The interaction of sodium monofluorophosphate (MFP) with artificial plaque and the underlying enamel was investigated. Streptococcus mutans Ingbritt (serotype c) and Streptococcus sobrinus 6715 (serotype g) cells were grown on windowed human enamel blocks to form artificial plaque. Plaque-covered enamel and clean enamel samples were incubated anaerobically with 5 ppm (as F-) of MFP or sodium fluoride (NaF) for 48 hr.
Fluoride concentration in the enamel was significantly increased in plaque-covered enamel in the presence of both MFP and NaF. More than 60% of the acquired fluoride from MFP in the enamel at a depth of 5μm was KOH-soluble fluoride. More fluoride was acquired from MFP in the enamel covered with the plaque of S. sobrinus 6715 than in that covered with the plaque of S. mutans Ingbritt.
Significant degradation of MFP was detected during the growth of S. sobrinus 6715 concomitant with the fall of broth pH around 5.4 within 48 hr incubation. Optimum MFP hydrolysis by S. sobrinus 6715 and S. mutans Ingbritt occurred at approximately pH 5.0, but no detectable MFP hydrolysis was found at alkaline pH. MFP hydrolyzing activity (MFPase) was sharply inhibited by low F- concentration and was not activated by Mg2+. These data indicated that acid phosphatase, and not alkaline phosphatase, is responsible for MFP hydrolysis in mutans streptococci.