The effect of Pb(2+) on the structure and hydroxyapatite binding properties of osteocalcin

Lead toxicity is a major environmental health problem in the United States. Bone is the major reservoir for body lead. Although lead has been shown to impair bone metabolism in animals and at the cellular level, the effect of Pb(2+) at the molecular level is largely unknown. We have used circular dichroism (CD), and a hydroxyapatite binding assay to investigate the effect of Pb(2+) on the structure and mineral binding properties of osteocalcin, a noncollagenous bone protein. The CD data indicate Pb(2+) induces a similar structure in osteocalcin as Ca(2+) but at 2 orders of magnitude lower concentration. These results were explained by the more than 4 orders of magnitude tighter binding of Pb(2+) to osteocalcin (K(d)=0.085 microM) than Ca(2+) (K(d)=1.25 mM). The hydroxyapatite binding assays show that Pb(2+) causes an increased adsorption to hydroxyapatite, similar to Ca(2+), but at 2-3 orders of magnitude lower concentration. Low Pb(2+) levels (1 microM) in addition to physiological Ca(2+) levels (1 mM) caused a significant (40%) increase in the amount of mineral bound osteocalcin as compared to 1 mM Ca(2+) alone. These results suggest a molecular mechanism of Pb(2+) toxicity where low Pb(2+) levels can inappropriately perturb Ca(2+) regulated processes. In-vivo, the increased mineral bound osteocalcin could play a role in the observed low bone formation rates and decreased bone density observed in Pb(2+)-intoxicated animals.