The effect of Pb2+ 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 Pb2+ at the molecular level is largely unknown. We have used circular dichroism (CD), and a hydroxyapatite binding assay to investigate the effect of Pb2+ on the structure and mineral binding properties of osteocalcin, a noncollagenous bone protein. The CD data indicate Pb2+ induces a similar structure in osteocalcin as Ca2+ but at 2 orders of magnitude lower concentration. These results were explained by the more than 4 orders of magnitude tighter binding of Pb2+ to osteocalcin (K-d = 0.085 muM) than Ca2+ (K-d = 1 25 mM). The hydroxyapatite binding assays show that Pb2+ causes an increased adsorption to hydroxyapatite, similar to Ca2+, but at 2-3 orders of magnitude lower concentration. Low Pb2+ levels (1 muM) in addition to physiological Ca2+ levels (1 mM) caused a significant (40%) increase in the amount of mineral bound osteocalcin as compared to 1 mM Ca2+ alone. These results suggest a molecular mechanism of Pb2+ toxicity where low Pb2+ levels can inappropriately perturb Ca2+ 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 Pb2+-intoxicated animals. (C) 2001 Elsevier Science B.V. All rights reserved.