Fluoride (F^-) ions at concentrations present in vivo at the plaque/enamel interface (0.05–10 mM) inhibited the 4activities of lactoperoxidase (LP), myeloperoxidase (MP) and total salivary peroxidase (TSP) in a pH- and dose-dependent way. The inhibition was observed only at pH ≤ 6.5 and with F concentrations ≥ 0.1 mM. At pH 5.5 LP activity was inhibited by 85% and MP by 34% with 10 mM F^-. TSP activity was also inhibited only at low pH (5.5) by approximately 25%. Furthermore, the generation of the actual antimicrobial agent in vivo, hypothiocyanite (HOSCN/OSCN), of the oral peroxidase systems was inhibited by F^-, again at low pH (5.0–5.5) both in buffer (by 45%) and in saliva (by 15%). This inhibition was observed only with the highest F^- concentrations studied (5–10 mM). Fluoridated toothpaste (with 0.10 or 0.14% F) mixed with saliva did not inhibit TSP or HOSCN/OSCN generation. This may have been due to the ‘buffering’ effect of toothpaste which did not allow salivary pH to drop below 5.9. We conclude that the F^- ions in acidic fluoride products, e.g. in gels or varnishes (but not in toothpastes), may have the potential to locally inhibit the generation of a nonimmune host defense factor, HOSCN/OSCN/SCN produced by oral peroxidase systems. The possible clinical significance of this finding remains to be shown.

The separate and combined effects of peroxidase-generated hypothiocyanite (HOSCN/OSCN-) and F- ions on glucose uptake and growth of Streptococcus mutans ATCC 25175 were investigated. S. mutans cells were grown to late exponential or stationary growth phase, harvested, washed and suspended in 2.0 ml of sterilized human whole saliva supplemented with 10 mM D-glucose. This saliva-bacteria mixture was supplemented with 5-150 microM H2O2 at pH 5.0 or 6.5. At pH 5.0, up to 103 +/- 21 microM HOSCN/OSCN- was generated. After 20 h of incubation at 37 degrees C, the saliva-bacteria suspension exposed to HOSCN/OSCN- were plated on mitis salivarius agar plates and incubated anaerobically for 2 days. Identical experiments were made with F- ions (0.5, 1.0 and 5.0 mM). Both HOSCN/OSCN- and F- caused a significant dose-dependent growth inhibition at pH 5.0, whereas no inhibition was observed at pH 6.5. When F- and HOSCN/OSCN- were added simultaneously at pH 5.0, an additive effect of growth inhibition was observed. In glucose incorporation experiments the bacteria-saliva mixture was exposed to 1 microM HOSCN/OSCN-, 0.5 mM F- or both. F-, HOSCN/OSCN- or their combination in sterilized whole saliva at pH 5.0 caused 14.2, 67.8 and 74.2% inhibition, respectively. These observations indicate that F- and HOSCN/OSCN- ions have an additive inhibitory effect on S. mutans and therefore their combination is likely to be more antibacterial than either agent alone.

Delmopinol is a new surface-active agent which can reduce plaque formation and gingivitis. This study was aimed to analyze whether delmopinol (0.0032-0.65 mM) interferes with the activity of two surface-active oral antimicrobial enzymes, salivary peroxidase and lysozyme. In addition to human whole saliva (pH 5.0 and 6.0), the experiments were done in 0.1 M phosphate buffer (pH 6.0) with purified lactoperoxidase (LPO) and myeloperoxidase (MPO). LPO and MPO were significantly inhibited in buffer by delmopinol concentrations > 6.5 mM and > or = 3.2 mM, respectively. No such inhibition was found for total peroxidase activity in mixed saliva. In vitro, delmopinol was found to desorb surface-bound peroxidases in an active form to the liquid phase. In further analyses, the possible effect of delmopinol on peroxidase-generated hypothiocyanite (HOSCN/OSCN-) was studied in saliva and buffer. No effect was found in buffer, but salivary HOSCN/OSCN- declined significantly with 6.5 mM delmopinol. This was obviously due to an enhanced decay of hypothiocyanite, rather than its reduced rate of formation. No delmopinol-related inhibition of lysozyme occurred in saliva or buffer. The results suggest that high concentration (6.4 mM -0.2%) of delmopinol may lower the concentrations of antimicrobial HOSCN/OSCN- in saliva but has no effect on human lysozyme.