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.

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