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 capacities) of Mytilus edulis blood plasma histidine-rich glycoprotein (HRG) were investigated by potentiometric titrations using a Cd ion-specific electrode. Titration data were applied to a single-component complexation model and expressed as Scatchard plots that were analyzed using the graphical curve peeling method and the algebraic statistical mechanical method. These sets of binding parameters, derived for the purified HRG, were subsequently entered into the geochemical speciation model MINTEQA2 and then used to simulate the experimental titration, thereby determining which set of log K and CL 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 5.41 +/- 0.06 M-1 and carrying capacities of 6.0 +/- 1.2 and 9.5 +/- 0.4 sites/molecule, respectively. At concentration of total Cd measured in the blood plasma of field-collected mussels (< or = 2 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 a high-capacity, apparently nonsaturable Cd transport system.
Nair
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Environmental, Coastal and Ocean Sciences Department, University of Massachusetts Boston, Harbor Campus, Boston, Massachusetts 02125-3393, USA.
Robinson
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D000818:Animals; D001665:Binding Sites; D049872:Bivalvia / Q000502:physiology*; D002104:Cadmium / Q000737:chemistry / Q000493:pharmacokinetics*; D010949:Plasma / Q000737:chemistry; D011506:Proteins / Q000737:chemistry*; D000099053:Histidine-Rich Glycoprotein
D011506:Proteins; D000099053:Histidine-Rich Glycoprotein; D002104:Cadmium