Journal of Dental Research, Vol 74, 691-697, Copyright © 1995 by International & American Associations for Dental Research Online Journals
The diffusion and enzymic hydrolysis of monofluorophosphate in dental plaque
E. I. Pearce and G. H. Dibdin
Dental Research Unit, HRC of NZ, Wellington.
Although the ability of dental plaque to hydrolyze sodium
monofluorophosphate (MFP) has been known for some time, its effect on the
F- concentration at the plaque-enamel interface is undefined. We have
determined enzyme kinetic values for MFP hydrolysis and diffusion
coefficients so that the penetration and degradation of MFP in plaque can
be modeled by computer. The KM and Vmax values for natural human plaque
were 1.77 mmol/L and 41.4 nmol/min/mg protein, respectively, at pH 8.0. At
pH 6.0, the Vmax was lower, 15.6 nmol/min/mg, but KM was not significantly
different. Competitive inhibition by orthophosphate gave a Ki of 4.55
mmol/L. The diffusion coefficient for MFP in artificial plaque was 1.91 x
10(-6) cm2/sec. When these data were used for mathematical modeling of the
effects of rinsing with MFP and F- solutions, compared with an equivalent
NaF application, the concentration of F- from MFP was lower at the inner
surface of plaque, and the peak occurred later. Both pH and plaque
thickness had a marked effect on the amount of MFP that could penetrate: At
pH 8.0, almost none reached the inner surface of a 1-mm-thick plaque
intact. At pH 6.0, however, more MFP was able to penetrate, due to lower
MFPase activity. While MFP diffusion is inherently slower than that of F-,
enzymic degradation increases the gradient for inward diffusion. If the
conventional view that MFP in toothpaste acts as a source of F- is true,
then MFP toothpaste should be formulated to optimize MFPase activity in
dental plaque.
