Calcium‐binding phosphoprotein particles in the extrapallial fluid and innermost shell lamella of clams

A calcium‐binding phosphoprotein has been isolated from the extrapallial fluid of clams where it occurs as discrete particles 30–40 nm in diameter at a concentration of 1.5–6.0 mM with respect to organic phosphate. The particles were isolated by nitration and differential centrifugation procedures. In particles obtained from Rangia cuneata, 90 mole % of the amino acid residues are aspartic acid (21.7%), serine (29.1%), histidine (34.6%), and lysine (4.6%). In Mercenaria mercenaria particles 95% of the residues are aspartic acid (20.1%), serine and threonine (26.9%), histidine (36.0%), and glycine (11.4%). The serine and threonine residues may be fully phosphorylated. Particles from Rangia contain 1.45 moles of calcium plus magnesium and 0.24 moles of inorganic phosphate per mole of organic phosphate, and in Mercenaria particles these values are 1.29 and 0.164 respectively. Calcium and magnesium probably bind to organic phosphate and aspartic acid residues, and inorganic phosphate may bind secondarily through calcium and magnesium. In Rangia, the innermost shell lamella (ISL) is a single continuous layer which forms the inner surface of the shell dorsal to the pallial attachment. It contains no crystalline mineral but is firmly attached to the mineral in the underlying calcined layer. The presence of phosphoprotein particles in the ISL was demonstrated by electron microscopy and amino acid analysis. Phosphoprotein particles are not present in the bulk shell; however, their occurrence in the extrapallial fluid and ISL coupled with their affinity for mineral ions suggests that the particles have an intermediary roll in shell deposition or dissolution.