Surface Of Hydroxyapatite Crystals Research Articles

The effect of chemical structure of carboxylate molecules on hydroxyapatite nanoparticles. A structural and morphological study

Being the most abundant non-macromolecular organic component of bone, the role of citrate (Cit) in hydroxyapatite (HA) crystallization is of high relevance. In this work we have investigated the influence of hydroxycitrate (CitOH) and glutarate (Glr) on HA crystallization in terms of particle growth, composition, and morphology in comparison to Cit. CitOH and Glr have been selected for this work because they share the same backbone structure of Cit but bear different functional groups in the central region. Our data has revealed that CitOH strongly inhibits HA crystallization more efficiently than Cit. CitOH-HA nanoparticles are composed of platy, elongated particles similar to those of Cit-HA but they are ca. twice smaller and have a lower crystal order. On the other hand, Glr does not inhibit HA crystallization as Cit, but leads to the formation of OCP platelets that convert with maturation time to HA nanorods with larger aspect ratio than Cit-HA. In comparison to Cit-HA samples, Glr-HA nanoparticles have bigger dimensions, and higher structural order. Overall, our data reveal that the central carboxyl group of Cit is involved in the selective binding with HA crystal surface and in regulating HA crystal growth. The results of this work highlight new possibilities to control the formation of HA for designing advanced bioactive materials and give new insights on the role of the structure of Cit in regulating the HA morphology.

P0884MYO-INOSITOL HEXAPHOSPHATE PEGYLATION IMPROVES BIOAVAILABILITY BUT REDUCES ITS ANTI-VASCULAR CALCIFICATION EFFECT IN RATS

Abstract Background and Aims Vascular calcification (VC) is a major contributor to increased morbidity and mortality in End Stage Kidney Disease (ESKD) patients undergoing dialysis. SNF472, a salt of inositol hexaphosphate (InsP6), is a selective calcification inhibitor that interferes in the formation and growth of ectopic hydroxyapatite (HAP). SNF472 is currently in Phase 3 clinical trials for the treatment of calciphylaxis in ESKD patients on dialysis. Inositol-1,2,3,5-tetraphosphate-4,6-bisPEG100 (InsP4bisPEG or INS3001) results from the PEGylation of inositol tetraphosphate (InsP4) with polyethylene glycol (PEG) 100. Our aim was to compare the relative bioavailability of SNF472 and InsP4bisPEG and their efficacy in the inhibition of calcification in silico, in vitro and in vivo. Method Subcutaneous (10 mg/kg) pharmacokinetics of InsP4bisPEG and SNF472 were assessed in Sprague Dawley (SD) rats. To evaluate the adsorption binding affinity (Eads) of SNF472, InsP4bisPEG and other inositol phosphates to the HAP crystal surface, computational studies were performed using Density Functional Theory calculations with DMOL3 (MS2016). The in vitro efficacy of the compounds was evaluated using a pharmacodynamic assay to measure the calcification potential of human plasma. An in vivo efficacy study (calcification induced by 3 consecutives daily s.c. administrations of 150 kIU/kg vitamin D3) was performed with SD rats receiving s.c. vehicle, or equimolar doses (36 µmol/kg) of SNF472 or InsP4bisPEG once daily. Results The PEGylation of inositol tetraphosphate in positions 4 and 6 increased the exposure and t1/2 of the compound when given subcutaneously compared to SNF472. Molecular modelling revealed that SNF472 binds to the HAP surface with higher affinity than InsP4bisPEG and INSP4 (ΔEads=-352 kcal/mol for SNF472, ΔEads=-177 kcal/mol for InsP4bisPEG and ΔEads=-146 Kcal/mol for InsP4, taking inositol as reference). These results were correlated with the inhibition of calcium phosphate crystallization in plasma in vitro. SNF472 treated animals presented significantly lower calcium levels in aorta, which were 38% and 55% lower than placebo and InsP4bisPEG treated animals, respectively. Conclusion The differential pharmacokinetic profile of InsP4bisPEG (INS3001) does not translate into higher, but lower, efficacy than SNF472 against vascular calcification when comparing equimolar doses.

The role of phosphorylation in dentin phosphoprotein peptide absorption to hydroxyapatite surfaces: a molecular dynamics study

Dentin phosphoprotein (DPP) is a protein expressed mainly in dentin and to a lesser extent in bone. DPP has a disordered structure, rich in glutamic acid, aspartic acid and phosphorylated serine/threonine residues. It has a high capacity for binding to calcium ions and to hydroxyapatite (HA) crystal surfaces. We used molecular dynamics (MD) simulations as a method for virtually screening interactions between DPP motifs and HA. The goal was to determine which motifs are absorbed to HA surfaces. For these simulations, we considered five peptides from the human DPP sequence. All-atom MD simulations were performed using GROMACS, the peptides were oriented parallel to the {100} HA crystal surface, the distance between the HA and the peptide was 3 nm. The system was simulated for 20 ns. Preliminary results show that for the unphosphorylated peptides, the acidic amino acids present an electrostatic attraction where their side chains are oriented towards HA. This attraction, however, is slow to facilitate bulk transport to the crystal surface. On the other hand, the phosphorylated (PP) peptides are rapidly absorbed on the surface of the HA with their centers of mass closer to the HA surface. More importantly, the root mean square fluctuation (RMSF) indicates that the average structures of the phosphorylated peptides are very inflexible and elongate, while that of the unphosphorylated peptides are flexible. Radius of gyration (Rg) analysis showed the compactness of un-phosphorylated peptides is lower than phosphorylated peptides. Phosphorylation of the DPP peptides is necessary for binding to HA surfaces.

Contrast-enhanced X-ray detection of breast microcalcifications in a murine model using targeted gold nanoparticles.

Microcalcifications are deposits of hydroxyapatite (HA) mineral within breast tissue and the most common abnormality detected by mammography when screening for breast cancer due to exhibiting greater X-ray attenuation than the surrounding tissue. However, the detection of microcalcifications is limited by the sensitivity and specificity of mammography. Therefore, the objective of this study was to investigate in vivo targeted delivery of bisphosphonate-functionalized gold nanoparticles (BP-Au NPs) for contrast-enhanced detection of microcalcifications using computed tomography (CT). A murine model was developed for precise, a priori control over the level of microcalcification burden by injecting varying concentrations of HA crystals in a Matrigel carrier into mammary glands. The measured X-ray attenuation of microcalcifications containing varying HA concentrations demonstrated that the model was reproducible and able to recapitulate varying levels of microcalcification burden, including levels undetectable by CT in the absence of contrast enhancement. After intramammary delivery, BP-Au NPs provided enhanced contrast for the detection of microcalcifications that were otherwise below the CT detection limit. BP-Au NPs targeted microcalcifications due to specific binding to HA crystal surfaces, resulting in contrast between the HA microcalcification site and surrounding tissue which was visibly apparent (∼30-135 HU) within 2 days after delivery. Therefore, targeted BP-Au NPs enabled improved sensitivity and specificity for the detection of microcalcifications.

Ab Initio Investigation of Strain Dependent Atomistic Interactions at Two Tropocollagen-Hydroxyapatite Interfaces

Tropocollagen (TC) and hydroxyapatite (HAP) interfaces are one of the main load bearing entities in bone family of materials. Atomistic interactions in such interfaces occur in a variety of chemical environments under a range of biomechanical loading conditions. It is challenging to investigate such interactions using traditional analytical or using classical molecular simulation approaches owing to their limitations in predicting bond strength change as a function of change in chemical environment. In the present work, 3D ab initio molecular dynamics simulations are used to understand such atomistic interactions by analyzing tensile strain dependent deformation mechanism and strength of two structurally distinct idealized TC-HAP interfaces in hydrated as well as unhydrated environments. Analyses suggest that the presence of water molecules leads to modification of H-bond density at the interfaces that also depends upon the level of strain. TC molecules become stiffer in the presence of water due to the presence of H-bonds. Bond forming-and-breaking cycle change as a function of H-bond density lies at the heart of TC-HAP interfacial shear deformation. Consequently, interfaces with TC molecule placed flat on the HAP crystal surface experience significantly higher shear stress during deformation in comparison to the interfaces with TC molecule placed with their axes perpendicular to the HAP surface.

The drug-carrier interactions, release behaviors and cell responses of hydroxyapaptite containing several Chinese medicines

The present work shows drug-carrier interactions, release behaviors and cell responses of hydroxyapatite (HA) containing salvianolic acid B (Sal B), astragalus polysaccharide (APS), and naringin. X-ray diffraction (XRD) showed that the crystallinity and crystal size of HA decreased significantly when Sal B was added (p<0.05). Transmission electron microscope (TEM) confirmed that the nano-acicular crystals of HA containing Sal B were the most fine among all specimens. It was conjectured that Sal B preferentially adsorbed on the positively charged surface of HA crystals to inhibit their growth. In vitro release of HA containing Chinese medicines followed the first-order equation. The drug-carrier affinity between HA and Sal B might have prolonged the release of Sal B. The proliferation and differentiation of osteoblasts were promoted by Chinese medicines containing HA in the time and dosage dependent manner. The osteoblasts displayed a polygonal morphology with cell-cell junctions in all cases. It is suggested that the contained Chinese medicines would promote the activities of the osteoblasts.

Retrospective dosimetry assessment using the 380 °C thermoluminescence peak of tooth enamel

Abstract The thermoluminescence (TL) response to gamma-ray irradiation of tooth enamel is reported. The tooth enamel was separated from dentine by using mechanical and physico-chemical procedures followed by grinding (grain size ∼100 μm) and etching. The TL was attributed to the recombination of CO 2 − radicals incorporated into or attached to the surface of hydroxyapatite crystals. The growth of the ∼380 °C TL peak with absorbed dose was examined with irradiated tooth enamel samples and reconstructed doses evaluated for tooth enamel samples from four human subjects.

Mechanism of promoted dipeptide formation on hydroxyapatite crystal surfaces

The formation of dipeptides from amino acids can be driven by hydroxyapatite at a relatively low temperature in air. For example, the formation of (Ala)2 from Ala is induced on hydroxyapatite at 110°C with considerable yield. Typically, condensing agents, high temperatures (>250°C) or high pressures (>25 MPa) are required to drive the condensation of amino acids. Similar effects are observed in the condensation of Gly, Glu and Asp. Experiments demonstrate that hydroxyapatite is an effective inorganic catalytic agent, reducing the activation barrier for the formation of dipeptides by more than 50%. HAP promotes condensation by adsorbing amino acid monomers in an organized manner, which decreases the distance between amino and carboxyl groups on neighboring molecules and extends the contact time of the reaction groups. This work provides a chemical understanding of the primitive condensation of amino acids and reveals a mechanism for enhancement of mineral catalysts. It is important that the conditions used for hydroxyapatite-assisted dipeptide formation are not harsh and can be readily achieved, revealing a possible mechanism for the chemical evolution of biomolecules over geologic ages.

Toward a Structure Determination Method for Biomineral-Associated Protein Using Combined Solid- State NMR and Computational Structure Prediction

Protein-biomineral interactions are paramount to materials production in biology, including the mineral phase of hard tissue. Unfortunately, the structure of biomineral-associated proteins cannot be determined by X-ray crystallography or solution nuclear magnetic resonance (NMR). Here we report a method for determining the structure of biomineral-associated proteins. The method combines solid-state NMR (ssNMR) and ssNMR-biased computational structure prediction. In addition, the algorithm is able to identify lattice geometries most compatible with ssNMR constraints, representing a quantitative, novel method for investigating crystal-face binding specificity. We use this method to determine most of the structure of human salivary statherin interacting with the mineral phase of tooth enamel. Computation and experiment converge on an ensemble of related structures and identify preferential binding at three crystal surfaces. The work represents a significant advance toward determining structure of biomineral-adsorbed protein using experimentally biased structure prediction. This method is generally applicable to proteins that can be chemically synthesized.

Molecular dynamics simulations of the adsorption of amino acids on the hydroxyapatite {100}-water interface

The understanding of interfaces and interaction of organic molecules and inorganic materials are the important issues in biomineralization. Experimentally, it has been found that amino acids (AA) can regulate the morphology of hydroxyapatite (HAP) crystals significantly. Inthis study, molecular dynamics simulation is employed to investigate the detailed adsorption behavior of polar, ionic, and hydrophobic AA on the {100} face of HAP at the atomic level. The results indicate that various AA are adsorbed on the HAP crystal surface mainly by amino and carboxylate groups at the specific sites. Multiple inter- action points are found for polar and ionic AA. The adsorbed AA molecules occupy the Ca and P sites of the HAP surfaces which may inhibit and regulate the HAP growth. The adsorbed amino acid layer can also change the interfacial hydration layer and influence the transporta- tion of ions in and out of HAP, which may be another strategy of biological control in biomineralization.

Effects of Acidic Amino Acids on Hydroxyapatite Morphology

Hydroxyapatite nanoparticles were synthesized in the presence of two kinds of acidic amino acids in order to investigate the effects of acidic amino acids on the morphology of hydroxyapatite obtained by homogeneous precipitation and hydrothermal treating. The results of X-ray diffraction analysis and microphotographs of transmission electron microscope showed that the morphology of hydroxyapatite nanoparticles changed significantly only with aspartic acid. Hydroxyapatite synthesized with aspartic acid showed smooth flake-like morphology at pH= 11 and flake-like morphology with stripes at pH= 8. However, hydroxyapatite synthesized in the present of glutamic acid showed rod-like morphology at both pH values. Variation in the morphology of nanoparticles could be consistent with the difference in the affinity of amino acids binding at the hydroxyapatite crystal surface. The adsorption process was revealed by computer simulation of molecular mechanics using energy minimizing method.

Effect of the Cysteine-Rich Mdm2 Peptide in the Crystal Growth of Hydroxyapatite in Aqueous Solution

The crystal growth of hydroxyapatite (HAP), the most stable calcium phosphate phase at 37 °C, pH 7.40, and ionic strength of 0.15 M NaCl, in the presence of the cysteine-rich Mdm2 peptide (containing 48 aminoacids in ring finger configuration) has been investigated by the constant composition technique. Crystallization took place exclusively on well-characterized HAP crystals in supersaturated solutions only with respect to this calcium phosphate salt. The kinetic results indicated a surface diffusion spiral growth mechanism. The presence of the Mdm2 peptide inhibited the crystal growth of HAP by 59−92% through adsorption onto the active growth sites of the HAP crystal surface. The kinetic results favored a Langmuir-type adsorption model, and the value of the calculated affinity constant was kaff = (258 ± 21) × 104 dm3 mol-1.

Nanoscale Probing of the Enamel Nanorod Surface Using Polyamidoamine Dendrimers

Although it is known that noncollagenous proteins of dental origin bind to the hydroxyapatite crystal surfaces, no measure of their binding strength has been calculated. This experiment used -COOH-capped generation 7 PAMAM dendrimers as nanoprobes of the biological hydroxyapatite nanorod surfaces. Dendrimer distribution was characterized using AFM. The results showed dendrimers to be spaced at intervals along the c-axis of the crystals. From these observations and assuming a fully ionized -COOH dendrimer, a mathematical model of the binding capacity of the crystal surface with the dendrimer was developed. The Monte Carlo method was used to simulate the binding process between the dendrimer and crystal surface, and the binding strength of the -COOH groups to the surface was calculated to be 90 +/- 20 kJ/mol. These results support the CFM studies which have described alternating bands of charge domains on the crystal surface and that the binding strength will be dependent on both the intensity of the charge on the protein and the crystal surface.

Kinetic Study on the Crystal Growth of Hydroxyapatite

The kinetics of hydroxyapatite (HAP) crystallization, which is thermodynamically the most stable calcium phosphate salt, has been investigated at physicochemically defined physiological conditions. The crystal growth process involved metastable supersaturated solutions, and for the study the constant composition method was employed. The kinetic results were tested over the most important crystal growth models. At the experimental conditions with the relatively low supersaturation level, the spiral growth model introduced by the Burton−Cabrera−Frank theory was found to better interpret the kinetic data. With the formulation of the model, thermodynamic and morphological parameters of hydroxyapatite were calculated. The surface kink site density was found to be 4.33 × 1014 kinks/m2, which means that the active growth sites represent less than 1 ‰ of the total number of HAP growth units on the crystal surface. The mean linear growth rate of the HAP crystal surface was found to be 1.34 monolayers per hour.

Hydroxyapatite Crystallization in the Presence of Amino Acids with Uncharged Polar Side Groups: Glycine, Cysteine, Cystine, and Glutamine

The crystal growth of hydroxyapatite (HAP), the most stable calcium phosphate phase at 37 °C, pH 7.40, and ionic strength 0.15 M, in the presence of four amino acids with uncharged polar side groups (glycine, cysteine, cystine, and glutamine) has been investigated by the constant composition technique. Crystallization took place exclusively on well-characterized HAP crystals in supersaturated solutions only with respect to this calcium phosphate salt. The kinetic results indicated a surface diffusion mechanism. The presence of the amino acids inhibited the crystal growth of HAP possibly through adsorption onto the active growth sites of the HAP crystal surface. Cysteine in particular caused a reduction in the crystal growth of HAP up to 81%. The kinetic results favored a Langmuir type adsorption model, and the value of the affinity constant for each amino acid was calculated. Each value was related to the nature of the amino acid side group.

The Surface Structure of Hydroxyapatite Single Crystal and the Accumulation of Arachidic Acid

Single crystals of hydroxyapatite (HAp) were grown by a flux method using β-tricalcium phosphate and Ca(OH)2 under hot isostatic pressure. After chemical etching by 0.05 N HCl aqueous solution, the HAp crystal surfaces were observed by an atomic force microscope (AFM) and shown dominantly to consist of steps with heights corresponding to a lattice distance d(100) or its 2/3. Arachidic acid films were accumulated on the etched HAp crystal by a Langmuir–Blodgett method. From section analyses by AFM, a distance between the carboxyl groups of arachidic acid and the HAp surface was estimated to be approximately 0.04 nm, sufficiently adjacent for a chemical interaction to take place.

Conformational analyses on soluble and surface bound osteopontin.

Immunohistology of calvarial sections revealed that staining with monoclonal anti-osteopontin antibodies (clone MPIIIB10) is minimal unless sections are first treated with EDTA. In contrast, following treatment of sections with EDTA, strong staining of mineralizing osteoid areas and osteoblast-like cells was noted (Fig. 1B). Immunostaining for osteopontin appeared to be specific in that controls which substituted rabbit IgG or normal mouse ascites fluid for monoclonal antibody, or which omitted monoclonal antibody uniformly gave background results (Fig. 1C). In an effort to circumvent problems of antibody accessibility we examined the immunoreactivity of OP when adsorbed to plastic and hydroxyapatite surfaces. Although OP bound to plastic surfaces is reactive with MPIIIB10 antibodies, OP adsorbed to hydroxyapatite crystal surfaces is not recognized by these antibodies as assessed by two detection methods. These results demonstrate that most or all of OP bound to hydroxyapatite exhibits a different conformation than when bound to plastic surfaces. On the basis of immunohistologic results with calvarial sections, we suggest that the conformation of native OP in bone and of isolated OP adsorbed to hydroxyapatite may be similar. Finally, solution circular dichroism and Fourier-transformed infrared spectroscopic studies indicate that the conformation of bone OP is dependent upon its concentration, and, secondarily to the presence or absence of calcium ion. With both spectroscopic methods, addition of calcium appeared to increase the extent of disordered structure. We suggest that these findings support our hypothesis that bone matrix proteins exhibit a different conformation when adsorbed on hydroxyapatite crystal surfaces. Assumption of a more organized secondary structure in concentrated OP solutions (i.e., 15 mg/ml) is consistent with these results in that local concentrations of OP within a semisolid matrix may approach or exceed levels used here.

Fine Structure and Elemental Analysis of the Enamel in Andrias davidianus (Cryptobranchidae) and Onychodactylus japonicus (Hynobiidae)

Functional teeth from Andrias davidianus and Onychodactylus japonicus were prepared for transmission electron microscopy (TEM) and electron microprobe analyzer (EMPA) of the elements of the enamel layer. The enamel layer is thin and the arrangement of the enamel crystals is regular in the outer enamel layer of Onychodactylus japonicus; on the other hand, the crystals of Andrias davidianus show irregularities in different tooth regions. Based on calcium and magnesium distribution levels using electron microprobe analyzer (EMPA), the enamel layer lacks enameloid in these species. Concentrations of iron and fluoride are located in the outer enamel layer. The presence of trace elements is related to the formation and mineralization of enamel. l of Herpetology, Vol. 25, No. 2, pp. 14 46, 1991 i t 1991 Society for the Study of Amphib ans and Reptiles cture and Elemental Analysi of the Enamel in i s avidianus (Cryptobranchidae) and actylus japonicus (Hynobiidae) O,1 KAZUYUKI SHIMADA,2 ATSUYUKI YOKOI,2 The enamel layers of the amphibian tooth form prismless structures, in contrast to mammalian teeth which have rods in the enamel layer. This material is known as aprismatic enamel, and though it is very thin, it is hypermineralized, as in mammals. In mammals, the enamel crystals are chiefly composed of hydroxyapatite, with iron, fluoride and other trace elements complexed in the surface enamel layer of the tooth (Halse and Selvig, 1974; Selvig and Halse, 1975; Weatherell et al., 1975; Heap et al., 1983; Nelson et al., 1989). A number of different trace elements may be involved in the formation of the apatite crystals. The pigmented layer of rat incisor enamel contains 1-10% iron (Halse and Selvig, 1974) bound to the surface of hydroxyapatite crystals (Selvig and Halse, 1975). Trace elements may also be associated with the mineral component of amphibian enamel. Based on observations using polarizing microscopy, Schmidt (1971) reported that the enamel of Andrias japonicus contains pigments that impart a ena el layers of the amphibian too h prismles structures, in contrast to ma brown color at the tip of the tooth, which he suggested to be an iron oxide. Using transmission electron microscopy (TEM), Kawasaki and Fearnhead (1983) reported the presence of enam loid, a substance previously reported to occur only in fish, beneath the enamel layer in the teeth of the salamanders Hynobius nigrescens and Cynops pyrrhogaster. Enameloid differs from enamel in having a different embryological origin (mesoderm vs. ectoderm), and in having a much lower mineral content (Poole, 1967). The purpose of this study was to look for an interrelationship between the fine structure, concentration, and distribution of trace elements of the enamel layer in Andrias davidianus and Onychodactylus japonicus, and to ascertain the presence or absence of enameloid in these species. MATERIALS AND METHODS Thirteen adult (total length ca. 80 cm) Andrias davidianus (Cryptobranchidae) and ten adult (tocolor at the tip of the tooth, w ich he ested to be an iron oxide. Using transmiselectron microscopy (TEM), K wasaki and head (1983) reported the presence of eloid, a substance previously reported to only in fish, beneath the enamel layer in teeth of the s lamanders Hynobius nigrescens nops py rhogaster. Enameloid differs from el in having a diff rent embryological or( esoderm vs. ectoderm), and in having a lower mineral content (Poole, 1967). urpose of thi study was t look for an relationship between the fine str cture, 141 This content downloaded from 157.55.39.59 on Mon, 17 Oct 2016 04:59:01 UTC All use subject to http://about.jstor.org/terms

Inhibition of apatite formation by titanium and vanadium ions

Ti4+ and V5+ ions were studied in two biologically relevant in vitro test systems to determine their effect on hydroxyapatite (HA) formation. System 1 involved direct HA precipitation from solution, and system 2 dealt with the growth of HA seed crystals. The experiments were carried out in a pH-stat by continuously recording NaOH uptake, which follows HA formation kinetics, at pH 7.4, 37 degrees C, and 0.15 M NaCl. In systems 1 and 2, Ti decreased HA formation kinetics in a dose-related manner without delaying the onset of HA formation. For V, the rate of HA proliferation decreased in system 1 in a dose-related manner, but the curves displayed a complicated shape. In system 2, V brought about a decrease in the HA seeded growth rate. Previous work has shown that Al blocks HA proliferation by adsorbing to active growth sites on the surface of HA crystals. By contrast, in this study V was found not to be adsorbed to the surface of HA. The mechanism of action of V probably involves the poorly understood hydrolysis and solution complex formation chemistry of the metal ion. We have shown here that V ions form V-PO4 complexes in solution in the HA formation systems; undoubtedly these are involved in the mechanism of V inhibition of HA formation. On the other hand, Ti was shown to bind to the surface of HA crystals in this study, which means that the ion may poison active crystal growth sites, as does aluminum. Ti-6Al-4V alloy is widely used in cementless total hip implants. Previous studies have shown that Ti concentrations 10 to 100 times higher than used here accumulate in osseous tissues around porous Ti implants in dogs 6 to 12 months after implantation. Ions leaching out over long periods of time into the implant interface could interfere with the normal osteoid mineralization and remodeling processes of bone in that region, which would result in subsequent loosening of the implant. This research suggests that further in vitro and animal studies should be carried out to determine the extent of Ti and V ion leaching from implants and their effect on tissue mineralization.

Effect of gallium on thein vitro formation, growth, and solubility of hydroxyapatite

Gallium (Ga) is an effective treatment for the hypercalcemia of malignancy. The mechanism of action of the metal in blocking bone resorption in humans is not well understood. This paper examines the effect of Ga on the in vitro formation of hydroxyapatite (HA) in three test systems that have possible biological relevance in a pH-stat at pH 7.4, 37 degrees C, and 0.15 M NaCl: (1) the direct precipitation of HA; (2) the transformation of amorphous calcium phosphate to HA; and (3) the growth of HA seeds. In addition, the effect of Ga on HA solubility was measured at pH 5.0, the approximate pH of osteoclastic bone resorption. Ga decreased the HA formation and/or growth kinetics in a dose-related manner in all three test systems. In addition, the time to the onset of HA formation was increased in systems 1 and 2. Also, the adsorption of Ga on the surface of HA crystals was measured. Ga reduced the dissolution kinetics of HA compared with Ga-free control. The mechanism reported herein--the significant adsorption of Ga on forming and growing HA nuclei and on the surface of HA crystals--is believed to be responsible for the effects of the metal on HA proliferation and solubility. Accumulation of the metal on newly formed metaphyseal bone can now be explained by this adsorption of Ga. These in vitro results partly explain the in vivo action of Ga in treating hypercalcemia by decreasing bone apatite solubility.(ABSTRACT TRUNCATED AT 250 WORDS)