The Diffusion and Enzymic Hydrolysis of Monofluorophosphate in Dental Plaque

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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.

Section: Discussionsupporting

confidence: 89%

mentioning

confidence: 99%

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

Pearce¹,

Dibdin

2003

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“…Here again the high levels of MFP found in plaque fluid ( fig. 1) are relevant, and in agreement with these measurements, a recent diffusion-hydrolysis model for MFP in plaque [Pearce and Dibdin, 1995] …”

Section: Discussionsupporting

confidence: 85%

“…Specifically, NaMFP requires a hydrolysis step to release fluoride to the mouth whereas NaF does not. This hydrolysis step has been postulated [Pearce and Dibdin, 1995] as a major reason for the observation that NaF dentifrices generally show higher salivary and whole plaque fluoride concentrations than NaMFP dentifrices [Duckworth et al, 1994]. In spite of these results, clinical studies have yielded somewhat ambiguous results …”

mentioning

confidence: 99%

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

Vogel

Y²,

Chow³

et al. 2000

Sodium monofluorophosphate (NaMFP) and sodium fluoride (NaF) are the two most common sources of fluoride used in currently marketed fluoride dentifrices. The purpose of this study was to investigate the effect of mouth rinses containing NaF or NaMFP on the concentrations of fluoride, or the MFP ion, in saliva, whole plaque, and plaque fluid. Twelve subjects abstained from tooth brushing for 48 h, fasted overnight, and then rinsed 1 min with 12 mmol/l (228 ppm [μg/g] F) NaF or NaMFP in the morning. Before the rinse and at 30, 60 and 120 min afterwards, upper and lower molar and premolar plaque samples and whole saliva samples were collected. Aliquots of plaque fluid and centrifuged saliva were obtained from these samples, and the whole plaque residue acid extracted. The F and MFP concentrations were then measured in these samples using ultramicro methods. For both rinses, a higher concentration of plaque fluid fluoride was found at lower molar sites while the reverse was true for the whole plaque fluoride. Furthermore, for both rinses, plaque fluid, whole plaque, but not salivary, fluoride concentrations were above baseline at 120 min. Following the NaMFP rinse, a substantial amount of unhydrolyzed MFP was found in plaque fluid and saliva. Although there was a very large range in these measurements, fluoride in plaque fluid (excluding fluoride in unhydrolyzed MFP) and whole plaque were significantly (p<0.05) greater after the NaF rinse at all time periods. In saliva, the NaF rinse produced a statistically significant greater salivary fluoride (excluding fluoride in unhydrolyzed MFP) only at 60 min. The lack of a clear correlation between these measurements and clinical studies suggest a novel mechanism may enhance the effectiveness of NaMFP dentifrices.

“…The mathematical model used is based on that described by Dibdin [1990] for modelling a cariogenic challenge and that of Pearce and Dibdin [1995] for modelling the diffusion of monofluorophosphate into dental plaque, and its subsequent hydrolysis. The model includes the salivary clearance of sugar from the mouth; site-dependent exchange between the bulk saliva and the plaque surface via a salivary film; sugar diffusion into plaque and pH-dependent acid formation; diffusion and dissociation equilibria for the acid end-products and other buffers (acetate, lactate, phosphate and carbonate); the diffusion of protons and other ions and their simultaneous equilibration with fixed and mobile buffers.…”

Section: Methodsmentioning

confidence: 99%

A Mathematical Model of the Influence of Salivary Urea on the pH of Fasted Dental Plaque and on the Changes Occurring during a Cariogenic Challenge

Dibdin

Dawes

1997

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Urea diffusing from saliva into dental plaque is converted to ammonia and carbon dioxide by bacterial ureases. The influence of normal salivary urea levels on the pH of fasted plaque and on the depth and duration of a Stephan curve is uncertain. A numerical model which simulates a cariogenic challenge (a 10% sucrose rinse alone or one followed by use of chewing-gum with or without sugar) was modified to include salivary urea levels from 0 to 30 mmol/l. It incorporated: site-dependent exchange between bulk saliva and plaque surfaces via a salivary film; sugar and urea diffusion into plaque; pH-dependent rates of acid formation and urea breakdown; diffusion and dissociation of end-products and other buffers (acetate, lactate, phosphate, ammonia and carbonate); diffusion of protons and other ions; equilibration with fixed and mobile buffers; and charge-coupling between ionic flows. The K_m (2.12 mmol/l) and V_max (0.11 μmol urea/min/mg dry weight) values for urease activity and the pH dependence of V_max were taken from the literature. From the results, it is predicted that urea concentrations normally present in saliva (3–5 mmol/l) will increase the pH at the base of a 0.5-mm-thick fasted plaque by up to 1 pH unit, and raise the pH minimum after a sucrose rinse or sugar-containing chewing-gum by at least half a pH unit. The results suggest that plaque cariogenicity may be inversely related to salivary urea concentrations, not only when the latter are elevated because of disease, but even when they are in the normal range.