Identification and quantification of histidine-rich glycoprotein (HRG) in the blood plasma of six marine bivalves.

Histidine-rich Glycoprotein (HRG) is a metal-binding protein described from the blood plasma of a pteriomorph bivalve, the marine mussel Mytilus edulis L. We demonstrate here, using Cd-Immobilized Metal Affinity Chromatography (IMAC), SDS-PAGE, Western Blotting, and ELISA, that HRG is present in three additional pteriomorphs and two heterodont bivalves. ELISA indicates that HRG is the predominant blood plasma protein in all six species (41 to 61% of total plasma proteins by weight). Thus, HRG appears to be a widespread metal-binding protein in the plasma of bivalves.

Histidine-rich glycoprotein from the hemolymph of the marine mussel Mytilus edulis L. binds Class A, Class B, and borderline metals.

Few studies have directly addressed the question of how metals (both essential and nonessential) are transported in the circulatory system of bivalve mollusks. One potential metal-transport protein, histidine-rich glycoprotein (HRG), has previously been isolated and characterized from the blood plasma of the marine mussel Mytilus edulis L. The present study was undertaken to investigate the extent to which mussel HRG can bind a variety of essential and nonessential metals in vitro, using immobilized metal-ion affinity chromatography (IMAC) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The equilibrium metal speciation model MINTEQA2 was used to compute the amount of metal that bound to the IMAC packing material during the charging and initial wash steps. Results demonstrated that HRG can bind all seven of the metals tested (Ca, Cd, Hg, Mg, Ni, Pd, and Zn) and that HRG is the only metal-binding protein in IMAC eluents. Because HRG-metal binding strengths (log K(a)) likely correspond with histidine-metal binding strengths, and because HRG is the predominant mussel plasma protein, the majority of each of the seven metals probably would be present in mussel blood as protein-bound metal rather than as free metal ion. The finding that a single mussel plasma protein may be responsible for binding all these metals raises important questions about how these different metals are subsequently transferred from HRG to different tissues of the mussel, where they may exhibit tissue-specific patterns of utilization, sequestration, elimination, and toxicity.