The Effect of pH, Temperature and Plaque Thickness on the Hydrolysis of Monofluorophosphate in Experimental Dental Plaque

Abstract

Monofluorophosphate (MFP), an anti-caries agent commonly used in toothpaste, is known to be degraded to fluoride and orthophosphate by bacterial phosphatases in dental plaque. We have examined the effect of pH, temperature, plaque thickness and some ions on this process. Both natural plaque and artificial microcosm plaque incubated with purified MFP at pH 4–10 showed an optimum pH of ∼8 for hydrolysis. Diffusion and concomitant hydrolysis were examined in an apparatus in which artificial plaque was held between rigid membranes separating two chambers. When MFP diffused through a plaque of 0.51-mm thickness over 4 h it was almost completely hydrolysed at pH 8, but hydrolysis on diffusion decreased as the pH deviated from 8. MFP in toothpaste extract showed a similar pH susceptibility to hydrolysis, according to the inherent pH of the toothpaste. Hydrolysis of MFP in the toothpaste was reduced by no more than 10% when compared with a matched-pH control, suggesting that other toothpaste ingredients had no major influence on hydrolysis. Transport was slower and hydrolysis at pH 6 more complete the thicker the plaque, but hydrolysis was not significantly slower at 23°C than at 37°C. The addition of various potential activating or inhibiting ions at 0.1 and 1.0 mmol/l had small and non-significant effects on hydrolysis. The results suggest that MFP toothpaste should be formulated and used to maximise enzymic hydrolysis of this complex anion, and that plaque pH control is probably the most important factor.

Effect of a Calcium Carbonate-Based Dentifrice on in situ Enamel Remineralization

“…This result agrees with Sullivan et al [1995] and is consistent with the accepted mechanism of action of MFP on caries control [Pearce and Dibdin, 2003], namely that hydrolysis during diffusion through dental plaque is essential.…”

“…The opposite fi ndings presented by the treatments with the dentifrices suggest that test plaque acted as a reservoir of MFP ions, as previously shown ]. The MFP molecule per se is not active against caries, but is hydrolysed by non-specifi c bacterial phosphatases [Pearce and Dibdin, 2003], releasing fl uoride ion, which enhances the ability of plaque fl uid to remineralize the enamel [Edgar and Higham, 1995]. In fact, we previously showed that test plaque was able to concentrate fl uoride and to hydrolyse MFP ].…”

“…The lack of statistical difference between the MFP dentifrices suggests that the superiority of the CaCO 3 abrasive compared with SiO 2 on inhibition of enamel demineralization does not seem to be relevant with respect to remineralization. The lack of difference between the dentifrices in terms of remineralization could be explained by the fact that the optimum pH for MFP hydrolysis is approximately 8.0 [Pearce and Dibdin, 2003], but the pH of the SiO 2 -and CaCO 3 -based dentifrices is 6.8 and 9.4, respectively [Ignácio et al, 1999]. Thus, neither have an ideal pH for F -release.…”

Effect of Calcium Glycerophosphate on Demineralization in an in vitro Biofilm Model

“…If such buffering did occur then it is possible that the enhanced effi cacy of CaGP when applied before acidic challenges was a pH-dependent effect. Whilst optimum activity would have been in the region of pH 8, recent fi ndings by Pearce and Dibdin [2003] suggest that hydrolysis would still be very much more effi cient at the resting pH in this model, typically between 6.2 and 6.5, than during acidic challenges below pH 5. However, this remains speculative.…”

Kinetics of Monofluorophosphate Hydrolysis in a Bacterial Test Plaque in situ

“…Phosphatase activity of the S. mutans IB-1600 strain was evaluated as described by Pearce and Dibdin [2003]. Briefly, 100 mg (wet weight) of test plaque prepared as described above was dispersed in 1 ml of 140 m M NaCl, and 2 ml of universal buffer (containing 0.2 M succinic acid, 0.1 M Tris and 0.2 M glycine, and 75 m M NaCl, 0.3 m M MgCl 2 and 0.02 m M ZnCl 2 to optimize phosphatase activity), adjusted to pH 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 or 10.0, was added to the suspension.…”