Binding of serum osteocalcin to hydroxyapatite in Paget's disease of bone

Serum osteocalcin (serum bone Gla protein, sBGP) is elevated in diseases characterized by an enhanced bone turnover. However, in Paget's disease of bone there is a discrepancy between the low increase of sBGP and the high increase of bone remodelling assessed by serum alkaline phosphatase and urinary hydroxyproline. It is possible that the relatively low levels of sBGP reflect an abnormal binding of the molecule to the bone due to an alteration of the BGP molecule and/or of the pagetic mineral phase of bone. Another possibility would be a disregulation of BGP synthesis. In the present work we studied the binding of pagetic sBGP to an experimental model of mineral phase of bone (HPLC hydroxyapatite column). Serum of 14 patients with Paget's disease of bone (sBGP = 7.8 +/- 3.5 ng/ml) and of 14 control subjects (sBGP = 3.3 +/- 0.9 ng/ml) was purified through a Sephadex G-50 medium column (2.6 x 100 cm, 5 mM NH4CO3H). The BGP peak was lyophilized and resuspended in 1 mM KH2PO4, 1 mM CaCl2, 0.02% NaN3, pH 8.4, and then injected into an HPLC gradient system (Waters 660), from 1 to 300 mM KH2PO4 through a hydroxyapatite column (BioGel HPHT, Bio-Rad, 0.78 x 10 cm). A single peak of sBGP was obtained with a retention time of 12 min in control and pagetic patients. From these results we conclude that BGP binding to hydroxyapatite is similar in pagetic patients and in control subjects, suggesting that an alteration in BGP molecule is not responsible for an abnormal binding to the bone. In order to validate our system, the binding of serum BGP from warfarin-treated rats to the hydroxyapatite HPLC column was also studied and compared to binding of serum BGP from normal rats.