Hydroxyapatite dissolution

N.H. de Leeuw, Resisting the onset of hydroxyapatite dissolution through incorporation of fluoride, J. Phys. Chem. B 108 (2004) 1809-1811. 

The influence of Na+, K+, calcite, and EDTA on complete hydroxyapatite dissolution and hydroxypyromorphite formation was also examined (Ma 1996). Only high EDTA concentrations (EDTA/Pb molar ratios up to 5) significantly affected the ability of dissolving hydroxyapatite from sequestering hydroxypyromorphite the strong bond between the EDTA ligand and the Pb2+ was fairly competitive with the tendency to precipitate out as hydroxypyromorphite. 

Fleisch, H., Russell, R. G. G., Francis, M. D. Diphosphonates inhibit hydroxyapatite dissolution in vitro and bone resorption in tissue culture and in vivo. Science 165, 1262 (1969)... 

Chin K. O. A., Johnsson M., Bergey E. J., Levine M. J., and Nancollas G. H. (1993) A constant composition kinetics study of the influence of salivary cystatins, statherin, amylase, and human serum albumin on hydroxyapatite dissolution. Coll. Surf. A Physicochem. Eng. Aspects 78, 229-234. 

Forward GC A new method of measuring hydroxyapatite dissolution rate. Caries Res 1977 11 9-15. 

Although bisphosphonates prevent hydroxyapatite dissolution, their antiresorptive action is due to direct inhibitory effects on osteoclasts rather than strictly physiochemical effects. The antiresorptive activity apparently involves two primary mechanisms osteoclast apoptosis and inhibition of components of the cholesterol biosynthetic pathway. 

Follmi KB (1995) 160 m.y. record of marine sedimentary phosphoras burial Coupling of climate and continental weathering under greenhouse and icehouse conditions. Geology 23 859-862 Fox JL, Higuchi WI, Fawz MB, Wu MS (1978) A new two-site model for hydroxyapatite dissolution in acidic media. J Colloid Interface Sci 67 312-330... 

Metabolic acidosis, a common complication of CKD, also contributes to ROD by altering the solubility of hydroxyapatite, promoting bone dissolution. Additionally, metabolic acidosis inhibits the activity of osteoblasts, which promote bone formation, while stimulating osteoclasts to promote bone resorption. Finally, metabolic acidosis can worsen sHPT by reducing the sensitivity of the parathyroid gland to serum calcium levels.38... 

Phosphate is remineralized during the oxidation of organic matter and dissolution of hard parts, such as bones and teeth, that are composed of the minerals hydroxyapatite and fluoroapatite. Unlike the other products of remineralization, pore-water phosphate concentrations are regulated only by mineral solubility, such as through vivianite (iron phosphate) and francolite (carbonate fluoroapatite). Redox reactions are not significant because phosphorus exists nearly entirely in the h-5 oxidation state. 

Bone dissolution is composed of two major processes mineral dissolution and protein degradation. Bone mineral is hydroxyapatite i.e. (Ca3(P04)2)3 x Ca(OH)2. It is dissolved into Ca, HP04, and H2O with the help of hydrochloric acid. For this, independently protons (H" ) and chloride (Cl ) are... 

The bisphosphonates are all analogues of pyrophosphate. They inhibit osteoclast resorption of bone and they are able to inhibit the formation and dissolution of hydroxyapatite crystals, however their exact mechanism is not well understood. Other effects which have relevance for bone homeostasis include inhibition of the activities of PTH, prostaglandins and 1,25-dihydroxy vitamin D. Bisphosphonates bind to bone with high affinity. They have therefore a duration of action that continues long after their use has been stopped. 

M.R. Christoffersen, J. Christoffersen, J. Arends, Kinetics of dissolution of calcium hydroxyapatite. VII. The effect of fluoride, J. Cryst. Growth 67 (1984) 107-114. 

In addition, the presence of low concentrations of fluoride in saliva also has the effect of preventing dissolution of hydroxyapatite from enamel at low pH, an effect that has been shown to apply at values of pH as a low as 4.2 [54,55], Thus, it is the fluoride in solution that has the effect of reducing solubility, rather than the fluoride in the mineral phase [51], This effect requires extremely small amounts of fluoride, typically in the sub-ppm range [51,56], and has the effect of shifting the balance between demineralisation and remineralisation so that loss of the hard tissue is inhibited. 

In the presence of fluoride, calcium ions have been found to be more firmly anchored than in pure hydroxyapatite [67]. This enhances the overall resistance to dissolution. Thus, the presence of a thin stable film of fluorapatite on the surface of hydroxyapatite crystals has two effects, namely (i) resistance to diffusion and dissolution of the anion and (ii) firmer binding of calcium ions into the surface. Both of these make the resulting apatite structure more resistant to dissolution, regardless of the pH of the external medium, and they thereby increase the resistance of the mineral phase to the onset of caries. 

More recently, it has been shown that topical fluoride preparations do not lead to fluoridation of the hydroxyapatite crystal [181]. Rather they form a calcium fluoride-like substance that is deposited onto the tooth surface and dissolves when the local pH is lowered [182]. The resulting dissolution adjacent to the tooth surface provides a source of soluble fluoride that can be incorporated into the mineral structure, and thus augment remineralisation. 

Ryan et al. (2001) studied the effects of hydroxyapatite addition to soils impacted by Pb from smelter operations. Dialysis experiment were conducted where the soil and hydroxyapatite solids were placed in separate dialysis bags suspended in 0.01 M NaN03. Chloropyromor-phite formed on the dialysis membrane containing the soil. The dissolution of solid-phase soil Pb was the rate-limiting step for pyromorphite formation. EXAFS showed that after the 240 day incubation the hydroxyapatite treatment caused a change in the average, local molecular bonding environment of soil Pb. 

The bisphosphonates are analogs of pyrophosphate in which the P-O-P bond has been replaced with a nonhydrolyzable P-C-P bond (Figure 42-4). Etidronate, pamidronate, and alendronate have now been joined by risedronate, tiludronate, ibandronate, and zoledronate for clinical use. The bisphosphonates owe at least part of their clinical usefulness and toxicity to their ability to retard formation and dissolution of hydroxyapatite crystals within and outside the skeletal system. They localize to regions of bone resorption and so exert their greatest effects on osteoclasts. However, the exact mechanism by which they selectively inhibit bone resorption is not clear. 

Mechanism of Action Inhibits formation, growth, and dissolution of hydroxyapatite crystals and their amorphous precursors by chemisorption to calcium phosphate surfaces ... 

Many biological and environmental processes involve the dissolution or precipitation of a sparingly soluble ionic compound. Tooth decay, for example, begins when tooth enamel, composed of the mineral hydroxyapatite, Cas PO OH,... 

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