The Role of Cation Bridging in Microbial Fluoride Binding

Abstract

The cation-bridged fluoride binding model proposed previously was tested by measuring fluoride binding to Streptococcus mutans R9 in the presence and absence of calcium, magnesium or zinc ions. The dissociation constant for fluoride binding to washed cells was 8.4 ± 7.9 mmol/l and the binding capacity was 4.3 ± 1.7 µmol/g wet weight. Binding was largely accounted for by residual bound divalent magnesium, with a small contribution from calcium. In the presence of 5 mmol/l divalent cation, dissociation constants for fluoride (mmol/l) were: 12.2 ± 3.8 (calcium), 9.9 ± 0.4 (magnesium) and 14.4 ± 0.5 (zinc). Binding capacities (µmol/g wet weight) were: 122 ± 26 (calcium), 130 ± 90 (magnesium) and 142 ± 56 (zinc). Fluoride produced a marked reduction in calcium binding affinity and approximately doubled the calcium binding capacity. In the absence of fluoride, divalent cation binding to plaque is bidentate. It is suggested that fluoride, by competing with macromolecular anionic groups, causes binding to become monodentate. This allows the binding of double the quantity of cations (and of further fluoride). Release of fluoride, bound by calcium bridging, into plaque fluid, as a result of fluoride clearance into saliva, or of a fall in pH, will always be accompanied by a release of calcium which will potentiate the cariostatic effect of fluoride.

Fluoride in Plaque Fluid, Plaque, and Saliva Measured for 2 hours after a Sodium Fluoride Monofluorophosphate Rinse

“…Since, as calculated from the free fluid volume of plaque (above) and the data of figures 1 and 2, the total plaque fluid fluoride was only about a mass fraction 03% of the total plaque fluoride, the release of even a small amount of fluoride from these reservoirs could maintain an elevated level of plaque fluid fluoride. Although the nature of this binding is unknown, it may include binding to cellular anion retention sites by cation bridging, notably calcium [Rose et al, 1996]. Another probable source is CaF 2 or 'CaF 2 -like' deposits [Arends and Christoffersen, 1990] which have been proposed as the source of labile fluoride reservoirs found on clean and plaque-covered enamel after in vitro and in vivo fluoride application [Hellwig et al, 1987;Bruun and Givskov, 1993;Rølla and Ekstrand, 1996].…”

A Continuum of Anionic Charge: Structures and Functions ofd-Alanyl-Teichoic Acids in Gram-Positive Bacteria

“…17). Rose et al (425) extended this model to include bidentate Ca 2ϩ bridging between cells in plaque. While this organization of bridging cations is difficult to comprehend in detail, let alone to demonstrate, it appears that the wall matrix, with its anionic constituents and metal cations, represents a supramolecular organization that is used by cells for growth and survival.…”

Remineralization Effect of a Low-Concentration Fluoride Rinse in an Intraoral Model

“…The precipitation of calcium fluoride (CaF 2 ) or 'CaF 2 -like' deposits has been suggested to be a major form of this loosely bound F after an NaF rinse [Rølla, 1998;Lagerlöf et al, 1988;Røl-la and Saxegaard, 1990]. F deposition in plaque may additionally be attributed to the formation of bacteria-Ca-F complexes [Rose et al, 1996].…”