The Mechanism of Precipitation of Biological Minerals. The Phosphates, Oxalates and Carbonates of Calcium

The precipitation of the phosphates, oxalates, and carbonates of calcium is complicated by the possible formation of different precursor phases involving polymorphs, hydrates, and acid salts. In order to elucidate the mechanisms of the reactions, it is necessary to study the kinetics under conditions of controlled supersaturation calculated from the activities of free ionic species. In general, the rates of formation of the salts are proportional to the (supersaturation)", where n = 1.25-2.0 suggesting a surface controlled process. However, in the case of the calcium phosphate phases, the precipitation of the thermodynamically most stable hydroxyapatite is often complicated by the formation of precursor phases which form and subsequently dissolve during the overall reactions. The sensitivity of the various solid phases to the presence of crystal growth inhibitors is markedly different. Thus in the case of calcium carbonate, it is possible to selectively inhibit calcite and aragonite by adding traces of phosphonate inhibitor, thereby encouraging the formation of vaterite, the most thermodynamically unstable phase. Such selective inhibition may explain the existence of thermodynamically unstable phases in biological systems

The Mechanism of Precipitation of Biological Minerals. The Phosphates, Oxalates and Carbonates of Calcium

The precipitation of the phosphates, oxalates, and carbonates of calcium is complicated by the possible formation of different precursor phases involving polymorphs, hydrates, and acid salts. In order to elucidate the mechanisms of the reactions, it is necessary to study the kinetics under conditions of controlled supersaturation calculated from the activities of free ionic species. In general, the rates of formation of the salts are proportional to the (supersaturation)", where n = 1.25-2.0 suggesting a surface controlled process. However, in the case of the calcium phosphate phases, the precipitation of the thermodynamically most stable hydroxyapatite is often complicated by the formation of precursor phases which form and subsequently dissolve during the overall reactions. The sensitivity of the various solid phases to the presence of crystal growth inhibitors is markedly different. Thus in the case of calcium carbonate, it is possible to selectively inhibit calcite and aragonite by adding traces of phosphonate inhibitor, thereby encouraging the formation of vaterite, the most thermodynamically unstable phase. Such selective inhibition may explain the existence of thermodynamically unstable phases in biological systems

The Precipitation of Calcium Phosphates in the Presence of Magnesium

The kinetics of growth of calcium ·phosphate on hydroxyapatite (HAP) seed crystals was studied at 25 °c and 37 °c and at constant physiological pH in stable supersaturated solutions of calcium phosphate containing various concentrations of magnesium ion. The pH \u27 was maintained constant by means of the pH-stat controlled addition of base and the growth was followed by analyzing the solutions for calcium, phosphate and magnesium. Grown material was characterized chemically, by subsequent dissolution kinetic experiments, by infrared spectroscopy and by specific surface area (SSA) measurement. Magnesium ion reduces the overall rate \u27of crystallization by stabilizing the precursor formed initially in the growth runs. Whereas the stoichiometry of the first formed surface phase corresponds to that of octacalcium phosphate (OCP) in the absence of magnesium, lower molar Ca/P ratios, possibly a mixture of dicalcium phosphate dihidrate (DCPD) and OCP are indicated in its presence. The inhibitory effect of magnesium on crystal growth of HAP and DCPD and some other inorganic compounds is described and discussed in terms of adsorption of magnesium ion at active growth sites and stabilization of growth precursor phases

The Precipitation of Calcium Phosphates in the Presence of Magnesium

The kinetics of growth of calcium ·phosphate on hydroxyapatite (HAP) seed crystals was studied at 25 °c and 37 °c and at constant physiological pH in stable supersaturated solutions of calcium phosphate containing various concentrations of magnesium ion. The pH \u27 was maintained constant by means of the pH-stat controlled addition of base and the growth was followed by analyzing the solutions for calcium, phosphate and magnesium. Grown material was characterized chemically, by subsequent dissolution kinetic experiments, by infrared spectroscopy and by specific surface area (SSA) measurement. Magnesium ion reduces the overall rate \u27of crystallization by stabilizing the precursor formed initially in the growth runs. Whereas the stoichiometry of the first formed surface phase corresponds to that of octacalcium phosphate (OCP) in the absence of magnesium, lower molar Ca/P ratios, possibly a mixture of dicalcium phosphate dihidrate (DCPD) and OCP are indicated in its presence. The inhibitory effect of magnesium on crystal growth of HAP and DCPD and some other inorganic compounds is described and discussed in terms of adsorption of magnesium ion at active growth sites and stabilization of growth precursor phases

Fluorescent Risedronate Analogues Reveal Bisphosphonate Uptake by Bone Marrow Monocytes and Localization Around Osteocytes In Vivo

Bisphosphonates are effective antiresorptive agents owing to their bone-targeting property and ability to inhibit osteoclasts. It remains unclear, however, whether any non-osteoclast cells are directly affected by these drugs in vivo. Two fluorescent risedronate analogues, carboxyfluorescein-labeled risedronate (FAM-RIS) and Alexa Fluor 647–labeled risedronate (AF647-RIS), were used to address this question. Twenty-four hours after injection into 3-month-old mice, fluorescent risedronate analogues were bound to bone surfaces. More detailed analysis revealed labeling of vascular channel walls within cortical bone. Furthermore, fluorescent risedronate analogues were present in osteocytic lacunae in close proximity to vascular channels and localized to the lacunae of newly embedded osteocytes close to the bone surface. Following injection into newborn rabbits, intracellular uptake of fluorescently labeled risedronate was detected in osteoclasts, and the active analogue FAM-RIS caused accumulation of unprenylated Rap1A in these cells. In addition, CD14high bone marrow monocytes showed relatively high levels of uptake of fluorescently labeled risedronate, which correlated with selective accumulation of unprenylated Rap1A in CD14+ cells, as well as osteoclasts, following treatment with risedronate in vivo. Similar results were obtained when either rabbit or human bone marrow cells were treated with fluorescent risedronate analogues in vitro. These findings suggest that the capacity of different cell types to endocytose bisphosphonate is a major determinant for the degree of cellular drug uptake in vitro as well as in vivo. In conclusion, this study shows that in addition to bone-resorbing osteoclasts, bisphosphonates may exert direct effects on bone marrow monocytes in vivo. © 2010 American Society for Bone and Mineral Researc

Surface Aggregation of Urinary Proteins and Aspartic Acid-Rich Peptides on the Faces of Calcium Oxalate Monohydrate Investigated by In Situ Force Microscopy

The growth of calcium oxalate monohydrate in the presence of Tamm-Horsfall protein (THP), osteopontin, and the 27-residue synthetic peptides (DDDS)6DDD and (DDDG)6DDD (D = aspartic acid, S = serine, and G = glycine) was investigated via in situ atomic force microscopy. The results show that these four growth modulators create extensive deposits on the crystal faces. Depending on the modulator and crystal face, these deposits can occur as discrete aggregates, filamentary structures, or uniform coatings. These proteinaceous films can lead to either the inhibition of or an increase in the step speeds (with respect to the impurity-free system), depending on a range of factors that include peptide or protein concentration, supersaturation, and ionic strength. While THP and the linear peptides act, respectively, to exclusively increase and inhibit growth on the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {\bar{1}01} \right)$$\end{document} face, both exhibit dual functionality on the (010) face, inhibiting growth at low supersaturation or high modulator concentration and accelerating growth at high supersaturation or low modulator concentration. Based on analyses of growth morphologies and dependencies of step speeds on supersaturation and protein or peptide concentration, we propose a picture of growth modulation that accounts for the observations in terms of the strength of binding to the surfaces and steps and the interplay of electrostatic and solvent-induced forces at the crystal surface