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 dich roism (CD), and a hydroxyapatite binding assay to investigate the effect of Pb2+ on the structure and mineral binding properties of osteocalcin, a non collagenous bone protein. The CD data indicate Pb2+ induces a similar struc ture in osteocalcin as Ca2+ but at 2 orders of magnitude lower concentratio n. These results were explained by the more than 4 orders of magnitude tigh ter 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 a dsorption to hydroxyapatite, similar to Ca2+, but at 2-3 orders of magnitud e lower concentration. Low Pb2+ levels (1 muM) in addition to physiological Ca2+ levels (1 mM) caused a significant (40%) increase in the amount of mi neral bound osteocalcin as compared to 1 mM Ca2+ alone. These results sugge st a molecular mechanism of Pb2+ toxicity where low Pb2+ levels can inappro priately perturb Ca2+ regulated processes. In-vivo, the increased mineral b ound 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.