1.2 and 9.5 +/- 0.4 rites/molecule, respectively. At concentrations of total Cd measured in the blood plasma of field-collected mussels (less than or equal to2 x 10(-7) M ), plasma Cd speciation would be dominated by the strong affinity sites of HRG (> 93.5% of total Cd binds to HRG), whereas HRG itself would only be 0. 05% saturated with Cd, indicating ii high-capacity, apparently nonsaturable Cd transport system.