Citrate Occurs Widely in Healthy and Pathological Apatitic Biomineral: Mineralized Articular Cartilage, and Intimal Atherosclerotic Plaque and Apatitic Kidney Stones

Abstract

There is continuing debate about whether abundant citrate plays an active role in biomineralization of bone. Using solid state NMR dipolar dephasing, we examined another normally mineralized hard tissue, mineralized articular cartilage, as well as biocalcifications arising in pathological conditions, mineralized intimal atherosclerotic vascular plaque, and apatitic uroliths (urinary stones). Residual nondephasing ¹³C NMR signal at 76 ppm in the spectra of mineralized cartilage and vascular plaque indicates that a quaternary carbon atom resonates at this frequency, consistent with the presence of citrate. The presence, and as yet unproven possible mechanistic involvement, of citrate in tissue mineralization extends the compositional, structural, biogenetic, and cytological similarities between these tissues and bone itself. Out of 10 apatitic kidney stones, five contained NMR-detectable citrate. Finding citrate in a high proportion of uroliths may be significant in view of the use of citrate in urolithiasis therapy and prophylaxis. Citrate may be essential for normal biomineralization (e.g., of cartilage), play a modulatory role in vascular calcification which could be a target for therapeutic intervention, and drive the formation of apatitic rather than other calcific uroliths, including more therapeutically intractable forms of calcium phosphate.

Spontaneous supersaturation of calcium citrate from simultaneous isothermal dissolution of sodium citrate and sparingly soluble calcium hydroxycarboxylates in water

“…However, precipitate was formed with a lag phase ranging for up to 2 hours depending on the degree of supersaturation. The speciation of calcium in these supersaturated solutions depends on the chemical equilibria of eqn (8) and (9). Since citrate has a thermodynamic association constant of 3.6 Â 10 4 at 25 C for binding calcium compared to 49 for lactate, citrate can be assumed to control the free calcium ion concentration.…”

“…The presence of citrate in calcied tissue has now been recognized. 7,8 The demonstration of the assistance of citrate in spontaneous supersaturation phenomena occurring by isothermal dissolution against a concentration gradient explains important part of biomineralization dynamics. This perspective of spontaneous supersaturation needs also to be studied for higher temperatures as they are relevant for physiological conditions in the human body.…”

“…5 A more specic role of citrate in apatite crystallization has now also been recognized during bone mineralization. [6][7][8] A common factor for these observations from experimental human nutrition and from medical treatment of osteoporosis may be related to the solubility overshooting phenomena recently observed for dissolution of calcium citrate tetrahydrate in aqueous solutions of sodium citrate but not in pure water. 9 Notably, the initial solubility of calcium citrate in aqueous sodium citrate was higher by approximately 10% than the equilibrium solubility established aer 48 hours of isothermal equilibration of solutions with solid calcium citrate in the temperature interval from 0 to 100 C. 9 Similarly spontaneous supersaturation of calcium D-gluconate in water by up to a factor of seven was observed by isothermal continuing dissolution of calcium L-lactate in an already saturated aqueous calcium L-lactate solution aer addition of solid sodium Dgluconate.…”

Citrate chemistry and biology for biomaterials design

“…As an anion, citrate serves as a key chelating agent of physiologically important divalent cations, especially Ca 2+ , Fe 2+ and Mg 2+ . It is well-known that more than 90% of total citrate is accumulated in human bone and citrate also concentrates in other healthy or pathological biomineralized tissues, such as teeth dentin [78], kidney stones and atheroscleroic plaques [79]. Besides, citrate is found in blood [80] and other soft tissues, such as prostate [81], kidney [82], and brain.…”

Pyrophosphate-Inhibition of Apatite Formation Studied by In Situ X-Ray Diffraction

“…Apatite biomineralization occurs under strict biological control enforced by the collective action of numerous additives, including small molecules, such as citrate [2][3][4][5], pyrophosphate, and polyphosphates [6,7], as well as larger macromolecules. Pyrophosphate is involved in controlling apatite formation during skeleton formation and is a potent inhibitor of apatite crystallization [8][9][10][11][12][13][14][15][16][17][18][19][20][21].…”