The binding of calcium and yttrium ions to a glycoprotein from bovine cortical bone

The binding of Ca(2+) and Y(3+) to an acidic glycoprotein from bovine cortical bone, bone sialoprotein, was determined from the titration curves at I 0.2 in the presence and absence of the cations. The binding of Y(3+) was greater than that of Ca(2+). The value for the association constant, k, for the interaction with Y(3+) increased with pH, from log k 2.93 at pH3.4 to log k 3.50 at pH4.4, and the number of binding sites/mol. increased from 4.6 at pH3.4 to 9.1 at pH4.4. It is proposed that the binding site consists of three carboxyl groups, but it is likely that the binding is a strong electrostatic interaction rather than a co-ordination linkage. A chondroitin sulphate-protein complex also extracted from bovine cortical bone interacted with Y(3+) and Ca(2+) to a similar extent as did bone sialoprotein. It is suggested that these materials are present in bone at the resting and resorbing surfaces and that they contribute to the deposition of yttrium, americium and plutonium at these sites.