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

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Abstract
The binding of Ca2+ and Y3+ 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 bindiag of Y3+ was greater than that of Ca2+. The value for the association constant, k, for the interaction with Y3+ increased with pH, from log k 2.93 at pH 3.4 to log k 3.50 at pH 4.4, and the number of binding sites/mol. increased from 4.6 at pH 3.4 to 9.1 at pH 4.4. The binding site consists of 3 carboxyl groups, but it is likely that the binding is a strong electrostatic interaction rather than a coordination linkage. A chondroitin sulfate-protein complex also extracted from bovine cortical bone interacted with Y3+ and Car+ to a similar extent as did bone sialoprotein. These materials may be present in bone at the resting and resorbing surfaces and they contribute to the deposition of Y, Am and Pu at these sites.